# ABSTRACT: A gentle introduction to Dancer2
package Dancer2::Manual;

__END__

=pod

=encoding UTF-8

=head1 NAME

Dancer2::Manual - A gentle introduction to Dancer2

=head1 VERSION

version 0.208001

=head1 DESCRIPTION

Dancer2 is a free and open source web application framework written in Perl.

It's a complete rewrite of L<Dancer>, based on L<Moo> and using a more
robust and extensible fully-OO design.

It's designed to be powerful and flexible, but also easy to use - getting up
and running with your web app is trivial, and an ecosystem of adaptors for
common template engines, session storage, logging methods, serializers, and
plugins to make common tasks easy means you can do what you want to do, your
way, easily.

=head1 INSTALL

Installation of Dancer2 is simple, using your favourite method to install from
CPAN, e.g.:

    perl -MCPAN -e 'install Dancer2'

Thanks to the magic of cpanminus, if you do not have CPAN.pm configured, or
just want a quickfire way to get running, the following should work, at
least on Unix-like systems:

    wget -O - http://cpanmin.us | sudo perl - Dancer2

(If you don't have root access, omit the 'sudo', and cpanminus will install
Dancer2 and prereqs into C<~/perl5>.)

Dancer2 is also available as a package from the package repository of several
distributions, for example on Debian/Ubuntu you should be able to just:

    apt-get install libdancer2-perl

Do be aware, though, that distribution-packaged versions sometimes lag behind
the most recent version on CPAN.

=head1 BOOTSTRAPPING A NEW APP

Create a web application using the dancer script:

    $ dancer2 -a MyApp && cd MyApp
    + MyApp
    + MyApp/config.yml
    + MyApp/Makefile.PL
    + MyApp/MANIFEST.SKIP
    + MyApp/.dancer
    + MyApp/cpanfile
    + MyApp/bin
    + MyApp/bin/app.psgi
    + MyApp/environments
    + MyApp/environments/development.yml
    + MyApp/environments/production.yml
    + MyApp/lib
    + MyApp/lib/MyApp.pm
    + MyApp/public
    + MyApp/public/favicon.ico
    + MyApp/public/500.html
    + MyApp/public/dispatch.cgi
    + MyApp/public/404.html
    + MyApp/public/dispatch.fcgi
    + MyApp/public/css
    + MyApp/public/css/error.css
    + MyApp/public/css/style.css
    + MyApp/public/images
    + MyApp/public/images/perldancer.jpg
    + MyApp/public/images/perldancer-bg.jpg
    + MyApp/public/javascripts
    + MyApp/public/javascripts/jquery.js
    + MyApp/t
    + MyApp/t/001_base.t
    + MyApp/t/002_index_route.t
    + MyApp/views
    + MyApp/views/index.tt
    + MyApp/views/layouts
    + MyApp/views/layouts/main.tt

It creates a directory named after the name of the app, along with a
configuration file, a views directory (where your templates and layouts
will live), an environments directory (where environment-specific
settings live), a module containing the actual guts of your application, and
a script to start it. A default skeleton is used to bootstrap the new
application, but you can use the C<-s> option to provide another skeleton.
For example:

    $ dancer2 -a MyApp -s ~/mydancerskel

For an example of a skeleton directory check the default one available in
the C<share/> directory of your Dancer2 distribution.

(In what follows we will refer to the directory in which you have created your
Dancer2 application -- I<e.g.,> what C<MyApp> was above -- as the
C<appdir>.)

Because Dancer2 is a L<PSGI> web application framework, you can use the
C<plackup> tool (provided by L<Plack>) for launching the application:

    plackup -p 5000 bin/app.psgi

View the web application at:

    http://localhost:5000

=head1 USAGE

When Dancer2 is imported to a script, that script becomes a webapp, and at
this point, all the script has to do is declare a list of B<routes>. A
route handler is composed by an HTTP method, a path pattern and a code
block. C<strict>, C<warnings> and C<utf8> pragmas are also imported with
Dancer2.

The code block given to the route handler has to return a string which will
be used as the content to render to the client.

Routes are defined for a given HTTP method. For each method supported, a
keyword is exported by the module.

=head2 HTTP Methods

Here are some of the standard HTTP methods which you can use to define your
route handlers.

=over 4

=item * B<GET> The GET method retrieves information, and is the most common

GET requests should be used for typical "fetch" requests - retrieving
information. They should not be used for requests which change data on the
server or have other effects.

When defining a route handler for the GET method, Dancer2 automatically
defines a route handler for the HEAD method (in order to honour HEAD
requests for each of your GET route handlers).

To define a GET action, use the L<get|Dancer2::Manual/get> keyword.

=item * B<POST> The POST method is used to create a resource on the server.

To define a POST action, use the L<post|Dancer2::Manual/post> keyword.

=item * B<PUT> The PUT method is used to replace an existing resource.

To define a PUT action, use the L<put|Dancer2::Manual/put> keyword.

a PUT request should replace the existing resource with that specified - for
instance - if you wanted to just update an email address for a user, you'd
have to specify all attributes of the user again; to make a partial update,
a PATCH request is used.

=item * B<PATCH> The PATCH method updates some attributes of an existing resource.

To define a PATCH action, use the L<patch|Dancer2::Manual/patch> keyword.

=item * B<DELETE> The DELETE method requests that the origin server delete the
resource identified by the Request-URI.

To define a DELETE action, use the L<del|Dancer2::Manual/del> keyword.

=back

=head3 Handling multiple HTTP request methods

Routes can use C<any> to match all, or a specified list of HTTP methods.

The following will match any HTTP request to the path C</myaction>:

    any '/myaction' => sub {
        # code
    }

The following will match GET or POST requests to C</myaction>:

    any ['get', 'post'] => '/myaction' => sub {
        # code
    };

For convenience, any route which matches GET requests will also match HEAD
requests.

=head2 Route Handlers

The route action is the code reference declared. It can access parameters
through the specific L<route_parameters|/route_parameters>, 
L<query_parameters|/query_parameters>, and  L<body_parameters|/body_parameters>
keywords, which return a L<Hash::MultiValue> object. 
This hashref is a merge of the route pattern matches and the request params.

You can find more details about how params are built and how to access them
in the L<Dancer2::Core::Request> documentation.

=head3 Declaring Routes

To control what happens when a web request is received by your webapp,
you'll need to declare C<routes>. A route declaration indicates which HTTP
method(s) it is valid for, the path it matches (e.g. C</foo/bar>), and a
coderef to execute, which returns the response.

    get '/hello/:name' => sub {
        return "Hi there " . route_parameters->get('name');
    };

The above route specifies that, for GET requests to C</hello/...>, the code
block provided should be executed.

=head3 Retrieving request parameters

The L<query_parameters|Dancer2::Manual/query_parameters>,
L<route_parameters|Dancer2::Manual/route_parameters>, and
L<body_parameters|Dancer2::Manual/body_parameters> keywords provide
a L<Hash::MultiValue> result from the three different parameters.

=head3 Named matching

A route pattern can contain one or more tokens (a word prefixed with ':').
Each token found in a route pattern is used as a named-pattern match. Any
match will be set in the route parameters.

    get '/hello/:name' => sub {
        "Hey " . route_parameters->get('name') . ", welcome here!";
    };

Tokens can be optional, for example:

    get '/hello/:name?' => sub {
        my $name = route_parameters->get('name') //= 'Whoever you are';
        "Hello there, $name";
    };

=head3 Wildcard Matching

A route can contain a wildcard (represented by a C<*>). Each wildcard match
will be placed in a list, which the C<splat> keyword returns.

    get '/download/*.*' => sub {
        my ($file, $ext) = splat;
        # do something with $file.$ext here
    };

An extensive, greedier wildcard represented by C<**> (A.K.A. "megasplat") can be
used to define a route. The additional path is broken down and returned as an
arrayref:

    get '/entry/*/tags/**' => sub {
        my ( $entry_id, $tags ) = splat;
        my @tags = @{$tags};
    };

The C<splat> keyword in the above example for the route F</entry/1/tags/one/two>
would set C<$entry_id> to C<1> and C<$tags> to C<['one', 'two']>.

=head3 Mixed named and wildcard matching

A route can combine named (token) matching and wildcard matching.
This is useful when chaining actions:

    get '/team/:team/**' => sub {
        var team => route_parameters->get('team');
        pass;
    };

    prefix '/team/:team';

    get '/player/*' => sub {
        my ($player) = splat;

        # etc...
    };

    get '/score' => sub {
        return score_for( vars->{'team'} );
    };

=head3 Regular Expression Matching

A route can be defined with a Perl regular expression.

In order to tell Dancer2 to consider the route as a real regexp, the route
must be defined explicitly with C<qr{}>, like the following:

    get qr{/hello/([\w]+)} => sub {
        my ($name) = splat;
        return "Hello $name";
    };

For Perl 5.10+, a route regex may use named capture groups. The C<captures>
keyword will return a reference to a copy of C<%+>.

=head3 Conditional Matching

Routes may include some matching conditions (on content_type, agent,
user_agent, content_length and path_info):

    get '/foo', {agent => 'Songbird (\d\.\d)[\d\/]*?'} => sub {
      'foo method for songbird'
    }

    get '/foo' => sub {
      'all browsers except songbird'
    }

=head2 Prefix

A prefix can be defined for each route handler, like this:

    prefix '/home';

From here, any route handler is defined to /home/*

    get '/page1' => sub {}; # will match '/home/page1'

You can unset the prefix value

    prefix '/'; # or: prefix undef;
    get '/page1' => sub {}; # will match /page1

Alternatively, to prevent you from ever forgetting to undef the prefix, you
can use lexical prefix like this:

    prefix '/home' => sub {
      get '/page1' => sub {}; # will match '/home/page1'
    }; ## prefix reset to previous value on exit

    get '/page1' => sub {}; # will match /page1

=head2 Delayed responses (Async/Streaming)

L<Dancer2> can provide delayed (otherwise known as I<asynchronous>) responses
using the C<delayed> keyword. These responses are streamed, although you can
set the content all at once, if you prefer.

    get '/status' => sub {
        delayed {
            response_header 'X-Foo' => 'Bar';

            # flush headers (in case of streaming)
            flush;

            # send content to the user
            content 'Hello, world!';

            # you can write more content
            # all streaming
            content 'Hello, again!';

            # when done, close the connection
            done;

            # do whatever you want else, asynchronously
            # the user socket closed by now
            ...
        };
    };

If you are streaming (calling C<content> several times), you must call
C<flush> first. If you're sending only once, you don't need to call C<flush>.

Here is an example of using delayed responses with L<AnyEvent>:

    use Dancer2;
    use AnyEvent;

    my %timers;
    my $count = 5;
    get '/drums' => sub {
        delayed {
            print "Stretching...\n";
            flush; # necessary, since we're streaming

            $timers{'Snare'} = AE::timer 1, 1, delayed {
                $timers{'HiHat'} ||= AE::timer 0, 0.5, delayed {
                    content "Tss...\n";
                };

                content "Bap!\n";

                if ( $count-- == 0 ) {
                    %timers = ();
                    content "Tugu tugu tugu dum!\n";
                    done;

                    print "<enter sound of applause>\n\n";
                    $timers{'Applause'} = AE::timer 3, 0, sub {
                        # the DSL will not available here
                        # because we didn't call the "delayed" keyword
                        print "<applause dies out>\n";
                    };
                }
            };
        };
    };

If an error happens during a write operation, a warning will be issued
to the logger.

You can handle the error yourself by providing an C<on_error> handler:

    get '/' => sub {
        delayed {
            flush;
            content "works";

            # ... user disconnected here ...

            content "fails";

            # ... error triggered ...

            done; # doesn't even get run
        } on_error => sub {
            # delayed{} not needed, DSL already available
            my ($error) = @_;
            # do something with $error
        };
    };

Here is an example that asynchronously streams the contents of a CSV file:

    use Dancer2;
    use Text::CSV_XS  qw< csv >;
    use Path::Tiny    qw< path >;
    use JSON::MaybeXS qw< encode_json >;
    # Create CSV parser
    my $csv = Text::CSV_XS->new({
        binary    => 1,
        auto_diag => 1,
    });
    get '/' => sub {
        # delayed response:
        delayed {
            # streaming content
            flush;
            # Read each row and stream it in JSON
            my $fh = path('filename.csv')->openr_utf8;
            while ( my $row = $csv->getline($fh) ) {
                content encode_json $row;
            }
            # close user connection
            done;
        } on_error => sub {
            my ($error) = @_;
            warning 'Failed to stream to user: ' . request->remote_address;
        };
    };

B<NOTE:> If you just want to send a file's contents asynchronously,
use C<send_file($filename)> instead of C<delayed>, as it will
automatically take advantage of any asynchronous capability.

=head2 Action Skipping

An action can choose not to serve the current request and ask Dancer2 to
process the request with the next matching route.

This is done with the B<pass> keyword, like in the following example

    get '/say/:word' => sub {
        pass if route_parameters->get('word') =~ /^\d+$/;
        "I say a word: " . route_parameters->get('word');
    };

    get '/say/:number' => sub {
        "I say a number: " . route_parameters->get('number');
    };

=head1 HOOKS

Hooks are code references (or anonymous subroutines) that are triggered at
specific moments during the resolution of a request.  They are set up using the
L<hook|/hook> keyword.

Many of them are provided by Dancer2's core, but plugins and engines can also
define their own.

=over 4

=item * C<before> hooks

C<before> hooks are evaluated before each request within the context of the
request and receives as argument the app (a L<Dancer2::Core::App> object).

It's possible to define variables which will be accessible in the action
blocks with the L<var keyword|/var>.

    hook before => sub {
        var note => 'Hi there';
    };

    get '/foo/*' => sub {
        my ($match) = splat; # 'oversee';
        vars->{note};        # 'Hi there'
    };

For another example, this can be used along with session support to easily
give non-logged-in users a login page:

    hook before => sub {
        if (!session('user') && request->path !~ m{^/login}) {
            # Pass the original path requested along to the handler:
            forward '/login', { requested_path => request->path };
        }
    };

The request keyword returns the current L<Dancer2::Core::Request> object
representing the incoming request.

=item * C<after> hooks

C<after> hooks are evaluated after the response has been built by a route
handler, and can alter the response itself, just before it's sent to the
client.

This hook runs after a request has been processed, but before the response
is sent.

It receives a L<Dancer2::Core::Response> object, which it can modify if it
needs to make changes to the response which is about to be sent.

The hook can use other keywords in order to do whatever it wants.

    hook after => sub {
        response->content(
            q{The "after" hook can alter the response's content here!}
        );
    };

=back

=head2 Templates

=over 4

=item * C<before_template_render>

C<before_template_render> hooks are called whenever a template is going to
be processed, they are passed the tokens hash which they can alter.

    hook before_template_render => sub {
        my $tokens = shift;
        $tokens->{foo} = 'bar';
    };

The tokens hash will then be passed to the template with all the
modifications performed by the hook. This is a good way to setup some
global vars you like to have in all your templates, like the name of the
user logged in or a section name.

=item * C<after_template_render>

C<after_template_render> hooks are called after the view has been rendered.
They receive as their first argument the reference to the content that has
been produced. This can be used to post-process the content rendered by the
template engine.

    hook after_template_render => sub {
        my $ref_content = shift;
        my $content     = ${$ref_content};

        # do something with $content
        ${$ref_content} = $content;
    };

=item * C<before_layout_render>

C<before_layout_render> hooks are called whenever the layout is going to be
applied to the current content. The arguments received by the hook are the
current tokens hashref and a reference to the current content.

    hook before_layout_render => sub {
        my ($tokens, $ref_content) = @_;
        $tokens->{new_stuff} = 42;
        $ref_content = \"new content";
    };

=item * C<after_layout_render>

C<after_layout_render> hooks are called once the complete content of the
view has been produced, after the layout has been applied to the content.
The argument received by the hook is a reference to the complete content
string.

    hook after_layout_render => sub {
        my $ref_content = shift;
        # do something with ${ $ref_content }, which reflects directly
        #   in the caller
    };

=back

=head2 Error Handling

Refer to L<Error Hooks|Dancer2::Manual/Error-Hooks>
for details about the following hooks:

=over 4

=item * C<init_error>

=item * C<before_error>

=item * C<after_error>

=item * C<on_route_exception>

=back

=head2 File Rendering

Refer to L<File Handler|Dancer2::Manual/File-Handler>
for details on the following hooks:

=over 4

=item * C<before_file_render>

=item * C<after_file_render>

=back

=head2 Serializers

=over 4

=item * C<before_serializer> is called before serializing the content, and receives
the content to serialize as an argument.

  hook before_serializer => sub {
    my $content = shift;
    ...
  };

=item * C<after_serializer> is called after the payload has been serialized, and
receives the serialized content as an argument.

  hook after_serializer => sub {
    my $serialized_content = shift;
    ...
  };

=back

=head1 HANDLERS

=head2 File Handler

Whenever a content is produced out of the parsing of a static file, the
L<Dancer2::Handler::File> component is used. This component provides two
hooks, C<before_file_render> and C<after_file_render>.

C<before_file_render> hooks are called just before starting to parse the
file, the hook receives as its first argument the file path that is going to
be processed.

    hook before_file_render => sub {
        my $path = shift;
    };

C<after_file_render> hooks are called after the file has been parsed and the
response content produced. It receives the response object
(L<Dancer2::Core::Response>) produced.

    hook after_file_render => sub {
       my $response = shift;
    };

=head2 Auto page

Whenever a page that matches an existing template needs to be served, the
L<Dancer2::Handler::AutoPage> component is used.

=head2 Writing your own

A route handler is a class that consumes the L<Dancer2::Core::Role::Handler>
role. The class must implement a set of methods: C<methods>, C<regexp> and
C<code> which will be used to declare the route.

Let's look at L<Dancer2::Handler::AutoPage> for example.

First, the matching methods are C<get> and C<head>:

    sub methods { qw(head get) }

Then, the C<regexp> or the I<path> we want to match:

    sub regexp { '/:page' }

Anything will be matched by this route, since we want to check if there's
a view named with the value of the C<page> token. If not, the route needs
to C<pass>, letting the dispatching flow to proceed further.

    sub code {
        sub {
            my $app = shift;
            my $prefix = shift;

            my $template = $app->template_engine;
            if ( !defined $template ) {
                $app->response->has_passed(1);
                return;
            }

            my $page       = $app->request->path;
            my $layout_dir = $template->layout_dir;
            if ( $page =~ m{^/\Q$layout_dir\E/} ) {
                $app->response->has_passed(1);
                return;
            }

            # remove leading '/', ensuring paths relative to the view
            $page =~ s{^/}{};
            my $view_path = $template->view_pathname($page);

            if ( ! $template->pathname_exists( $view_path ) ) {
                $app->response->has_passed(1);
                return;
            }

            my $ct = $template->process( $page );
            return ( $app->request->method eq 'GET' ) ? $ct : '';
        };
    }

The C<code> method passed the L<Dancer2::Core::App> object which provides
access to anything needed to process the request.

A C<register> is then implemented to add the route to the registry and if
the C<auto_page setting> is off, it does nothing.

    sub register {
        my ($self, $app) = @_;

        return unless $app->config->{auto_page};

        $app->add_route(
            method => $_,
            regexp => $self->regexp,
            code   => $self->code,
        ) for $self->methods;
    }

The config parser looks for a C<route_handlers> section and any handler defined
there is loaded. Thus, any random handler can be added to your app.
For example, the default config file for any Dancer2 application is as follows:

    route_handlers:
      File:
        public_dir: /path/to/public
      AutoPage: 1

=head1 ERRORS

=head2 Error Pages

When an HTTP error occurs (i.e. the action responds with a status code other
than 200), this is how Dancer2 determines what page to display.

=over

=item * Looks in the C<views/> directory for a corresponding template file
matching the error code (e.g. C<500.tt> or C<404.tt>). If such a file exists,
it's used to report the error.

=item * Next, looks in the C<public/> directory for a corresponding HTML file
matching the error code (e.g. C<500.html> or C<404.html>). If such a file
exists, it's used to report the error. (Note, however, that if B<show_errors>
is set to true, in the case of a 500 error the static HTML page will not be
shown, but will be replaced with a default error page containing more
informative diagnostics. For more information see L<Dancer2::Config>.)

=item * As default, render a generic error page on the fly.

=back

=head2 Execution Errors

When an error occurs during the route execution, Dancer2 will render an
error page with the HTTP status code 500.

It's possible either to display the content of the error message or to hide
it with a generic error page. This is a choice left to the end-user and can
be controlled with the B<show_errors> setting (see above).

Note that you can also choose to consider all warnings in your route
handlers as errors when the setting B<warnings> is set to 1.

=head2 Error Hooks

When an error is caught by Dancer2's core, an exception object is built (of
the class L<Dancer2::Core::Error>). This class provides a hook to let the
user alter the error workflow if needed.

C<init_error> hooks are called whenever an error object is built, the object
is passed to the hook.

    hook init_error => sub {
        my $error = shift;
        # do something with $error
    };

I<This hook was named B<before_error_init> in Dancer, both names currently
are synonyms for backward-compatibility.>

C<before_error> hooks are called whenever an error is going to be thrown, it
receives the error object as its sole argument.

    hook before_error => sub {
        my $error = shift;
        # do something with $error
    };

I<This hook was named B<before_error_render> in Dancer, both names currently
are synonyms for backward-compatibility.>

C<after_error> hooks are called whenever an error object has been thrown, it
receives a L<Dancer2::Core::Response> object as its sole argument.

    hook after_error => sub {
        my $response = shift;
    };

I<This hook was named B<after_error_render> in Dancer, both names currently
are synonyms for backward-compatibility.>

C<on_route_exception> is called when an exception has been caught, at the
route level, just before rethrowing it higher. This hook receives a
L<Dancer2::Core::App> and the error as arguments.

  hook on_route_exception => sub {
    my ($app, $error) = @_;
  };

=head1 SESSIONS

=head2 Handling sessions

It's common to want to use sessions to give your web applications state; for
instance, allowing a user to log in, creating a session, and checking that
session on subsequent requests.

By default Dancer 2 has L<Simple|Dancer2::Session::Simple> sessions enabled.
It implements a very simple in-memory session storage. This will be fast and
useful for testing, but such sessions will not persist between restarts of
your app.

If you'd like to use a different session engine you must declare it in the
configuration file.

For example to use YAML file base sessions you need to add the following
to your F<config.yml>:

    session: YAML

Or, to enable session support from within your code,

    set session => 'YAML';

(However, controlling settings is best done from your config file.)

The L<Dancer2::Session::YAML> backend implements a file-based YAML session
storage to help with debugging, but shouldn't be used on production systems.

There are other session backends, such as L<Dancer2::Session::Memcached>,
which are recommended for production use.

You can then use the L<session|Dancer2::Manual/session> keyword to manipulate the
session:

=head3 Storing data in the session

Storing data in the session is as easy as:

    session varname => 'value';

=head3 Retrieving data from the session

Retrieving data from the session is as easy as:

    session('varname')

Or, alternatively,

    session->read("varname")

=head3 Controlling where sessions are stored

For disc-based session backends like L<Dancer2::Session::YAML>,
session files are written to the session dir specified by
the C<session_dir> setting, which defaults to C<./sessions>
if not specifically set.

If you need to control where session files are created, you can do so
quickly and easily within your config file, for example:

    session: YAML
    engines:
      session:
        YAML:
          session_dir: /tmp/dancer-sessions

If the directory you specify does not exist, Dancer2 will attempt to create
it for you.

=head3 Changing session ID

If you wish to change the session ID (for example on privilege level change):

    my $new_session_id = app->change_session_id

=head3 Destroying a session

When you're done with your session, you can destroy it:

    app->destroy_session

=head2 Sessions and logging in

A common requirement is to check the user is logged in, and, if not, require
them to log in before continuing.

This can easily be handled using a before hook to check their session:

    use Dancer2;
    set session => "Simple";

    hook before => sub {
        if (!session('user') && request->path !~ m{^/login}) {
            forward '/login', { requested_path => request->path };
        }
    };

    get '/' => sub { return "Home Page"; };

    get '/secret' => sub { return "Top Secret Stuff here"; };

    get '/login' => sub {
        # Display a login page; the original URL they requested is available as
        # query_parameters->get('requested_path'), so could be put in a hidden field in the form
        template 'login', { path => query_parameters->get('requested_path') };
    };

    post '/login' => sub {
        # Validate the username and password they supplied
        if (body_parameters->get('user') eq 'bob' && body_parameters->get('pass') eq 'letmein') {
            session user => body_parameters->get('user');
            redirect body_parameters->get('path') || '/';
        } else {
            redirect '/login?failed=1';
        }
    };

    dance();

Here is what the corresponding C<login.tt> file should look like. You should
place it in a directory called C<views/>:

    <html>
      <head>
        <title>Session and logging in</title>
      </head>
      <body>
        <form action='/login' method='POST'>
            User Name : <input type='text' name='user'/>
            Password: <input type='password' name='pass' />

            <!-- Put the original path requested into a hidden
                       field so it's sent back in the POST and can be
                       used to redirect to the right page after login -->
            <input type='hidden' name='path' value='<% path %>'/>

            <input type='submit' value='Login' />
        </form>
      </body>
    </html>

Of course, you'll probably want to validate your users against a database
table, or maybe via IMAP/LDAP/SSH/POP3/local system accounts via PAM etc.
L<Authen::Simple> is probably a good starting point here!

A simple working example of handling authentication against a database table
yourself (using L<Dancer2::Plugin::Database> which provides the C<database>
keyword, and L<Crypt::SaltedHash> to handle salted hashed passwords (well,
you wouldn't store your users passwords in the clear, would you?)) follows:

    post '/login' => sub {
        my $user_value = body_parameters->get('user');
        my $pass_value = body_parameters->get('pass');

        my $user = database->quick_select('users',
            { username => $user_value }
        );
        if (!$user) {
            warning "Failed login for unrecognised user $user_value";
            redirect '/login?failed=1';
        } else {
            if (Crypt::SaltedHash->validate($user->{password}, $pass_value))
            {
                debug "Password correct";
                # Logged in successfully
                session user => $user;
                redirect body_parameters->get('path') || '/';
            } else {
                debug("Login failed - password incorrect for " . $user_value);
                redirect '/login?failed=1';
            }
        }
    };

=head3 Retrieve complete hash stored in session

Get complete hash stored in session:

    my $hash = session;

=head2 Writing a session engine

In Dancer 2, a session backend consumes the role
L<Dancer2::Core::Role::SessionFactory>.

The following example using the Redis session demonstrates how session
engines are written in Dancer 2.

First thing to do is to create the class for the session engine,
we'll name it C<Dancer2::Session::Redis>:

     package Dancer2::Session::Redis;
     use Moo;
     with 'Dancer2::Core::Role::SessionFactory';

we want our backend to have a handle over a Redis connection.
To do that, we'll create an attribute C<redis>

     use JSON;
     use Redis;
     use Dancer2::Core::Types; # brings helper for types

     has redis => (
         is => 'rw',
         isa => InstanceOf['Redis'],
         lazy => 1,
         builder => '_build_redis',
     );

The lazy attribute says to Moo that this attribute will be
built (initialized) only when called the first time. It means that
the connection to Redis won't be opened until necessary.

     sub _build_redis {
         my ($self) = @_;
         Redis->new(
             server => $self->server,
             password => $self->password,
             encoding => undef,
         );
     }

Two more attributes, C<server> and C<password> need to be created.
We do this by defining them in the config file. Dancer2 passes anything
defined in the config to the engine creation.

     # config.yml
     ...
     engines:
       session:
         Redis:
           server: foo.mydomain.com
           password: S3Cr3t

The server and password entries are now passed to the constructor
of the Redis session engine and can be accessed from there.

     has server => (is => 'ro', required => 1);
     has password => (is => 'ro');

Next, we define the subroutine C<_retrieve> which will return a session
object for a session ID it has passed. Since in this case, sessions are
going to be stored in Redis, the session ID will be the key, the session the value.
So retrieving is as easy as doing a get and decoding the JSON string returned:

     sub _retrieve {
         my ($self, $session_id) = @_;
         my $json = $self->redis->get($session_id);
         my $hash = from_json( $json );
         return bless $hash, 'Dancer2::Core::Session';
     }

The C<_flush> method is called by Dancer when the session needs to be stored in
the backend. That is actually a write to Redis. The method receives a C<Dancer2::Core::Session>
object and is supposed to store it.

     sub _flush {
         my ($self, $session) = @_;
         my $json = encode_json( { %{ $session } } );
         $self->redis->set($session->id, $json);
     }

For the C<_destroy> method which is supposed to remove a session from the backend,
deleting the key from Redis is enough.

     sub _destroy {
         my ($self, $session_id) = @_;
         $self->redis->del($session_id);
     }

The C<_sessions> method which is supposed to list all the session IDs currently
stored in the backend is done by listing all the keys that Redis has.

     sub _sessions {
         my ($self) = @_;
         my @keys = $self->redis->keys('*');
         return \@keys;
     }

The session engine is now ready.

=head3 The Session keyword

Dancer2 maintains two session layers.

The first layer, L<Dancer2::Core::Session> provides a session object
which represents the current session. You can read from it as many
times as you want, and write to it as many times as you want.

The second layer is the session engine (L<Dancer2::Session::Simple>
is one example), which is used in order to implement the reading and
writing from the actual storage. This is read only once, when a request
comes in (using a cookie whose value is C<dancer.session> by default).
At the end of a request, all the data you've written will be flushed
to the engine itself, which will do the actual write to the storage
(whether it's in a hash in memory, in Memcache, or in a database).

=head1 TEMPLATES

Returning plain content is all well and good for examples or trivial apps,
but soon you'll want to use templates to maintain separation between your
code and your content. Dancer2 makes this easy.

Your route handlers can use the L<template|Dancer2::Manual/template> keyword
to render templates.

=head2 Views

In Dancer2, a file which holds a template is called a I<view>.  Views are
located in the C<appdir/views> directory.

You can change this location by changing the setting 'views'. For instance
if your templates are located in the 'templates' directory, do the
following:

    set views => path( app->location , 'templates' );

By default, the internal template engine L<Dancer2::Template::Simple> is
used, but you may want to upgrade to L<Template
Toolkit|http://www.template-toolkit.org/>. If you do so, you have to enable
this engine in your settings as explained in
L<Dancer2::Template::TemplateToolkit> and you'll also have to install the
L<Template> module.

In order to render a view, just call the
L<template|Dancer2::Manual/template> keyword at the end of the action by
giving the view name and the HASHREF of tokens to interpolate in the view
(note that for convenience, the request, session, params and vars are
automatically accessible in the view, named C<request>, C<session>, C<params>,
and C<vars>) - for example:

    hook before => sub { var time => scalar(localtime) };

    get '/hello/:name' => sub {
        my $name = route_parameters->get('name');
        template 'hello.tt', { name => $name };
    };

The template C<hello.tt> could contain, for example:

    <p>Hi there, [% name %]!</p>
    <p>You're using [% request.user_agent %]</p>
    [% IF session.username %]
        <p>You're logged in as [% session.username %]</p>
    [% END %]
    It's currently [% vars.time %]

For a full list of the tokens automatically added to your template (like
C<session>, C<request>, and C<vars>, refer to
L<Dancer2::Core::Role::Template>).

By default, views use a F<.tt> extension. This can be overridden by setting
the C<extension> attribute in the template engine configuration:

    set engines => {
        template => {
            template_toolkit => {
                extension => 'foo',
            },
        },
    };

=head2 Layouts

A layout is a special view, located in the F<layouts> directory (inside the
views directory) which must have a token named C<content>. That token marks
the place where to render the action view. This lets you define a global
layout for your actions, and have each individual view contain only
specific content. This is a good thing and helps avoid lots of needless
duplication of HTML. :)

For example, the layout F<views/layouts/main.tt>:

    <html>
        <head>...</head>
        <body>
        <div id="header">
        ...
        </div>

        <div id="content">
        [% content %]
        </div>

        </body>
    </html>

You can tell your app which layout to use with C<layout: name> in the config
file, or within your code:

    set layout => 'main';

You can control which layout to use (or whether to use a layout at all) for
a specific request without altering the layout setting by passing an options
hashref as the third param to the template keyword:

    template 'index.tt', {}, { layout => undef };

If your application is not mounted under root (C</>), you can use a
C<before_template_render> hook instead of hardcoding the path into your
application for your CSS, images and JavaScript:

    hook before_template_render => sub {
        my $tokens = shift;
        $tokens->{uri_base} = request->base->path;
    };

Then in your layout, modify your CSS inclusion as follows:

    <link rel="stylesheet" href="[% uri_base %]/css/style.css" />

From now on you can mount your application wherever you want, without any
further modification of the CSS inclusion.

=head2 Encoding

If you use L<Plack> and have a Unicode problem with your Dancer2
application, don't forget to check if you have set your template engine to
use Unicode, and set the default charset to UTF-8. So, if you are using
template toolkit, your config file will look like this:

    charset: UTF-8
    engines:
      template:
        template_toolkit:
          ENCODING: utf8

=head2 Default Template Variables

Every template knows about the following variables, which are provided by
L<Dancer2::Core::Role::Template>. Some are similar to the keywords you can
use in the Perl part of your Dancer2 application.

=over 4

=item * B<perl_version>

Current version of perl, effectively
L<C<$^V>|http://perldoc.perl.org/perlvar.html#%24%5eV>.

=item * B<dancer_version>

Current version of Dancer2, effectively C<< Dancer2->VERSION >>.

=item * B<settings>

A hash of the application configuration. This is like
the L<config|Dancer2::Manual/config> keyword.

=item * B<request>

The current request object. This is like the L<request|Dancer2::Manual/request> keyword.

=item * B<params>

A hash reference of all the parameters.

Currently the equivalent of C<< $request->params >>, and like the
L<params|Dancer2::Manual/params> keyword.

=item * B<vars>

The list of request variables, which is what you would get if you
called the L<vars|Dancer2::Manual/vars> keyword.

=item * B<session>

The current session data, if a session exists. This is like
the L<session|Dancer2::Manual/session> keyword.

=back

=head1 STATIC FILES

=head2 Static Directory

Static files are served from the F<./public> directory. You can specify a
different location by setting the C<public_dir> option:

    set public_dir => path( app->location , 'static' );

When you modify default public_dir you have to set C<static_handler> option.

    set static_handler => true;

Note that the public directory name is not included in the URL. A file
F<./public/css/style.css> is made available as
L<http://example.com/css/style.css>.

=head2 Static File from a Route Handler

It's possible for a route handler to send a static file, as follows:

    get '/download/*' => sub {
        my ($file) = splat;

        send_file $file;
    };

Or even if you want your index page to be a plain old F<index.html> file,
just do:

    get '/' => sub {
        send_file '/index.html'
    };

=head1 FILE UPLOADS

Files are uploaded in Dancer2 using the class L<Dancer2::Core::Request::Upload>.
The objects are accessible within the route handlers using the C<upload>
keyword:

    post '/upload' => sub {
        my $upload     = upload('file_input_name');    # upload object
        $upload->copy_to('Uploads/');
    };

=head1 CONFIGURATION

=head2 Configuration and environments

Configuring a Dancer2 application can be done in many ways. The easiest one
(and maybe the dirtiest) is to put all your settings statements at the top
of your script, before calling the C<dance()> method.

Other ways are possible: for example, you can define all your settings in the file
C<appdir/config.yml>. For this, you must have installed the L<YAML> module, and
of course, write the config file in YAML.

That's better than the first option, but it's still not perfect as you can't
switch easily from an environment to another without rewriting the config
file.

A better solution is to have one F<config.yml> file with default global
settings, like the following:

    # appdir/config.yml
    logger: 'file'
    layout: 'main'

And then write as many environment files as you like in
C<appdir/environments>. That way, the appropriate environment config file
will be loaded according to the running environment (if none is specified,
it will be 'development').

You can change the running environment when starting your app using the
C<plackup> command's C<--env> or C<--E> switch:

    plackup -E production bin/app.psgi

Altenatively, you can set the
L<C<DANCER_ENVIRONMENT>|https://metacpan.org/pod/Dancer2::Config#DANCER_ENVIRONMENT>
environment variable in the shell or in your web server's configuration file.

Typically, you'll want to set the following values in a development config
file:

    # appdir/environments/development.yml
    log: 'debug'
    startup_info: 1
    show_errors:  1

And in a production one:

    # appdir/environments/production.yml
    log: 'warning'
    startup_info: 0
    show_errors:  0

Please note that you are not limited to writing configuration files in YAML.
Dancer2 supports any file format that is supported by L<Config::Any>, such
as JSON, XML, INI files, and Apache-style config files. See the Dancer2
L<configuration guide|https://metacpan.org/pod/Dancer2::Config#MANIPULATING-SETTINGS-VIA-CONFIGURATION-FILES>
for more information.

=head2 Accessing configuration information

A Dancer2 application can use the C<config> keyword to easily access the
settings within its config file, for instance:

    get '/appname' => sub {
        return "This is " . config->{appname};
    };

This makes keeping your application's settings all in one place simple and
easy - you shouldn't need to worry about implementing all that yourself. :)

=head2 Settings

It's possible to change almost every parameter of the application via the
settings mechanism.

A setting is a key/value pair assigned by the keyword B<set>:

    set setting_name => 'setting_value';

More usefully, settings can be defined in a configuration file.
Environment-specific settings can also be defined in environment-specific
files (for instance, you do not want to show error stacktraces in
production, and might want extra logging in development).

=head2 Serializers

When writing a webservice, data serialization/deserialization is a common
issue to deal with. Dancer2 can automatically handle that for you, via a
serializer.

When setting up a serializer, a new behaviour is authorized for any route
handler you define: any non-scalar response will be rendered as a serialized
string, via the current serializer.

Here is an example of a route handler that will return a hashref:

    use Dancer2;
    set serializer => 'JSON';

    get '/user/:id/' => sub {
        { foo => 42,
          number => 100234,
          list => [qw(one two three)],
        }
    };

Dancer2 will render the response via the current serializer.

Hence, with the JSON serializer set, the route handler above would result in
a content like the following:

    {"number":100234,"foo":42,"list":["one","two","three"]}

If you send a value which is validated serialized data, but is not in the
form a key and value pair (such as a serialized string or a JSON array), the
data will not be available in C<params> but will be available in
C<< request->data >>.

The following serializers are available, be aware they dynamically depend on
Perl modules you may not have on your system.

=over 4

=item * B<JSON>

Requires L<JSON>.

=item * B<YAML>

Requires L<YAML>,

=item * B<XML>

Requires L<XML::Simple>.

=item * B<Mutable>

Will try to find the appropriate serializer using the B<Content-Type> and
B<Accept-type> header of the request.

=back

=head2 Importing using Appname

An app in Dancer2 uses the class name (defined by the C<package> function) to
define the App name. Thus separating the App to multiple files, actually means
creating multiple applications. This means that any engine defined in an application,
because the application is a complete separate scope, will not be available to a
different application:

     package MyApp::User {
         use Dancer2;
         set serializer => 'JSON';
         get '/view' => sub {...};
     }

     package MyApp::User::Edit {
         use Dancer2;
         get '/edit' => sub {...};
     }

These are two different Dancer2 Apps. They have different scopes, contexts,
and thus different engines. While C<MyApp::User> has a serializer defined,
C<MyApp::User::Edit> will not have that configuration.

By using the import option C<appname>, we can ask Dancer2 to extend an
App without creating a new one:

     package MyApp::User {
         use Dancer2;
         set serializer => 'JSON';
         get '/view' => sub {...};
     }

     package MyApp::User::Edit {
         use Dancer2 appname => 'MyApp::User'; # extending MyApp::User
         get '/edit' => sub {...};
     }

The import option C<appname> allows you to seamlessly extend Dancer2 Apps
without creating unnecessary additional applications or repeat any definitions.
This allows you to spread your application routes across multiple files and allow
ease of mind when developing it, and accommodate multiple developers working
on the same codebase.

     # app.pl
     use MyApp::User;
     use MyApp::User::Edit;

     # single application composed of routes provided in multiple files
     MyApp::User->to_app;

This way only one class needs to be loaded while creating an app:

     # app.pl:
     use MyApp::User;
     MyApp::User->to_app;

=head1 LOGGING

=head2 Configuring logging

It's possible to log messages generated by the application and by Dancer2
itself.

To start logging, select the logging engine you wish to use with the
C<logger> setting; Dancer2 includes built-in log engines named C<file> and
C<console>, which log to a logfile and to the console respectively.

To enable logging to a file, add the following to your config file:

    logger: 'file'

Then you can choose which kind of messages you want to actually log:

    log: 'core'      # will log debug, info, warnings, errors,
                     #   and messages from Dancer2 itself
    log: 'debug'     # will log debug, info, warning and errors
    log: 'info'      # will log info, warning and errors
    log: 'warning'   # will log warning and errors
    log: 'error'     # will log only errors

If you're using the C<file> logging engine, a directory C<appdir/logs> will
be created and will host one logfile per environment. The log message
contains the time it was written, the PID of the current process, the
message and the caller information (file and line).

=head2 Logging your own messages

Just call L<debug|https://metacpan.org/pod/Dancer2::Manual#debug>,
L<info|https://metacpan.org/pod/Dancer2::Manual#info>,
L<warning|https://metacpan.org/pod/Dancer2::Manual#warning> or
L<error|https://metacpan.org/pod/Dancer2::Manual#error> with your message:

    debug "This is a debug message from my app.";

=head1 TESTING

=head2 Using Plack::Test

L<Plack::Test> receives a common web request (using standard L<HTTP::Request>
objects), fakes a web server in order to create a proper PSGI request, and sends it
to the web application. When the web application returns a PSGI response
(which Dancer applications do), it will then convert it to a common web response
(as a standard L<HTTP::Response> object).

This allows you to then create requests in your test, create the code reference
for your web application, call them, and receive a response object, which can
then be tested.

=head3 Basic Example

Assuming there is a web application:

     # MyApp.pm
     package MyApp;
     use Dancer2;
     get '/' => sub {'OK'};
     1;

The following test I<base.t> is created:

     # base.t
     use strict;
     use warnings;
     use Test::More tests => 2;
     use Plack::Test;
     use HTTP::Request;
     use MyApp;

Creating a coderef for the application using the C<to_app> keyword:

     my $app = MyApp->to_app;

Creating a test object from L<Plack::Test> for the application:

     my $test = Plack::Test->create($app);

Creating the first request object and sending it to the test object
to receive a response:

     my $request  = HTTP::Request->new( GET => '/' );
     my $response = $test->request($request);

It can now be tested:

     ok( $response->is_success, '[GET /] Successful request' );
     is( $response->content, 'OK', '[GET /] Correct content' );

=head3 Putting it together

     # base.t
     use strict;
     use warnings;
     use Test::More;
     use Plack::Test;
     use HTTP::Request::Common;
     use MyApp;

     my $test     = Plack::Test->create( MyApp->to_app );
     my $response = $test->request( GET '/' );

     ok( $response->is_success, '[GET /] Successful request' );
     is( $response->content, 'OK', '[GET /] Correct content' );

     done_testing();

=head3 Subtests

Tests can be separated using L<Test::More>'s C<subtest> functionality,
thus creating multiple self-contained tests that don't overwrite each other.

Assuming we have a different app that has two states we want to test:

     # MyApp.pm
     package MyApp;
     use Dancer2;
     set serializer => 'JSON';

     get '/:user' => sub {
         my $user = route_parameters->get('user');

         $user and return { user => $user };

         return {};
     };

     1;

This is a contrived example of a route that checks for a user
parameter. If it exists, it returns it in a hash with the key
'user'. If not, it returns an empty hash

     # param.t
     use strict;
     use warnings;
     use Test::More;
     use Plack::Test;
     use HTTP::Request::Common;
     use MyApp;

     my $test = Plack::Test->create( MyApp->to_app );

     subtest 'A empty request' => sub {
         my $res = $test->request( GET '/' );
         ok( $res->is_success, 'Successful request' );
         is( $res->content '{}', 'Empty response back' );
     };

     subtest 'Request with user' => sub {
         my $res = $test->request( GET '/?user=sawyer_x' );
         ok( $res->is_success, 'Successful request' );
         is( $res->content '{"user":"sawyer_x"}', 'Empty response back' );
     };

     done_testing();

=head3 Cookies

To handle cookies, which are mostly used for maintaining sessions,
the following modules can be used:

=over 4

=item * L<Test::WWW::Mechanize::PSGI>

=item * L<LWP::Protocol::PSGI>

=item * L<HTTP::Cookies>

=back

Taking the previous test, assuming it actually creates and uses
cookies for sessions:

     # ... all the use statements
     use HTTP::Cookies;

     my $jar  = HTTP::Cookies->new;
     my $test = Plack::Test->create( MyApp->to_app );

     subtest 'A empty request' => sub {
         my $res = $test->request( GET '/' );
         ok( $res->is_success, 'Successful request' );
         is( $res->content '{}', 'Empty response back' );
         $jar->extract_cookies($res);
         ok( $jar->as_string, 'We have cookies!' );
     };

     subtest 'Request with user' => sub {
         my $req = GET '/?user=sawyer_x';
         $jar->add_cookie_header($req);
         my $res = $test->request($req);
         ok( $res->is_success, 'Successful request' );
         is( $res->content '{"user":"sawyer_x"}', 'Empty response back' );
         $jar->extract_cookies($res);

         ok( ! $jar->as_string, 'All cookies deleted' );
     };

     done_testing();

Here a cookie jar is created, all requests and responses, existing
cookies, as well as cookies that were deleted by the response, are checked.

=head3 Accessing the configuration file

By importing Dancer2 in the command line scripts, there is full
access to the configuration using the imported keywords:

     use strict;
     use warnings;
     use Test::More;
     use Plack::Test;
     use HTTP::Request::Common;
     use MyApp;
     use Dancer2;

     my $appname = config->{'appname'};
     diag "Testing $appname";

     # ...

=head1 PACKAGING

=head2 Carton

=head3 What it does

L<Carton> sets up a local copy of your project prerequisites. You only
need to define them in a file and ask Carton to download all of them
and set them up.
When you want to deploy your app, you just carry the git clone and ask
Carton to set up the environment again and you will then be able to run it.

The benefits are multifold:

=over 4

=item * Local Directory copy

By putting all the dependencies in a local directory, you can make
sure they aren't updated by someone else by accident and their versions
locked to the version you picked.

=item * Sync versions

Deciding which versions of the dependent modules your project needs
allows you to sync this with other developers as well. Now you're all
using the same version and they don't change unless you want update the
versions you want. When updated everyone again uses the same new version
of everything.

=item * Carry only the requirement, not bundled modules

Instead of bundling the modules, you only actually bundle the requirements.
Carton builds them for you when you need it.

=back

=head3 Setting it up

First set up a new app:

     $ dancer2 -a MyApp
     ...

Delete the files that are not needed:

     $ rm -f Makefile.PL MANIFEST MANIFEST.SKIP

Create a git repo:

     $ git init && git add . && git commit -m "initial commit"

Add a requirement using the L<cpanfile> format:

     $ cat > cpanfile
     requires 'Dancer2' => 0.155000;
     requires 'Template' => 0;
     recommends 'URL::Encode::XS' => 0;
     recommends 'CGI::Deurl::XS' => 0;
     recommends 'HTTP::Parser::XS' => 0;

Ask carton to set it up:

     $ carton install
     Installing modules using [...]
     Successfully installed [...]
     ...
     Complete! Modules were install into [...]/local

Now we have two files: I<cpanfile> and I<cpanfile.snapshot>. We
add both of them to our Git repository and we make sure we don't
accidentally add the I<local/> directory Carton created which holds
the modules it installed:

     $ echo local/ >> .gitignore
     $ git add .gitignore cpanfile cpanfile.snapshot
     $ git commit -m "Start using carton"

When we want to update the versions on the production machine,
we simply call:

     $ carton install --deployment

By using --deployment we make sure we only install the modules
we have in our cpanfile.snapshot file and do not fallback to querying
the CPAN.

=head2 FatPacker

L<App::FatPacker> (using its command line interface, L<fatpack>) packs
dependencies into a single file, allowing you to carry a single file
instead of a directory tree.

As long as your application is pure-Perl, you could create a single
file with your application and all of Dancer2 in it.

The following example will demonstrate how this can be done:

Assuming we have an application in I<lib/MyApp.pm>:

     package MyApp;
     use Dancer2;
     get '/' => sub {'OK'};
     1;

And we have a handler in I<bin/app.pl>:

     use strict;
     use warnings;
     use FindBin;
     use lib "$FindBin::Bin/../lib";
     use MyApp;

     MyApp->to_app;

To fatpack it, we begin by tracing the script:

     $ fatpack trace bin/app.pl

This creates a I<fatpacker.trace> file. From this we create the packlists:

     $ fatpack packlists-for `cat fatpacker.trace` > packlists

The packlists are stored in a file called I<packlists>.

Now we create the tree using the following command:

     $ fatpack tree `cat packlists`

The tree is created under the directory I<fatlib>.

Now we create a file containing the dependency tree, and add our script to it,
using the following command:

     $ (fatpack file; cat bin/app.pl) > myapp.pl

This creates a file called I<myapp.pl> with everything in it. Dancer2 uses
L<MIME::Types> which has a database of all MIME types and helps translate those.
The small database file containing all of these types is a binary and therefore
cannot be fatpacked. Hence, it needs to be copied to the current directory so our
script can find it:

     $ cp fatlib/MIME/types.db .

=head1 MIDDLEWARES

=head2 Plack middlewares

If you want to use Plack middlewares, you need to enable them using
L<Plack::Builder> as such:

    # in app.psgi or any other handler
    use MyApp;
    use Plack::Builder;

    builder {
        enable 'Deflater';
        enable 'Session', store => 'File';
        enable 'Debug', panels => [ qw<DBITrace Memory Timer> ];
        MyApp->to_app;
    };

The nice thing about this setup is that it will work seamlessly through
Plack or through the internal web server.

    # load dev web server (without middlewares)
    perl -Ilib app.psgi

    # load plack web server (with middlewares)
    plackup -I lib app.psgi

You do not need to provide different files for either server.

=head3 Path-based middlewares

If you want to set up a middleware for a specific path, you can do that using
L<Plack::Builder> which uses L<Plack::App::URLMap>:

    # in your app.psgi or any other handler
    use MyApp;
    use Plack::Builder;

    my $special_handler = sub { ... };

    builder {
        mount '/special' => $special_handler;
        mount '/'        => MyApp->to_app;
    };

=head3 Removing default middlewares

By default, a Dancer2 app is automatically wrapped with the following middleware

=over 4

=item * L<Plack::Middleware::FixMissingBodyInRedirect>

=item * L<Plack::Middleware::Head>

=back

You can configure the setting C<no_default_middleware> to a true value to stop your
Dancer2 app being wrapped with these default middleware layers.

    # in you Dancer2 app or config.yml
    package MyApp;
    use Dancer2

    set no_default_middleware => true;

This is necessary if you need to add eTag or ContentMD5 headers to
C<HEAD> requests, and you are encouraged to manually add those default
middleware back into your PSGI stack.

=head3 Running on Perl web servers with plackup

A number of Perl web servers supporting PSGI are available on CPAN:

=over 4

=item * L<Starman>

C<Starman> is a high performance web server, with support for preforking,
signals, multiple interfaces, graceful restarts and dynamic worker pool
configuration.

=item * L<Twiggy>

C<Twiggy> is an C<AnyEvent> web server, it's light and fast.

=item * L<Corona>

C<Corona> is a C<Coro> based web server.

=back

To start your application, just run plackup (see L<Plack> and specific
servers above for all available options):

   $ plackup bin/app.psgi
   $ plackup -E deployment -s Starman --workers=10 -p 5001 -a bin/app.psgi

As you can see, the scaffolded Perl script for your app can be used as a
PSGI startup file.

=head4 Enabling content compression

Content compression (gzip, deflate) can be easily enabled via a Plack
middleware (see L<Plack/Plack::Middleware>): L<Plack::Middleware::Deflater>.
It's a middleware to encode the response body in gzip or deflate, based on the
C<Accept-Encoding> HTTP request header.

Enable it as you would enable any Plack middleware. First you need to
install L<Plack::Middleware::Deflater>, then in the handler (usually
F<app.psgi>) edit it to use L<Plack::Builder>, as described above:

    use Dancer2;
    use MyApp;
    use Plack::Builder;

    builder {
        enable 'Deflater';
        MyApp->to_app;
    };

To test if content compression works, trace the HTTP request and response
before and after enabling this middleware. Among other things, you should
notice that the response is gzip or deflate encoded, and contains a header
C<Content-Encoding> set to C<gzip> or C<deflate>.

=head3 Running multiple apps with Plack::Builder

You can use L<Plack::Builder> to mount multiple Dancer2 applications on a
L<PSGI> webserver like L<Starman>.

Start by creating a simple app.psgi file:

    use OurWiki;  # first app
    use OurForum; # second app
    use Plack::Builder;

    builder {
        mount '/wiki'  => OurWiki->to_app;
        mount '/forum' => OurForum->to_app;
    };

and now use L<Starman>

    plackup -a app.psgi -s Starman

Currently this still demands the same appdir for both (default circumstance)
but in a future version this will be easier to change while staying very
simple to mount.

=head3 Running from Apache with Plack

You can run your app from Apache using PSGI (Plack), with a config like the
following:

    <VirtualHost myapp.example.com>
        ServerName www.myapp.example.com
        ServerAlias myapp.example.com
        DocumentRoot /websites/myapp.example.com

        <Directory /home/myapp/myapp>
            AllowOverride None
            Order allow,deny
            Allow from all
        </Directory>

        <Location />
            SetHandler perl-script
            PerlResponseHandler Plack::Handler::Apache2
            PerlSetVar psgi_app /websites/myapp.example.com/app.psgi
        </Location>

        ErrorLog  /websites/myapp.example.com/logs/error_log
        CustomLog /websites/myapp.example.com/logs/access_log common
    </VirtualHost>

To set the environment you want to use for your application (production or
development), you can set it this way:

    <VirtualHost>
        ...
        SetEnv DANCER_ENVIRONMENT "production"
        ...
    </VirtualHost>

=head1 PLUGINS

=head2 Writing a plugin

See L<Dancer2::Plugin/Writing the plugin> for information on how to author
a new plugin for Dancer2.

=head1 EXPORTS

By default, C<use Dancer2> exports all the DSL keywords and sets up the
webapp under the name of the current package. The following tags control
exports and webapp namespace.

=over 4

=item * B<!keyword>

If you want to prevent Dancer2 from exporting specific keywords (perhaps you
plan to implement them yourself in a different way, or they clash with
another module you're loading), you can simply exclude them:

    use Test::More;
    use Dancer2 qw(!pass);

The above would import all keywords as usual, with the exception of C<pass>.

=item * B<appname>

A larger application may split its source between several packages to aid
maintainability. Dancer2 will create a separate application for each
package, each having separate hooks, config and/or engines. You can force
Dancer2 to collect the route and hooks into a single application with the
C<appname> tag; e.g.

    package MyApp;
    use Dancer2;
    get '/foo' => sub {...};

    package MyApp::Private;
    use Dancer2 appname => MyApp;
    get '/bar' => sub {...};

The above would add the C<bar> route to the MyApp application. Dancer2 will
I<not> create an application with the name C<MyApp::Private>.

=item * B<:nopragmas>

By default L<Dancer2> will import three pragmas: L<strict>, L<warnings>,
and L<utf8>. If you require control over the imported pragmas, you can add
B<:nopragmas> to the importing flags, in which case Dancer2 will not import
any pragmas:

    use strict;
    use warnings;
    no warnings 'experimental::smartmatch'; # for example...
    use Dancer2 ':nopragmas'; # do not touch the existing pragmas

This way importing C<Dancer2> does not change the existing pragmas setup
you have.

=back

When you C<use Dancer2>, you get an C<import> method added into the current
package. This B<will> override previously declared import methods from other
sources, such as L<Exporter>. Dancer2 applications support the following
tags on import:

=over 4

=item * B<with>

The C<with> tag allows an app to pass one or more config entries to another
app, when it C<use>s it.

    package MyApp;
    use Dancer2;

    BEGIN { set session => 'YAML' };
    use Blog with => { session => engine('session') };

In this example, the session engine is passed to the C<Blog> app. That way,
anything done in the session will be shared between both apps.

Anything that is defined in the config entry can be passed that way. If we
want to pass the whole config object, it can be done like so:

    use SomeApp with => { %{config()} };

=back

=head1 DSL KEYWORDS

Dancer2 provides you with a DSL (Domain-Specific Language) which makes
implementing your web application trivial.

For example, take the following example:

    use Dancer2;

    get '/hello/:name' => sub {
        my $name = route_parameters->get('name');
    };
    dance;

C<get> and C<route_parameters> are keywords provided by Dancer2.

This document lists all keywords provided by Dancer2. It does not cover
additional keywords which may be provided by loaded plugins; see the
documentation for plugins you use to see which additional keywords they make
available to you.

=head2 any

Defines a route for multiple HTTP methods at once:

    any ['get', 'post'] => '/myaction' => sub {
        # code
    };

Or even, a route handler that would match any HTTP methods:

    any '/myaction' => sub {
        # code
    };

=head2 cookies

Accesses cookies values, it returns a hashref of L<Dancer2::Core::Cookie>
objects:

    get '/some_action' => sub {
        my $cookie = cookies->{name};
        return $cookie->value;
    };

In case you have stored something other than a scalar in your cookie:

    get '/some_action' => sub {
        my $cookie = cookies->{oauth};
        my %values = $cookie->value;
        return ($values{token}, $values{token_secret});
    };

=head2 cookie

Accesses a cookie value (or sets it). Note that this method will eventually
be preferred over C<set_cookie>.

    cookie lang => "fr-FR";              # set a cookie and return its value
    cookie lang => "fr-FR", expires => "2 hours";   # extra cookie info
    cookie "lang"                        # return a cookie value

If your cookie value is a key/value URI string, like

    token=ABC&user=foo

C<cookie> will only return the first part (C<token=ABC>) if called in scalar
context. Use list context to fetch them all:

    my @values = cookie "name";

=head2 config

Accesses the configuration of the application:

    get '/appname' => sub {
        return "This is " . config->{appname};
    };

=head2 content

Sets the content for the response. This B<only> works within a delayed
response.

This will crash:

    get '/' => sub {
        # THIS WILL CRASH
        content 'Hello, world!';
    };

But this will work just fine:

    get '/' => sub {
        delayed {
            content 'Hello, world!';
            ...
        };
    };

=head2 content_type

Sets the B<content-type> rendered, for the current route handler:

    get '/cat/:txtfile' => sub {
        content_type 'text/plain';

        # here we can dump the contents of route_parameters->get('txtfile')
    };

You can use abbreviations for content types. For instance:

    get '/svg/:id' => sub {
        content_type 'svg';

        # here we can dump the image with id route_parameters->get('id')
    };

Note that if you want to change the default content-type for every route,
it is easier to change the C<content_type> setting instead.

=head2 dance

Alias for the C<start> keyword.

=head2 dancer_version

Returns the version of Dancer. If you need the major version, do something
like:

  int(dancer_version);

=head2 debug

Logs a message of debug level:

    debug "This is a debug message";

See L<Dancer2::Core::Role::Logger> for details on how to configure where log
messages go.

=head2 decode_json ($string)

Deserializes a JSON structure from an UTF-8 binary string.

=head2 dirname

Returns the dirname of the path given:

    my $dir = dirname($some_path);

=head2 encode_json ($structure)

Serializes a structure to a UTF-8 binary JSON string.

Calling this function will B<not> trigger the serialization's hooks.

=head2 engine

Given a namespace, returns the current engine object

    my $template_engine = engine 'template';
    my $html = $template_engine->apply_renderer(...);
    $template_engine->apply_layout($html);

=head2 error

Logs a message of error level:

    error "This is an error message";

See L<Dancer2::Core::Role::Logger> for details on how to configure where log
messages go.

=head2 false

Constant that returns a false value (0).

=head2 forward

Runs an "internal redirect" of the current route to another route. More
formally; when C<forward> is executed, the current dispatch of the route is
aborted, the request is modified (altering query params or request method),
and the modified request following a new route is dispatched again. Any
remaining code (route and hooks) from the current dispatch will never be run
and the modified route's dispatch will execute hooks for the new route normally.

It effectively lets you chain routes together in a clean manner.

    get '/demo/articles/:article_id' => sub {

        # you'll have to implement this next sub yourself :)
        change_the_main_database_to_demo();

        forward "/articles/" . route_parameters->get('article_id');
    };

In the above example, the users that reach I</demo/articles/30> will
actually reach I</articles/30> but we've changed the database to demo
before.

This is pretty cool because it lets us retain our paths and offer a demo
database by merely going to I</demo/...>.

You'll notice that in the example we didn't indicate whether it was B<GET>
or B<POST>. That is because C<forward> chains the same type of route the
user reached. If it was a B<GET>, it will remain a B<GET> (but if you do
need to change the method, you can do so; read on below for details.)

Also notice that C<forward> only redirects to a new route. It does not redirect
the requests involving static files. This is because static files are handled
before L<Dancer2> tries to match the request to a route - static files take
higher precedence.

This means that you will not be able to C<forward> to a static file. If you
wish to do so, you have two options: either redirect (asking the browser to
make another request, but to a file path instead) or use C<send_file> to
provide a file.

B<WARNING:> Any code after a C<forward> is ignored, until the end of the
route. It's not necessary to use C<return> with C<forward> anymore.

    get '/foo/:article_id' => sub {
        if ($condition) {
            forward "/articles/" . route_parameters->get('article_id');
            # The following code WILL NOT BE executed
            do_stuff();
        }

        more_stuff();
    };

Note that C<forward> doesn't parse GET arguments. So, you can't use
something like:

    forward '/home?authorized=1';

But C<forward> supports an optional hashref with parameters to be added to
the actual parameters:

    forward '/home', { authorized => 1 };

Finally, you can add some more options to the C<forward> method, in a third
argument, also as a hashref. That option is currently only used to change
the method of your request. Use with caution.

    forward '/home', { auth => 1 }, { method => 'POST' };

=head2 from_dumper ($structure)

Deserializes a Data::Dumper structure.

=head2 from_json ($string, \%options)

Deserializes a JSON structure from a string. You should probably use
C<decode_json> which expects a UTF-8 encoded binary string and
handles decoding it for you.

=head2 from_yaml ($structure)

Deserializes a YAML structure.

=head2 get

Defines a route for HTTP B<GET> requests to the given path:

    get '/' => sub {
        return "Hello world";
    }

Note that a route to match B<HEAD> requests is automatically created as well.

=head2 halt

Sets a response object with the content given.

When used as a return value from a hook, this breaks the execution flow and
renders the response immediately:

    hook before => sub {
        if ($some_condition) {
            halt("Unauthorized");

            # this code is not executed
            do_stuff();
        }
    };

    get '/' => sub {
        "hello there";
    };

B<WARNING:> Issuing a halt immediately exits the current route, and performs
the halt. Thus, any code after a halt is ignored, until the end of the route.
Hence, it's not necessary anymore to use C<return> with halt.

=head2 response_headers

Adds custom headers to response:

    get '/send/headers', sub {
        response_headers 'X-Foo' => 'bar', 'X-Bar' => 'foo';
    }

=head2 response_header

Adds a custom header to response:

    get '/send/header', sub {
        response_header 'x-my-header' => 'shazam!';
    }

Note that it will overwrite the old value of the header, if any. To avoid
that, see L</push_response_header>.

=head2 push_response_header

Do the same as C<response_header>, but allow for multiple headers with the same
name.

    get '/send/header', sub {
        push_response_header 'x-my-header' => '1';
        push_response_header 'x-my-header' => '2';
        # will result in two headers "x-my-header" in the response
    }

=head2 prepare_app

You can introduce code you want to run when your app is loaded, similar to the
C<prepare_app> in L<Plack::Middleware>.

    prepare_app {
        my $app = shift;

        ... # do your thing
    };

You should not close over the App instance, since you receive it as a first
argument. If you close over it, you B<will> have a memory leak.

    my $app = app();

    prepare_app {
        do_something_with_app($app); # MEMORY LEAK
    };

=head2 hook

Adds a hook at some position. For example :

  hook before_serializer => sub {
    my $content = shift;
    ...
  };

There can be multiple hooks assigned to a given position, and each will be
executed in order.

See the L<HOOKS|/HOOKS> section for a list of available hooks.

=head2 info

Logs a message of C<info> level:

    info "This is an info message";

See L<Dancer2::Core::Role::Logger> for details on how to configure where log
messages go.

=head2 mime

Shortcut to access the instance object of L<Dancer2::Core::MIME>. You should
read the L<Dancer2::Core::MIME> documentation for full details, but the most
commonly-used methods are summarized below:

    # set a new mime type
    mime->add_type( foo => 'text/foo' );

    # set a mime type alias
    mime->add_alias( f => 'foo' );

    # get mime type for an alias
    my $m = mime->for_name( 'f' );

    # get mime type for a file (based on extension)
    my $m = mime->for_file( "foo.bar" );

    # get current defined default mime type
    my $d = mime->default;

    # set the default mime type using config.yml
    # or using the set keyword
    set default_mime_type => 'text/plain';

=head2 params

I<This method should be called from a route handler>.
It's an alias for the L<Dancer2::Core::Request params
accessor|Dancer2::Core::Request/"params($source)">. It returns a hash (in
list context) or a hash reference (in scalar context) to all defined
parameters. Check C<param> below to access quickly to a single parameter
value.

    post '/login' => sub {
        # get all parameters as a single hash
        my %all_parameters = params;

        // request all parmameters from a specific source: body, query, route
        my %body_parameters  = params('body');
        my %route_parameters = params('route');
        my %query_parameters = params('query');

        # any $source that is not body, query, or route generates an exception
        params('fake_source'); // Unknown source params "fake_source"
    };

We now recommend using one of the specific keywords for parameters
(C<route_parameters>, C<query_parameters>, and C<body_parameters>)
instead of C<params> or C<param>.

=head2 param

I<This method should be called from a route handler>.
This method is an accessor to the parameters hash table.

   post '/login' => sub {
       my $username = param "user";
       my $password = param "pass";
       # ...
   };

We now recommend using one of the specific keywords for parameters
(C<route_parameters>, C<query_parameters>, and C<body_parameters>)
instead of C<params> or C<param>.

=head2 route_parameters

Returns a L<Hash::MultiValue> object from the route parameters.

    # /hello
    get '/:foo' => sub {
        my $foo = route_parameters->get('foo');
    };

=head2 query_parameters

Returns a L<Hash::MultiValue> object from the request parameters.

    /?foo=hello
    get '/' => sub {
        my $name = query_parameters->get('foo');
    };

    /?name=Alice&name=Bob
    get '/' => sub {
        my @names = query_parameters->get_all('name');
    };

=head2 body_parameters

Returns a L<Hash::MultiValue> object from the body parameters.

    post '/' => sub {
        my $last_name = body_parameters->get('name');
        my @all_names = body_parameters->get_all('name');
    };

=head2 pass

I<This method should be called from a route handler>.
Tells Dancer2 to pass the processing of the request to the next matching
route.

B<WARNING:> Issuing a pass immediately exits the current route, and performs
the pass. Thus, any code after a pass is ignored, until the end of the
route. Hence, it's not necessary anymore to use C<return> with pass.

    get '/some/route' => sub {
        if (...) {
            # we want to let the next matching route handler process this one
            pass(...);

            # this code will be ignored
            do_stuff();
        }
    };

B<WARNING:> You cannot set the content before passing and have it remain,
even if you use the C<content> keyword or set it directly in the response
object.

=head2 patch

Defines a route for HTTP B<PATCH> requests to the given URL:

    patch '/resource' => sub { ... };

(C<PATCH> is a relatively new and not-yet-common HTTP verb, which is
intended to work as a "partial-PUT", transferring just the changes; please
see L<RFC5789|http://tools.ietf.org/html/rfc5789> for further details.)

=head2 path

Concatenates multiple paths together, without worrying about the underlying
operating system:

    my $path = path(dirname($0), 'lib', 'File.pm');

It also normalizes (cleans) the path aesthetically. It does not verify whether
the path exists, though.

=head2 post

Defines a route for HTTP B<POST> requests to the given URL:

    post '/' => sub {
        return "Hello world";
    }

=head2 prefix

Defines a prefix for each route handler, like this:

    prefix '/home';

From here, any route handler is defined to /home/*:

    get '/page1' => sub {}; # will match '/home/page1'

You can unset the prefix value:

    prefix undef;
    get '/page1' => sub {}; # will match /page1

For a safer alternative you can use lexical prefix like this:

    prefix '/home' => sub {
        ## Prefix is set to '/home' here

        get ...;
        get ...;
    };
    ## prefix reset to the previous version here

This makes it possible to nest prefixes:

   prefix '/home' => sub {
       ## some routes

      prefix '/private' => sub {
         ## here we are under /home/private...

         ## some more routes
      };
      ## back to /home
   };
   ## back to the root

B<Notice:> Once you have a prefix set, do not add a caret to the regex:

    prefix '/foo';
    get qr{^/bar} => sub { ... } # BAD BAD BAD
    get qr{/bar}  => sub { ... } # Good!

=head2 del

Defines a route for HTTP B<DELETE> requests to the given URL:

    del '/resource' => sub { ... };

=head2 options

Defines a route for HTTP B<OPTIONS> requests to the given URL:

    options '/resource' => sub { ... };

=head2 put

Defines a route for HTTP B<PUT> requests to the given URL:

    put '/resource' => sub { ... };

=head2 redirect

Generates a HTTP redirect (302). You can either redirect to a complete
different site or within the application:

    get '/twitter', sub {
        redirect 'http://twitter.com/me';
        # Any code after the redirect will not be executed.
    };

B<WARNING:> Issuing a C<redirect> immediately exits the current route.
Thus, any code after a C<redirect> is ignored, until the end of the route.
Hence, it's not necessary anymore to use C<return> with C<redirect>.

You can also force Dancer to return a specific 300-ish HTTP response code:

    get '/old/:resource', sub {
        redirect '/new/' . route_parameters->get('resource'), 301;
    };

=head2 request

Returns a L<Dancer2::Core::Request> object representing the current request.

See the L<Dancer2::Core::Request> documentation for the methods you can
call, for example:

    request->referer;         # value of the HTTP referer header
    request->remote_address;  # user's IP address
    request->user_agent;      # User-Agent header value

=head2 request_header

Returns request header(s).

    get '/get/headers' => sub {
        my $xfoo = request_header 'X-Foo';
        ...
    };

=head2 send_as

Allows the current route handler to return specific content types to the
client using either a specified serializer or as html.

Any Dancer2 serializer may be used. The specified serializer class will
be loaded if required, or an error generated if the class can not be found.
Serializer configuration may be added to your apps C<engines> configuration.

If C<html> is specified, the content will be returned assuming it is HTML with
appropriate C<Content-Type> headers and encoded using the apps configured
C<charset> (or UTF-8).

    set serializer => 'YAML';
    set template   => 'TemplateToolkit';

    # returns html (not YAML)
    get '/' => sub { send_as html => template 'welcome.tt' };

    # return json (not YAML)
    get '/json' => sub {
        send_as JSON => [ some => { data => 'structure' } ];
    };

C<send_as> uses L</send_file> to return the content immediately. You may
pass any option C<send_file> supports as an extra option. For example:

    # return json with a custom content_type header
    get '/json' => sub {
        send_as JSON => [ some => { data => 'structure' } ],
                { content_type => 'application/json; charset=UTF-8' },
    };

B<WARNING:> Issuing a send_as immediately exits the current route, and
performs the C<send_as>. Thus, any code after a C<send_as> is ignored,
until the end of the route. Hence, it's not necessary to use C<return>
with C<send_as>.

    get '/some/route' => sub {
        if (...) {
            send_as JSON => $some_data;

            # this code will be ignored
            do_stuff();
        }
    };

=head2 send_error

Returns a HTTP error. By default the HTTP code returned is 500:

    get '/photo/:id' => sub {
        if (...) {
            send_error("Not allowed", 403);
        } else {
           # return content
        }
    }

B<WARNING:> Issuing a send_error immediately exits the current route, and
performs the C<send_error>. Thus, any code after a C<send_error> is ignored,
until the end of the route. Hence, it's not necessary anymore to use C<return>
with C<send_error>.

    get '/some/route' => sub {
        if (...) {
            # Something bad happened, stop immediately!
            send_error(..);

            # this code will be ignored
            do_stuff();
        }
    };

=head2 send_file

Lets the current route handler send a file to the client. Note that the path
of the file must be relative to the B<public> directory unless you use the
C<system_path> option (see below).

    get '/download/:file' => sub {
        return send_file(route_parameters->get('file'));
    }

B<WARNING:> Issuing a C<send_file> immediately exits the current route, and
performs the C<send_file>. Thus, any code after a C<send_file> is ignored,
until the end of the route. Hence, it's not necessary anymore to use C<return>
with C<send_file>.

    get '/some/route' => sub {
        if (...) {
            # OK, send her what she wants...
            send_file(...);

            # this code will be ignored
            do_stuff();
        }
    };

C<send_file> will use PSGI streaming if the server supports it (most, if
not all, do). You can explicitly disable streaming by passing
C<streaming =E<gt> 0> as an option to C<send_file>.

    get '/download/:file' => sub {
        send_file( route_parameters->get('file'), streaming => 0 );
    }

The content-type will be set depending on the current MIME types definition
(see C<mime> if you want to define your own).

If your filename does not have an extension, you are passing in a filehandle,
or you need to force a specific mime type, you can pass it to C<send_file>
as follows:

    send_file(route_parameters->get('file'), content_type => 'image/png');
    send_file($fh, content_type => 'image/png');

Also, you can use your aliases or file extension names on C<content_type>,
like this:

    send_file(route_parameters->get('file'), content_type => 'png');

The encoding of the file or filehandle may be specified by passing both
the C<content_type> and C<charset> options. For example:

    send_file($fh, content_type => 'text/csv', charset => 'utf-8' );

For files outside your B<public> folder, you can use the C<system_path>
switch. Just bear in mind that its use needs caution as it can be dangerous.

   send_file('/etc/passwd', system_path => 1);

If you have your data in a scalar variable, C<send_file> can be useful as
well. Pass a reference to that scalar, and C<send_file> will behave as if
there was a file with that contents:

   send_file( \$data, content_type => 'image/png' );

Note that Dancer is unable to guess the content type from the data contents.
Therefore you might need to set the C<content_type> properly. For this kind
of usage an attribute named C<filename> can be useful. It is used as the
Content-Disposition header, to hint the browser about the filename it should
use.

   send_file( \$data, content_type => 'image/png'
                      filename     => 'onion.png' );

By default the Content-Disposition header uses the "attachment" type, which
triggers a "Save" dialog in some browsers. Supply a C<content_disposition>
attribute of "inline" to have the file displayed inline by the browser.

=head2 set

Defines a setting:

    set something => 'value';

You can set more than one value at once:

    set something => 'value', otherthing => 'othervalue';

=head2 setting

Returns the value of a given setting:

    setting('something'); # 'value'

=head2 session

Provides access to all data stored in the user's session (if any).

It can also be used as a setter to store data in the session:

    # getter example
    get '/user' => sub {
        if (session('user')) {
            return "Hello, ".session('user')->name;
        }
    };

    # setter example
    post '/user/login' => sub {
        ...
        if ($logged_in) {
            session user => $user;
        }
        ...
    };

You may also need to clear a session:

    # destroy session
    get '/logout' => sub {
        ...
        app->destroy_session;
        ...
    };

If you need to fetch the session ID being used for any reason:

    my $id = session->id;

=head2 splat

Returns the list of captures made from a route handler with a route pattern
which includes wildcards:

    get '/file/*.*' => sub {
        my ($file, $extension) = splat;
        ...
    };

There is also the extensive splat (A.K.A. "megasplat"), which allows
extensive greedier matching, available using two asterisks. The additional
path is broken down and returned as an arrayref:

    get '/entry/*/tags/**' => sub {
        my ( $entry_id, $tags ) = splat;
        my @tags = @{$tags};
    };

The C<splat> keyword in the above example for the route F</entry/1/tags/one/two>
would set C<$entry_id> to C<1> and C<$tags> to C<['one', 'two']>.

=head2 start

Starts the application or the standalone server (depending on the deployment
choices).

This keyword should be called at the very end of the script, once all routes
are defined. At this point, Dancer2 takes over.

=head2 to_app

Returns the PSGI coderef for the current (and only the current) application.

You can call it as a method on the class or as a DSL:

    my $app = MyApp->to_app;

    # or

    my $app = to_app;

There is a
L<Dancer Advent Calendar article|http://advent.perldancer.org/2014/9> covering
this keyword and its usage further.

=head2 psgi_app

Provides the same functionality as C<to_app> but uses the deprecated
Dispatcher engine. You should use C<to_app> instead.

=head2 status

Changes the status code provided by an action. By default, an action will
produce an C<HTTP 200 OK> status code, meaning everything is OK:

    get '/download/:file' => {
        if (! -f route_parameters->get('file')) {
            status 'not_found';
            return "File does not exist, unable to download";
        }
        # serving the file...
    };

In that example, Dancer will notice that the status has changed, and will
render the response accordingly.

The C<status> keyword receives either a numeric status code or its name in
lower case, with underscores as a separator for blanks - see the list in
L<Dancer2::Core::HTTP/"HTTP CODES">. As an example, The above call translates
to setting the code to C<404>.

=head2 template

Returns the response of processing the given template with the given
parameters (and optional settings), wrapping it in the default or specified
layout too, if layouts are in use.

An example of a  route handler which returns the result of using template to
build a response with the current template engine:

    get '/' => sub {
        ...
        return template 'some_view', { token => 'value'};
    };

Note that C<template> simply returns the content, so when you use it in a
route handler, if execution of the route handler should stop at that point,
make sure you use C<return> to ensure your route handler returns the content.

Since C<template> just returns the result of rendering the template, you can
also use it to perform other templating tasks, e.g. generating emails:

    post '/some/route' => sub {
        if (...) {
            email {
                to      => 'someone@example.com',
                from    => 'foo@example.com',
                subject => 'Hello there',
                msg     => template('emails/foo', { name => body_parameters->get('name') }),
            };

            return template 'message_sent';
        } else {
            return template 'error';
        }
    };

Compatibility notice: C<template> was changed in version 1.3090 to
immediately interrupt execution of a route handler and return the content,
as it's typically used at the end of a route handler to return content.
However, this caused issues for some people who were using C<template> to
generate emails etc, rather than accessing the template engine directly, so
this change has been reverted in 1.3091.

The first parameter should be a template available in the views directory,
the second one (optional) is a hashref of tokens to interpolate, and the
third (again optional) is a hashref of options.

For example, to disable the layout for a specific request:

    get '/' => sub {
        template 'index', {}, { layout => undef };
    };

Or to request a specific layout, of course:

    get '/user' => sub {
        template 'user', {}, { layout => 'user' };
    };

Some tokens are automatically added to your template (C<perl_version>,
C<dancer_version>, C<settings>, C<request>, C<vars> and, if you
have sessions enabled, C<session>). Check L<Default Template
Variables|Dancer2::Manual/Default-Template-Variables>
for further details.

=head2 to_dumper ($structure)

Serializes a structure with Data::Dumper.

Calling this function will B<not> trigger the serialization's hooks.

=head2 to_json ($structure, \%options)

Serializes a structure to JSON. You should probably use C<encode_json> instead
which handles encoding the result for you.

=head2 to_yaml ($structure)

Serializes a structure to YAML.

Calling this function will B<not> trigger the serialization's hooks.

=head2 true

Constant that returns a true value (1).

=head2 upload

Provides access to file uploads. Any uploaded file is accessible as a
L<Dancer2::Core::Request::Upload> object. You can access all parsed uploads
via:

    post '/some/route' => sub {
        my $file = upload('file_input_foo');
        # $file is a Dancer2::Core::Request::Upload object
    };

If you named multiple inputs of type "file" with the same name, the C<upload>
keyword would return an Array of L<Dancer2::Core::Request::Upload> objects:

    post '/some/route' => sub {
        my ($file1, $file2) = upload('files_input');
        # $file1 and $file2 are Dancer2::Core::Request::Upload objects
    };

You can also access the raw hashref of parsed uploads via the current
C<request> object:

    post '/some/route' => sub {
        my $all_uploads = request->uploads;
        # $all_uploads->{'file_input_foo'} is a Dancer2::Core::Request::Upload object
        # $all_uploads->{'files_input'} is an arrayref of Dancer2::Core::Request::Upload objects
    };

Note that you can also access the filename of the upload received via the
C<body_parameters> keyword:

    post '/some/route' => sub {
        # body_parameters->get('files_input') is the filename of the file uploaded
    };

See L<Dancer2::Core::Request::Upload> for details about the interface provided.

=head2 uri_for

Returns a fully-qualified URI for the given path:

    get '/' => sub {
        redirect uri_for('/path');
        # can be something like: http://localhost:5000/path
    };

Query string parameters can be provided by passing a hashref as a second param:

    uri_for('/path', { foo => 'bar' });
    # would return e.g. http://localhost:5000/path?foo=bar

By default, the parameters will be URL encoded:

    uri_for('/path', { foo => 'hope;faith' });
    # would return http://localhost:5000/path?foo=hope%3Bfaith

If desired (for example, if you've already encoded your query
parameters and you want to prevent double encoding) you can disable
URL encoding via a third parameter:

    uri_for('/path', { foo => 'qux%3Dquo' }, 1);
    # would return http://localhost:5000/path?foo=qux%3Dquo

=head2 captures

Returns a reference to a copy of C<%+>, if there are named captures in the
route's regular expression.

Named captures are a feature of Perl 5.10, and are not supported in earlier
versions:

    get qr{
        / (?<object> user   | ticket | comment )
        / (?<action> delete | find )
        / (?<id> \d+ )
        /?$
    }x
    , sub {
        my $value_for = captures;
        "i don't want to $$value_for{action} the $$value_for{object} $$value_for{id} !"
    };

=head2 var

Provides an accessor for variables shared between hooks and route
handlers. Given a key/value pair, it sets a variable:

    hook before => sub {
        var foo => 42;
    };

Later, route handlers and other hooks will be able to read that variable:

    get '/path' => sub {
        my $foo = var 'foo';
        ...
    };

=head2 vars

Returns the hashref of all shared variables set during the hook/route
chain with the C<var> keyword:

    get '/path' => sub {
        if (vars->{foo} eq 42) {
            ...
        }
    };

=head2 warning

Logs a warning message through the current logger engine:

    warning "This is a warning";

See L<Dancer2::Core::Role::Logger> for details on how to configure where log
messages go.

=head1 AUTHOR

Dancer Core Developers

=head1 COPYRIGHT AND LICENSE

This software is copyright (c) 2019 by Alexis Sukrieh.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut
