Module ActionView::Helpers::CaptureHelper
In: vendor/rails/actionpack/lib/action_view/helpers/capture_helper.rb

Capture lets you extract parts of code into instance variables which can be used in other points of the template or even layout file.

Capturing a block into an instance variable

  <% @script = capture do %>
    [some html...]
  <% end %>

Add javascript to header using content_for

content_for("name") is a wrapper for capture which will store the fragment in a instance variable similar to @content_for_layout.

layout.rhtml: 

  <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
      <title>layout with js</title>
      <script type="text/javascript">
      <%= @content_for_script %>
          </script>
  </head>
  <body>
    <%= @content_for_layout %>
  </body>
  </html>

view.rhtml 

  This page shows an alert box!

  <% content_for("script") do %>
    alert('hello world')
  <% end %>

  Normal view text

Methods

capture   content_for  

Public Instance methods

capture(*args, &block)

Capture allows you to extract a part of the template into an instance variable. You can use this instance variable anywhere in your templates and even in your layout.

Example: 

  <% @greeting = capture do %>
    Welcome To my shiny new web page!
  <% end %>

[Source]

    # File vendor/rails/actionpack/lib/action_view/helpers/capture_helper.rb, line 51
51:       def capture(*args, &block)
52:         # execute the block
53:         buffer = eval("_erbout", block.binding)
54:         pos = buffer.length
55:         block.call(*args)
56:         
57:         # extract the block 
58:         data = buffer[pos..-1]
59:         
60:         # replace it in the original with empty string
61:         buffer[pos..-1] = ''
62:         
63:         data
64:       end
content_for(name, &block)

Content_for will store the given block in an instance variable for later use in another template or in the layout.

The name of the instance variable is content_for_<name> to stay consistent with @content_for_layout which is used by ActionView’s layouts

Example: 

  <% content_for("header") do %>
    alert('hello world')
  <% end %>

You can use @content_for_header anywhere in your templates.

NOTE: Beware that content_for is ignored in caches. So you shouldn’t use it for elements that are going to be fragment cached.

[Source]

    # File vendor/rails/actionpack/lib/action_view/helpers/capture_helper.rb, line 84
84:       def content_for(name, &block)                      
85:         base = controller.instance_variable_get(instance_var_name(name)) || ""
86:         data = capture(&block)
87:         controller.instance_variable_set(instance_var_name(name), base + data)
88:         data
89:       end

[Validate]