Object::Pad::MOP::FieldAttr(3pm)

NAME
"Object::Pad::MOP::FieldAttr" - meta-object representation of a field attribute for "Object::Pad"

DESCRIPTION
This API provides a way for pure-perl implementations of field attributes to be provided. Pure-perl attributes cannot currently add new behaviour to the way that fields work, but they do provide a means for class authors to annotate extra metadata onto fields, that can be queried by other code.

Primilarily this is done by using the "get_attribute_value" in Object::Pad::MOP::Field accessor method on a field metadata instance.

This API should be considered experimental, and will emit warnings to that effect. They can be silenced with

   use Object::Pad qw( :experimental(custom_field_attr) );

METHODS

  register
   Object::Pad::MOP::FieldAttr->register( $name, %args );

Since version 0.60.

Creates a new field attribute of the given name. The name must begin with a capital letter, in order to distinguish this from any of the built-in core attributes, whose names are lowercase.

The attribute is only available if the hints hash contains a key of the name given by the attribute's "permit_hintkey" argument. This would typically be set in the hints hash by the "import" method of the module implementing it, and would be named based on the name of the module providing the attribute:

   sub import { $^H{"Some::Package::Name/Attrname"} }

Takes the following additional named arguments:

permit_hintkey => STRING
    Required. A string giving a key that must be found in the hints hash ("%^H") for this attribute name to be visible.

no_value => BOOL
    An optional flag; if set to true then no value is permitted on the attribute's declaration. A compiletime error will be generated if a value is provided

must_value => BOOL
    An optional flag; if set to true then a value is required on the attribute's declaration. A compiletime error will be generated if a value is not provided.

    If neither of these flags are provided, then a value is optional. It is not permitted to set both flags at once.

apply => CODE
    An optional code reference for a callback function to invoke when the attribute is applied to a field. If present, it is passed the field metadata instance as a Object::Pad::MOP::Field reference, and a string containing the contents of the attribute's parenthesized value. The return value of the callback will be stored as the attribute's value and can be accessed by the "get_attribute_value" method on the field metadata.

       $result = $apply->( $fieldmeta, $value )

    If the "apply" callback is absent then the string value itself is stored.

AUTHOR
Paul Evans <leonerd@leonerd.org.uk>