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>