Object::InsideOut::Metadata(3)

NAME
Object::InsideOut::Metadata - Introspection for Object::InsideOut classes

VERSION
This document describes Object::InsideOut::Metadata version 4.05

SYNOPSIS
 package My::Class; {
     use Object::InsideOut;
     use Object::InsideOut::Metadata;

     my @data :Field :Arg('data') :Get('data') :Set('put_data');
     my @misc :Field;

     my %init_args :InitArgs = (
         'INFO' => '',
     );

     sub _init :Init
     {
         my ($self, $args) = @_;
         if (exists($args->{'INFO'})) {
             $misc[$$self] = 'INFO: ' . $args->{'INFO'};
         }
     }

     sub misc :lvalue :Method
     {
         my $self = shift;
         $misc[$$self];
     }
     add_meta(__PACKAGE__, 'misc', { 'lvalue' => 1 });
 }

 package main;

 # Obtain a metadata object for a class
 my $meta = My::Class->meta();

 # ... or obtain a metadata object for an object
 my $obj = My::Class->new();
 my $meta = $obj->meta();

 # Obtain the class hierarchy from the metadata object
 my @classes = $meta->get_classes();

 # Obtain information on the parameters for a class's construction
 my %args = $meta->get_args();

 # Obtain information on a class's methods
 my %methods = $meta->get_methods();

DESCRIPTION
Object::InsideOut provides an introspection API that allows you to obtain metadata on a class's hierarchy, constructor parameters, and methods. This is done through the use of metadata objects that are generated by this class.

In addition, developers can specify metadata data for methods they write for their classes.

METADATA OBJECT
To obtain metadata on an Object::InsideOut class or object, you must first generate a metadata object for that class or object. Using that metadata object, one can then obtain information on the class hierarchy, constructor parameters, and methods.

my $meta = My::Class->meta();
my $meta = $obj->meta();
    The "->meta()" method, which is exported by Object::InsideOut to each class, returns an Object::InsideOut::Metadata object which can then be queried for information about the invoking class or invoking object's class.

CLASS HIERARCHY
Any Object::InsideOut class potentially has four categories of classes associated with it:

1.  Object::InsideOut
    While the basis for all Object::InsideOut classes it is not an object class per se because you can create objects from it (i.e., you can't do "Object::InsideOut-"new()>). While "My::Class-"isa('Object::InsideOut')> will return true, because Object::InsideOut is not an object class, it is not considered to be part of a class's hierarchy.

2.  The class itself
    A class's hierarchy always includes itself.

3.  Parent classes
    These are all the Object::InsideOut classes up the inheritance tree that a class is derived from.

4.  Foreign classes
    These are non-Object::InsideOut classes that a class inherits from. (See "FOREIGN CLASS INHERITANCE" in Object::InsideOut.) Because of implementation details, foreign classes do not appear in a class's @ISA array. However, Object::InsideOut implements a version of "->isa()" that handles foreign classes.

A class's hierarchy consists of any classes in the latter three categories.

$meta->get_classes();
    When called in an array context, returns a list that constitutes the class hierarchy for the class or object used to generate the metadata object. When called in a scalar context, returns an array ref.

My::Class->isa();
$obj->isa();
    When called in an array context, calling "->isa()" without any arguments on an Object::InsideOut class or object returns a list of the classes in the class hierarchy for that class or object, and is equivalent to:

     my @classes = $obj->meta()->get_classes();

    When called in a scalar context, it returns an array ref containing the classes.

CONSTRUCTOR PARAMETERS
Constructor parameters are the arguments given to a class's "->new()" call.

$meta->get_args();
    Returns a hash (hash ref in scalar context) containing information on the parameters that can be used to construct an object from the class associated with the metadata object. Here's an example of such a hash:

     {
         'My::Class' => {
             'data' => {
                 'field' => 1,
                 'type' => 'numeric',
             },
             'misc' => {
                 'mandatory' => 1,
             },
         },
         'My::Parent' => {
             'info' => {
                 'default' => '<none>',
             },
         },
     }

    The keys for this hash are the Object::IsideOut classes in the class hierarchy. These class keys are paired with hash refs, the keys of which are the names of the parameters for that class (e.g., 'data' and 'misc' for My::Class, and 'info' for My::Parent). The hashes paired to the parameters contain information about the parameter:

    field
        The parameter corresponds directly to a class field, and is automatically processed during object creation. See "Field-Specific Parameters" in Object::InsideOut.

    mandatory
        The parameter is required for object creation. See "Mandatory Parameters" in Object::InsideOut.

    default
        The default value assigned to the parameter if it is not found in the arguments to "->new()". See "Default Values" in Object::InsideOut.

    preproc
        The code ref for the subroutine that is used to preprocess a parameter's value. See "Parameter Preprocessing" in Object::InsideOut

    type
        The form of type checking performed on the parameter. See "TYPE CHECKING" in Object::InsideOut for more details.

        'numeric'
            Parameter takes a numeric value as recognized by Scalar::Util::looks_like_number().

        'list'
        'list(_subtype_)'
            Parameter takes a single value (which is then placed in an array ref) or an array ref.

            When specified, the contents of the resulting array ref must be of the specified subtype:

            'numeric'
                Same as for the basic type above.

            A class name
                Same as for the basic type below.

            A reference type
                Any reference type as returned by ref()).

        'ARRAY(_subtype_)'
            Parameter takes an array ref with contents of the specified subtype as per the above.

        A class name
            Parameter takes an object of a specified class, or one of its sub-classes as recognized by "->isa()".

        Other reference type
            Parameter takes a reference of the specified type as returned by ref().

        A code ref
            Parameter takes a value that is type-checked by the code ref paired to the 'type' key.

METHODS METADATA
The methods returned by a metadata object are those that are currently available at the time of the "->get_methods()" call.

The presence of ":Automethod" subroutines in an Object::InsideOut class, or "AUTOLOAD" in a foreign class means that the methods supported by the class may not be determinable. The presence of "AUTOLOAD" in the list of methods for a class should alert the programmer to the fact that more methods may be supported than are listed.

Methods that are excluded are private and hidden methods (see "PERMISSIONS" in Object::InsideOut), methods that begin with an underscore (which, by convention, means they are private), and subroutines named "CLONE", "CLONE_SKIP", and "DESTROY" (which are not methods). While technically a method, "import" is also excluded as it is generally not invoked directly (i.e., it's usually called as part of "use").

$meta->get_methods();
    Returns a hash (hash ref in scalar context) containing information on the methods for the class associated with the metadata object. The keys in the hash are the method names. Paired to the names are hash refs containing metadata about the methods. Here's an example:

     {
         # Methods exported by Object::InsideOut
         'new' => {
            'class' => 'My::Class',
            'kind'  => 'constructor'
         },
         'clone' => {
             'class' => 'My::Class',
             'kind'  => 'object'
         },
         'meta'  => {
             'class' => 'My::Class'
         },
         'set' => {
             'class' => 'My::Class',
             'kind'  => 'object',
             'restricted' => 1
         },
         # Methods provided by Object::InsideOut
         'dump' => {
             'class' => 'Object::InsideOut',
             'kind'  => 'object'
         },
         'pump' => {
             'class' => 'Object::InsideOut',
             'kind'  => 'class'
         },
         'inherit' => {
             'class' => 'Object::InsideOut',
             'kind'  => 'object',
             'restricted' => 1
         },
         'heritage' => {
             'class' => 'Object::InsideOut',
             'kind'  => 'object',
             'restricted' => 1
         },
         'disinherit' => {
             'class' => 'Object::InsideOut',
             'kind'  => 'object',
             'restricted' => 1
         },
         # Methods generated by Object::InsideOut for My::Class
         'set_data' => {
             'class'  => 'My::Class',
             'kind'   => 'set',
             'type'   => 'ARRAY',
             'return' => 'new'
         },
         'get_data' => {
             'class' => 'My::Class',
             'kind'  => 'get'
         }
         # Class method provided by My::Class
         'my_method' => {
             'class' => 'My::Class',
             'kind'  => 'class'
         }
     }

    Here are the method metadata that are provided:

    class
        The class in whose symbol table the method resides. The method may reside in the classes code, it may be exported by another class, or it may be generated by Object::InsideOut.

        Methods that are overridden in child classes are represented as being associated with the most junior class for which they appear.

    kind
        Designation of the characteristic of the method:

        constructor
            The "->new()" method, of course.

        get, set or accessor
            A get, set, or combined accessor generated by Object::InsideOut. See "AcCESSOR GENERATION" in Object::InsideOut.

        cumulative, or cumulative (bottom up)
        chained, or chained (bottom up)
            A cumulative or chained method. See "CUMULATIVE METHODS" in Object::InsideOut, and "CHAINED METHODS" in Object::InsideOut. The class associated with these methods is the most junior class in which they appears.

        class
            A method that is callable only on a class (e.g., "My::Class->my_method()").

        object
            A method that is callable only on a object (e.g. "$obj->get_data()").

        foreign
            A subroutine found in a foreign class's symbol table. Programmers must check the class's documentation to determine which are actually methods, and what kinds of methods they are.

        overload
            A subroutine used for object coercion. These may be called as methods, but this is not normally how they are used.

        automethod
            Associated with an AUTOLOAD method for an Object::InsideOut class that implements an ":Automethod" subroutine. See "AUTOMETHODS" in Object::InsideOut.

    type
        The type checking that is done on arguments to set/combined accessors generated by Object::InsideOut. See "TYPE CHECKING" in Object::InsideOut

    return
        The value returned by a set/combined accessor generated by Object::InsideOut. See "Set Accessor Return Value" in Object::InsideOut

    lvalue
        The method is an :lvalue accessor.

    restricted
        The method is restricted (i.e., callable only from within the class hierarchy; not callable from application code). See "PERMISSIONS" in Object::InsideOut.

My::Class->can();
$obj->can();
    When called in an array context, calling "->can()" without any arguments on an Object::InsideOut class or object returns a list of the method names for that class or object, and is equivalent to:

     my %meths = $obj->meta()->get_methods();
     my @methods = keys(%meths);

    When called in a scalar context, it returns an array ref containing the method names.

  METADATA ATTRIBUTES
Class authors may add the ":Method" attribute to subroutines in their classes to specifically designate them as OO-callable methods. If a method is only a class method or only an object method, this may be added as a parameter to the attribute:

 sub my_method :Method(class)
 {
     ...

The class or object parameter will appear in the metadata for the method when listed using "->get_methods()".

CAUTION: Be sure not to use ":method" (all lowercase) except as appropriate (see "ARGUMENT VALIDATION" in Object::InsideOut) as this is a Perl reserved attribute.

The ":Sub" attribute can be used to designate subroutines that are not OO-callable methods. These subroutines will not show up as part of the methods listed by "->get_methods()", etc..

Subroutine names beginning with an underscore are, by convention, considered private, and will not show up as part of the methods listed by "->get_methods()", etc..

  ADDING METADATA
Class authors may add additional metadata to their methods using the add_meta() subroutine which is exported by this package. For example, if the class implements it own ":lvalue" method, it should add that metadata so that it is picked up the "->get_methods()":

 package My::Class; {
     use Object::InsideOut;
     use Object::InsideOut::Metadata;

     sub my_method :lvalue :Method(object)
     {
         ....
     }
     add_meta(__PACKAGE__, 'my_method', 'lvalue', 1);
 }

The arguments to add_meta() are:

Class name
    This can usually be designated using the special literal C__PACKAGE__>.

Method name
Metadata name
    This can be any of the metadata names under "METHODS METADATA", or can be whatever additional name the programmer chooses to implement.

Metadata value

When adding multiple metadata for a method, they may be enclosed in a single hash ref:

 add_meta(__PACKAGE__, 'my_method', { 'lvalue' => 1,
                                      'return' => 'old' });

If adding metadata for multiple methods, another level of hash may be used:

 add_meta(__PACKAGE__, { 'my_method' => { 'lvalue' => 1,
                                          'return' => 'old' },
                         'get_info'  => { 'my_meta' => 'true' } });

TO DO
Provide filtering capabilities on the method information returned by "->get_methods()".

REQUIREMENTS
Perl 5.8.0 or later

SEE ALSO
Object::InsideOut

Perl 6 introspection: <http://dev.perl.org/perl6/doc/design/apo/A12.html#Introspection>, and <http://dev.perl.org/perl6/rfc/335.html>

AUTHOR
Jerry D. Hedden, <jdhedden AT cpan DOT org>

COPYRIGHT AND LICENSE
Copyright 2006 - 2012 Jerry D. Hedden. All rights reserved.

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