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Class::MOP::Class(3pm) User Contributed Perl Documentation Class::MOP::Class(3pm)
NAME
Class::MOP::Class - Class Meta Object
SYNOPSIS
# assuming that class Foo
# has been defined, you can
# use this for introspection ...
# add a method to Foo ...
Foo->meta->add_method( 'bar' => sub {...} )
# get a list of all the classes searched
# the method dispatcher in the correct order
Foo->meta->class_precedence_list()
# remove a method from Foo
Foo->meta->remove_method('bar');
# or use this to actually create classes ...
Class::MOP::Class->create(
'Bar' => (
version => '0.01',
superclasses => ['Foo'],
attributes => [
Class::MOP::Attribute->new('$bar'),
Class::MOP::Attribute->new('$baz'),
],
methods => {
calculate_bar => sub {...},
construct_baz => sub {...}
}
)
);
DESCRIPTION
The Class Protocol is the largest and most complex part of the Class::MOP meta-object
protocol. It controls the introspection and manipulation of Perl 5 classes, and it can
create them as well. The best way to understand what this module can do is to read the
documentation for each of its methods.
INHERITANCE
"Class::MOP::Class" is a subclass of Class::MOP::Module.
METHODS
Class construction
These methods all create new "Class::MOP::Class" objects. These objects can represent
existing classes or they can be used to create new classes from scratch.
The metaclass object for a given class is a singleton. If you attempt to create a
metaclass for the same class twice, you will just get the existing object.
Class::MOP::Class->create($package_name, %options)
This method creates a new "Class::MOP::Class" object with the given package name. It
accepts a number of options:
o version
An optional version number for the newly created package.
o authority
An optional authority for the newly created package.
o superclasses
An optional array reference of superclass names.
o methods
An optional hash reference of methods for the class. The keys of the hash
reference are method names and values are subroutine references.
o attributes
An optional array reference of Class::MOP::Attribute objects.
Class::MOP::Class->create_anon_class(%options)
This method works just like "Class::MOP::Class->create" but it creates an "anonymous"
class. In fact, the class does have a name, but that name is a unique name generated
internally by this module.
It accepts the same "superclasses", "methods", and "attributes" parameters that
"create" accepts.
Anonymous classes are destroyed once the metaclass they are attached to goes out of
scope, and will be removed from Perl's internal symbol table.
All instances of an anonymous class keep a special reference to the metaclass object,
which prevents the metaclass from going out of scope while any instances exist.
This only works if the instance is based on a hash reference, however.
Class::MOP::Class->initialize($package_name, %options)
This method will initialize a "Class::MOP::Class" object for the named package. Unlike
"create", this method will not create a new class.
The purpose of this method is to retrieve a "Class::MOP::Class" object for
introspecting an existing class.
If an existing "Class::MOP::Class" object exists for the named package, it will be
returned, and any options provided will be ignored!
If the object does not yet exist, it will be created.
The valid options that can be passed to this method are "attribute_metaclass",
"method_metaclass", "wrapped_method_metaclass", and "instance_metaclass". These are
all optional, and default to the appropriate class in the "Class::MOP" distribution.
Object instance construction and cloning
These methods are all related to creating and/or cloning object instances.
$metaclass->clone_object($instance, %params)
This method clones an existing object instance. Any parameters you provide are will
override existing attribute values in the object.
This is a convenience method for cloning an object instance, then blessing it into the
appropriate package.
You could implement a clone method in your class, using this method:
sub clone {
my ($self, %params) = @_;
$self->meta->clone_object($self, %params);
}
$metaclass->rebless_instance($instance, %params)
This method changes the class of $instance to the metaclass's class.
You can only rebless an instance into a subclass of its current class. If you pass any
additional parameters, these will be treated like constructor parameters and used to
initialize the object's attributes. Any existing attributes that are already set will
be overwritten.
Before reblessing the instance, this method will call "rebless_instance_away" on the
instance's current metaclass. This method will be passed the instance, the new
metaclass, and any parameters specified to "rebless_instance". By default,
"rebless_instance_away" does nothing; it is merely a hook.
$metaclass->new_object(%params)
This method is used to create a new object of the metaclass's class. Any parameters
you provide are used to initialize the instance's attributes. A special "__INSTANCE__"
key can be passed to provide an already generated instance, rather than having
Class::MOP generate it for you. This is mostly useful for using Class::MOP with
foreign classes which generate instances using their own constructors.
$metaclass->instance_metaclass
Returns the class name of the instance metaclass. See Class::MOP::Instance for more
information on the instance metaclass.
$metaclass->get_meta_instance
Returns an instance of the "instance_metaclass" to be used in the construction of a
new instance of the class.
Informational predicates
These are a few predicate methods for asking information about the class itself.
$metaclass->is_anon_class
This returns true if the class was created by calling
"Class::MOP::Class->create_anon_class".
$metaclass->is_mutable
This returns true if the class is still mutable.
$metaclass->is_immutable
This returns true if the class has been made immutable.
$metaclass->is_pristine
A class is not pristine if it has non-inherited attributes or if it has any generated
methods.
Inheritance Relationships
$metaclass->superclasses(@superclasses)
This is a read-write accessor which represents the superclass relationships of the
metaclass's class.
This is basically sugar around getting and setting @ISA.
$metaclass->class_precedence_list
This returns a list of all of the class's ancestor classes. The classes are returned
in method dispatch order.
$metaclass->linearized_isa
This returns a list based on "class_precedence_list" but with all duplicates removed.
$metaclass->subclasses
This returns a list of all subclasses for this class, even indirect subclasses.
$metaclass->direct_subclasses
This returns a list of immediate subclasses for this class, which does not include
indirect subclasses.
Method introspection
See "Method introspection and creation" in Class::MOP::Package for methods that operate
only on the current class. Class::MOP::Class adds introspection capabilities that take
inheritance into account.
$metaclass->get_all_methods
This will traverse the inheritance hierarchy and return a list of all the
Class::MOP::Method objects for this class and its parents.
$metaclass->find_method_by_name($method_name)
This will return a Class::MOP::Method for the specified $method_name. If the class
does not have the specified method, it returns "undef"
Unlike "get_method", this method will look for the named method in superclasses.
$metaclass->get_all_method_names
This will return a list of method names for all of this class's methods, including
inherited methods.
$metaclass->find_all_methods_by_name($method_name)
This method looks for the named method in the class and all of its parents. It returns
every matching method it finds in the inheritance tree, so it returns a list of
methods.
Each method is returned as a hash reference with three keys. The keys are "name",
"class", and "code". The "code" key has a Class::MOP::Method object as its value.
The list of methods is distinct.
$metaclass->find_next_method_by_name($method_name)
This method returns the first method in any superclass matching the given name. It is
effectively the method that "SUPER::$method_name" would dispatch to.
Attribute introspection and creation
Because Perl 5 does not have a core concept of attributes in classes, we can only return
information about attributes which have been added via this class's methods. We cannot
discover information about attributes which are defined in terms of "regular" Perl 5
methods.
$metaclass->get_attribute($attribute_name)
This will return a Class::MOP::Attribute for the specified $attribute_name. If the
class does not have the specified attribute, it returns "undef".
NOTE that get_attribute does not search superclasses, for that you need to use
"find_attribute_by_name".
$metaclass->has_attribute($attribute_name)
Returns a boolean indicating whether or not the class defines the named attribute. It
does not include attributes inherited from parent classes.
$metaclass->get_attribute_list
This will return a list of attributes names for all attributes defined in this class.
$metaclass->get_all_attributes
This will traverse the inheritance hierarchy and return a list of all the
Class::MOP::Attribute objects for this class and its parents.
$metaclass->find_attribute_by_name($attribute_name)
This will return a Class::MOP::Attribute for the specified $attribute_name. If the
class does not have the specified attribute, it returns "undef".
Unlike "get_attribute", this attribute will look for the named attribute in
superclasses.
$metaclass->add_attribute(...)
This method accepts either an existing Class::MOP::Attribute object or parameters
suitable for passing to that class's "new" method.
The attribute provided will be added to the class.
Any accessor methods defined by the attribute will be added to the class when the
attribute is added.
If an attribute of the same name already exists, the old attribute will be removed
first.
$metaclass->remove_attribute($attribute_name)
This will remove the named attribute from the class, and Class::MOP::Attribute object.
Removing an attribute also removes any accessor methods defined by the attribute.
However, note that removing an attribute will only affect future object instances
created for this class, not existing instances.
$metaclass->attribute_metaclass
Returns the class name of the attribute metaclass for this class. By default, this is
Class::MOP::Attribute.
Class Immutability
Making a class immutable "freezes" the class definition. You can no longer call methods
which alter the class, such as adding or removing methods or attributes.
Making a class immutable lets us optimize the class by inlining some methods, and also
allows us to optimize some methods on the metaclass object itself.
After immutabilization, the metaclass object will cache most informational methods that
returns information about methods or attributes. Methods which would alter the class, such
as "add_attribute" and "add_method", will throw an error on an immutable metaclass object.
The immutabilization system in Moose takes much greater advantage of the inlining features
than Class::MOP itself does.
$metaclass->make_immutable(%options)
This method will create an immutable transformer and use it to make the class and its
metaclass object immutable.
This method accepts the following options:
o inline_accessors
o inline_constructor
o inline_destructor
These are all booleans indicating whether the specified method(s) should be
inlined.
By default, accessors and the constructor are inlined, but not the destructor.
o immutable_trait
The name of a class which will be used as a parent class for the metaclass
object being made immutable. This "trait" implements the post-immutability
functionality of the metaclass (but not the transformation itself).
This defaults to Class::MOP::Class::Immutable::Trait.
o constructor_name
This is the constructor method name. This defaults to "new".
o constructor_class
The name of the method metaclass for constructors. It will be used to generate
the inlined constructor. This defaults to "Class::MOP::Method::Constructor".
o replace_constructor
This is a boolean indicating whether an existing constructor should be
replaced when inlining a constructor. This defaults to false.
o destructor_class
The name of the method metaclass for destructors. It will be used to generate
the inlined destructor. This defaults to "Class::MOP::Method::Denstructor".
o replace_destructor
This is a boolean indicating whether an existing destructor should be replaced
when inlining a destructor. This defaults to false.
$metaclass->immutable_options
Returns a hash of the options used when making the class immutable, including both
defaults and anything supplied by the user in the call to
"$metaclass->make_immutable". This is useful if you need to temporarily make a class
mutable and then restore immutability as it was before.
$metaclass->make_mutable
Calling this method reverse the immutabilization transformation.
Method Modifiers
Method modifiers are hooks which allow a method to be wrapped with before, after and
around method modifiers. Every time a method is called, its modifiers are also called.
A class can modify its own methods, as well as methods defined in parent classes.
How method modifiers work?
Method modifiers work by wrapping the original method and then replacing it in the class's
symbol table. The wrappers will handle calling all the modifiers in the appropriate order
and preserving the calling context for the original method.
The return values of "before" and "after" modifiers are ignored. This is because their
purpose is not to filter the input and output of the primary method (this is done with an
around modifier).
This may seem like an odd restriction to some, but doing this allows for simple code to be
added at the beginning or end of a method call without altering the function of the
wrapped method or placing any extra responsibility on the code of the modifier.
Of course if you have more complex needs, you can use the "around" modifier which allows
you to change both the parameters passed to the wrapped method, as well as its return
value.
Before and around modifiers are called in last-defined-first-called order, while after
modifiers are called in first-defined-first-called order. So the call tree might looks
something like this:
before 2
before 1
around 2
around 1
primary
around 1
around 2
after 1
after 2
What is the performance impact?
Of course there is a performance cost associated with method modifiers, but we have made
every effort to make that cost directly proportional to the number of modifier features
you use.
The wrapping method does its best to only do as much work as it absolutely needs to. In
order to do this we have moved some of the performance costs to set-up time, where they
are easier to amortize.
All this said, our benchmarks have indicated the following:
simple wrapper with no modifiers 100% slower
simple wrapper with simple before modifier 400% slower
simple wrapper with simple after modifier 450% slower
simple wrapper with simple around modifier 500-550% slower
simple wrapper with all 3 modifiers 1100% slower
These numbers may seem daunting, but you must remember, every feature comes with some
cost. To put things in perspective, just doing a simple "AUTOLOAD" which does nothing but
extract the name of the method called and return it costs about 400% over a normal method
call.
$metaclass->add_before_method_modifier($method_name, $code)
This wraps the specified method with the supplied subroutine reference. The modifier
will be called as a method itself, and will receive the same arguments as are passed
to the method.
When the modifier exits, the wrapped method will be called.
The return value of the modifier will be ignored.
$metaclass->add_after_method_modifier($method_name, $code)
This wraps the specified method with the supplied subroutine reference. The modifier
will be called as a method itself, and will receive the same arguments as are passed
to the method.
When the wrapped methods exits, the modifier will be called.
The return value of the modifier will be ignored.
$metaclass->add_around_method_modifier($method_name, $code)
This wraps the specified method with the supplied subroutine reference.
The first argument passed to the modifier will be a subroutine reference to the
wrapped method. The second argument is the object, and after that come any arguments
passed when the method is called.
The around modifier can choose to call the original method, as well as what arguments
to pass if it does so.
The return value of the modifier is what will be seen by the caller.
Introspection
Class::MOP::Class->meta
This will return a Class::MOP::Class instance for this class.
It should also be noted that Class::MOP will actually bootstrap this module by
installing a number of attribute meta-objects into its metaclass.
AUTHORS
Stevan Little <stevan AT iinteractive.com>
COPYRIGHT AND LICENSE
Copyright 2006-2009 by Infinity Interactive, Inc.
<http://www.iinteractive.com>
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
perl v5.10.0 2009-12-18 Class::MOP::Class(3pm)
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