Class::Meta::AccessorBuilder::Affordance - Affordance style accessor generation
package MyApp::TypeDef;
use strict;
use Class::Meta::Type;
use IO::Socket;
my $type = Class::Meta::Type->add(
key => 'io_socket',
builder => 'affordance',
desc => 'IO::Socket object',
name => 'IO::Socket Object'
);
This module provides the an affordance style accessor builder for Class::Meta.
Affordance accessors are attribute accessor methods that separate the getting
and setting of an attribute value into distinct methods. The approach both
eliminates the overhead of checking to see whether an accessor is called as a
getter or a setter, which is common for Perl style accessors, while also
creating a psychological barrier to accidentally misusing an attribute.
Class::Meta::AccessorBuilder::Affordance create two different types of
accessors: getters and setters. The type of accessors created depends on the
value of the "authz" attribute of the Class::Meta::Attribute for
which the accessor is being created.
For example, if the "authz" is Class::Meta::RDWR, then two accessor
methods will be created:
my $value = $obj->get_io_socket;
$obj->set_io_socket($value);
If the value of "authz" is Class::Meta::READ, then only the get method
will be created:
my $value = $obj->io_socket;
And finally, if the value of "authz" is Class::Meta::WRITE, then only
the set method will be created (why anyone would want this is beyond me, but I
provide for the sake of completeness):
my $value = $obj->io_socket;
Class::Meta::AccessorBuilder::Affordance uses all of the validation checks
passed to it to validate new values before assigning them to an attribute. It
also checks to see if the attribute is required, and if so, adds a check to
ensure that its value is never undefined. It does not currently check to
ensure that private and protected methods are used only in their appropriate
contexts, but may do so in a future release.
If the "context" attribute of the attribute object for which accessors
are to be built is "Class::Meta::CLASS",
Class::Meta::AccessorBuilder will build accessors for a class attribute
instead of an object attribute. Of course, this means that if you change the
value of the class attribute in any context--whether via a an object, the
class name, or an an inherited class name or object, the value will be changed
everywhere.
For example, for a class attribute "count", you can expect the
following to work:
MyApp::Custom->set_count(10);
my $count = MyApp::Custom->get_count; # Returns 10.
my $obj = MyApp::Custom->new;
$count = $obj->get_count; # Returns 10.
$obj->set_count(22);
$count = $obj->get_count; # Returns 22.
my $count = MyApp::Custom->get_count; # Returns 22.
MyApp::Custom->set_count(35);
$count = $obj->get_count; # Returns 35.
my $count = MyApp::Custom->get_count; # Returns 35.
Currently, class attribute accessors are not designed to be inheritable in the
way designed by Class::Data::Inheritable, although this might be changed in a
future release. For now, I expect that the current simple approach will cover
the vast majority of circumstances.
Note: Class attribute accessors will not work accurately in multiprocess
environments such as mod_perl. If you change a class attribute's value in one
process, it will not be changed in any of the others. Furthermore, class
attributes are not currently shared across threads. So if you're using
Class::Meta class attributes in a multi-threaded environment (such as iThreads
in Perl 5.8.0 and later) the changes to a class attribute in one thread will
not be reflected in other threads.
Any attributes that have their "view" attribute set to
Class::Meta::Private or Class::Meta::Protected get additional validation
installed to ensure that they're truly private and protected. This includes
when they are set via parameters to constructors generated by Class::Meta. The
validation is performed by checking the caller of the accessors, and throwing
an exception when the caller isn't the class that owns the attribute (for
private attributes) or when it doesn't inherit from the class that owns the
attribute (for protected attributes).
As an implementation note, this validation is performed for parameters passed to
constructors created by Class::Meta by ignoring looking for the first caller
that isn't Class::Meta::Constructor:
my $caller = caller;
# Circumvent generated constructors.
for (my $i = 1; $caller eq 'Class::Meta::Constructor'; $i++) {
$caller = caller($i);
}
This works because Class::Meta::Constructor installs the closures that become
constructors, and thus, when those closures call accessors to set new values
for attributes, the caller is Class::Meta::Constructor. By going up the stack
until we find another package, we correctly check to see what context is
setting attribute values via a constructor, rather than the constructor method
itself being the context.
This is a bit of a hack, but since Perl uses call stacks for checking security
in this way, it's the best I could come up with. Other suggestions welcome. Or
see Class::Meta::Type to create your own accessor generation code
The following functions must be implemented by any Class::Meta accessor
generation module.
build_attr_get
my $code = Class::Meta::AccessorBuilder::Affordance::build_attr_get();
This function is called by "Class::Meta::Type::make_attr_get()" and
returns a code reference that can be used by the "get()" method of
Class::Meta::Attribute to return the value stored for that attribute for the
object passed to the code reference.
build_attr_set
my $code = Class::Meta::AccessorBuilder::Affordance::build_attr_set();
This function is called by "Class::Meta::Type::make_attr_set()" and
returns a code reference that can be used by the "set()" method of
Class::Meta::Attribute to set the value stored for that attribute for the
object passed to the code reference.
build
Class::Meta::AccessorBuilder::Affordance::build(
$pkg, $attribute, $create, @checks
);
This method is called by the "build()" method of Class::Meta::Type,
and does the work of actually generating the accessors for an attribute
object. The arguments passed to it are:
- $pkg
- The name of the class to which the accessors will be
added.
- $attribute
- The Class::Meta::Attribute object that specifies the
attribute for which the accessors will be created.
- $create
- The value of the "create" attribute of the
Class::Meta::Attribute object, which determines what accessors, if any,
are to be created.
- @checks
- A list of code references that validate the value of an
attribute. These will be used in the set accessor (mutator) to validate
new attribute values.
This module is stored in an open GitHub repository
<
http://github.com/theory/class-meta/>. Feel free to fork and
contribute!
Please file bug reports via GitHub Issues
<
http://github.com/theory/class-meta/issues/> or by sending mail to
[email protected] <mailto:
[email protected]>.
David E. Wheeler <
[email protected]>
- Class::Meta
- This class contains most of the documentation you need to
get started with Class::Meta.
- Class::Meta::AccessorBuilder
- This module generates Perl style accessors.
- Class::Meta::Type
- This class manages the creation of data types.
- Class::Meta::Attribute
- This class manages Class::Meta class attributes, most of
which will have generated accessors.
Copyright (c) 2002-2011, David E. Wheeler. Some Rights Reserved.
This module is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.