Class::Container(3) User Contributed Perl Documentation Class::Container(3)NAMEClass::Container - Glues object frameworks together transparently
SYNOPSIS
package Car;
use Class::Container;
@ISA = qw(Class::Container);
__PACKAGE__->valid_params
(
paint => {default => 'burgundy'},
style => {default => 'coupe'},
windshield => {isa => 'Glass'},
radio => {isa => 'Audio::Device'},
);
__PACKAGE__->contained_objects
(
windshield => 'Glass::Shatterproof',
wheel => { class => 'Vehicle::Wheel',
delayed => 1 },
radio => 'Audio::MP3',
);
sub new {
my $package = shift;
# 'windshield' and 'radio' objects are created automatically by
# SUPER::new()
my $self = $package->SUPER::new(@_);
$self->{right_wheel} = $self->create_delayed_object('wheel');
... do any more initialization here ...
return $self;
}
DESCRIPTION
This class facilitates building frameworks of several classes that
inter-operate. It was first designed and built for "HTML::Mason", in
which the Compiler, Lexer, Interpreter, Resolver, Component, Buffer,
and several other objects must create each other transparently, passing
the appropriate parameters to the right class, possibly substituting
other subclasses for any of these objects.
The main features of "Class::Container" are:
· Explicit declaration of containment relationships (aggregation,
factory creation, etc.)
· Declaration of constructor parameters accepted by each member in a
class framework
· Transparent passing of constructor parameters to the class that
needs them
· Ability to create one (automatic) or many (manual) contained
objects automatically and transparently
Scenario
Suppose you've got a class called "Parent", which contains an object of
the class "Child", which in turn contains an object of the class
"GrandChild". Each class creates the object that it contains. Each
class also accepts a set of named parameters in its "new()" method.
Without using "Class::Container", "Parent" will have to know all the
parameters that "Child" takes, and "Child" will have to know all the
parameters that "GrandChild" takes. And some of the parameters
accepted by "Parent" will really control aspects of "Child" or
"GrandChild". Likewise, some of the parameters accepted by "Child"
will really control aspects of "GrandChild". So, what happens when you
decide you want to use a "GrandDaughter" class instead of the generic
"GrandChild"? "Parent" and "Child" must be modified accordingly, so
that any additional parameters taken by "GrandDaughter" can be
accommodated. This is a pain - the kind of pain that object-oriented
programming was supposed to shield us from.
Now, how can "Class::Container" help? Using "Class::Container", each
class ("Parent", "Child", and "GrandChild") will declare what arguments
they take, and declare their relationships to the other classes
("Parent" creates/contains a "Child", and "Child" creates/contains a
"GrandChild"). Then, when you create a "Parent" object, you can pass
"Parent->new()" all the parameters for all three classes, and they will
trickle down to the right places. Furthermore, "Parent" and "Child"
won't have to know anything about the parameters of its contained
objects. And finally, if you replace "GrandChild" with
"GrandDaughter", no changes to "Parent" or "Child" will likely be
necessary.
METHODSnew()
Any class that inherits from "Class::Container" should also inherit its
"new()" method. You can do this simply by omitting it in your class,
or by calling "SUPER::new(@_)" as indicated in the SYNOPSIS. The
"new()" method ensures that the proper parameters and objects are
passed to the proper constructor methods.
At the moment, the only possible constructor method is "new()". If you
need to create other constructor methods, they should call "new()"
internally.
__PACKAGE__->contained_objects()
This class method is used to register what other objects, if any, a
given class creates. It is called with a hash whose keys are the
parameter names that the contained class's constructor accepts, and
whose values are the default class to create an object of.
For example, consider the "HTML::Mason::Compiler" class, which uses the
following code:
__PACKAGE__->contained_objects( lexer => 'HTML::Mason::Lexer' );
This defines the relationship between the "HTML::Mason::Compiler" class
and the class it creates to go in its "lexer" slot. The
"HTML::Mason::Compiler" class "has a" "lexer". The
"HTML::Mason::Compiler->new()" method will accept a "lexer" parameter
and, if no such parameter is given, an object of the
"HTML::Mason::Lexer" class should be constructed.
We implement a bit of magic here, so that if
"HTML::Mason::Compiler->new()" is called with a "lexer_class"
parameter, it will load the indicated class (presumably a subclass of
"HTML::Mason::Lexer"), instantiate a new object of that class, and use
it for the Compiler's "lexer" object. We're also smart enough to
notice if parameters given to "HTML::Mason::Compiler->new()" actually
should go to the "lexer" contained object, and it will make sure that
they get passed along.
Furthermore, an object may be declared as "delayed", which means that
an object won't be created when its containing class is constructed.
Instead, these objects will be created "on demand", potentially more
than once. The constructors will still enjoy the automatic passing of
parameters to the correct class. See the "create_delayed_object()" for
more.
To declare an object as "delayed", call this method like this:
__PACKAGE__->contained_objects( train => { class => 'Big::Train',
delayed => 1 } );
__PACKAGE__->valid_params(...)
Specifies the parameters accepted by this class's "new()" method as a
set of key/value pairs. Any parameters accepted by a
superclass/subclass will also be accepted, as well as any parameters
accepted by contained objects. This method is a get/set accessor
method, so it returns a reference to a hash of these key/value pairs.
As a special case, if you wish to set the valid params to an empty set
and you previously set it to a non-empty set, you may call
"__PACKAGE__->valid_params(undef)".
"valid_params()" is called with a hash that contains parameter names as
its keys and validation specifications as values. This validation
specification is largely the same as that used by the
"Params::Validate" module, because we use "Params::Validate"
internally.
As an example, consider the following situation:
use Class::Container;
use Params::Validate qw(:types);
__PACKAGE__->valid_params
(
allow_globals => { type => ARRAYREF, parse => 'list', default => [] },
default_escape_flags => { type => SCALAR, parse => 'string', default => '' },
lexer => { isa => 'HTML::Mason::Lexer' },
preprocess => { type => CODEREF, parse => 'code', optional => 1 },
postprocess_perl => { type => CODEREF, parse => 'code', optional => 1 },
postprocess_text => { type => CODEREF, parse => 'code', optional => 1 },
);
__PACKAGE__->contained_objects( lexer => 'HTML::Mason::Lexer' );
The "type", "default", and "optional" parameters are part of the
validation specification used by "Params::Validate". The various
constants used, "ARRAYREF", "SCALAR", etc. are all exported by
"Params::Validate". This means that any of these six parameter names,
plus the "lexer_class" parameter (because of the "contained_objects()"
specification given earlier), are valid arguments to the Compiler's
"new()" method.
Note that there are also some "parse" attributes declared. These have
nothing to do with "Class::Container" or "Params::Validate" - any extra
entries like this are simply ignored, so you are free to put extra
information in the specifications as long as it doesn't overlap with
what "Class::Container" or "Params::Validate" are looking for.
$self->create_delayed_object()
If a contained object was declared with "delayed => 1", use this method
to create an instance of the object. Note that this is an object
method, not a class method:
my $foo = $self->create_delayed_object('foo', ...); # YES!
my $foo = __PACKAGE__->create_delayed_object('foo', ...); # NO!
The first argument should be a key passed to the "contained_objects()"
method. Any additional arguments will be passed to the "new()" method
of the object being created, overriding any parameters previously
passed to the container class constructor. (Could I possibly be more
alliterative? Veni, vedi, vici.)
$self->delayed_object_params($name, [params])
Allows you to adjust the parameters that will be used to create any
delayed objects in the future. The first argument specifies the "name"
of the object, and any additional arguments are key-value pairs that
will become parameters to the delayed object.
When called with only a $name argument and no list of parameters to
set, returns a hash reference containing the parameters that will be
passed when creating objects of this type.
$self->delayed_object_class($name)
Returns the class that will be used when creating delayed objects of
the given name. Use this sparingly - in most situations you shouldn't
care what the class is.
__PACKAGE__->decorates()
Version 0.09 of Class::Container added [as yet experimental] support
for so-called "decorator" relationships, using the term as defined in
Design Patterns by Gamma, et al. (the Gang of Four book). To declare a
class as a decorator of another class, simply set @ISA to the class
which will be decorated, and call the decorator class's "decorates()"
method.
Internally, this will ensure that objects are instantiated as
decorators. This means that you can mix & match extra add-on
functionality classes much more easily.
In the current implementation, if only a single decoration is used on
an object, it will be instantiated as a simple subclass, thus avoiding
a layer of indirection.
$self->validation_spec()
Returns a hash reference suitable for passing to the "Params::Validate"
"validate" function. Does not include any arguments that can be passed
to contained objects.
$class->allowed_params(\%args)
Returns a hash reference of every parameter this class will accept,
including parameters it will pass on to its own contained objects. The
keys are the parameter names, and the values are their corresponding
specifications from their "valid_params()" definitions. If a parameter
is used by both the current object and one of its contained objects,
the specification returned will be from the container class, not the
contained.
Because the parameters accepted by "new()" can vary based on the
parameters passed to "new()", you can pass any parameters to the
"allowed_params()" method too, ensuring that the hash you get back is
accurate.
$self->container()
Returns the object that created you. This is remembered by storing a
reference to that object, so we use the "Scalar::Utils" "weakref()"
function to avoid persistent circular references that would cause
memory leaks. If you don't have "Scalar::Utils" installed, we don't
make these references in the first place, and calling "container()"
will result in a fatal error.
If you weren't created by another object via "Class::Container",
"container()" returns "undef".
In most cases you shouldn't care what object created you, so use this
method sparingly.
$object->show_containers
$package->show_containers
This method returns a string meant to describe the containment
relationships among classes. You should not depend on the specific
formatting of the string, because I may change things in a future
release to make it prettier.
For example, the HTML::Mason code returns the following when you do
"$interp->show_containers":
HTML::Mason::Interp=HASH(0x238944)
resolver -> HTML::Mason::Resolver::File
compiler -> HTML::Mason::Compiler::ToObject
lexer -> HTML::Mason::Lexer
request -> HTML::Mason::Request (delayed)
buffer -> HTML::Mason::Buffer (delayed)
Currently, containment is shown by indentation, so the Interp object
contains a resolver and a compiler, and a delayed request (or several
delayed requests). The compiler contains a lexer, and each request
contains a delayed buffer (or several delayed buffers).
$object->dump_parameters
Returns a hash reference containing a set of parameters that should be
sufficient to re-create the given object using its class's "new()"
method. This is done by fetching the current value for each declared
parameter (i.e. looking in $object for hash entries of the same name),
then recursing through all contained objects and doing the same.
A few words of caution here. First, the dumped parameters represent
the current state of the object, not the state when it was originally
created.
Second, a class's declared parameters may not correspond exactly to its
data members, so it might not be possible to recover the former from
the latter. If it's possible but requires some manual fudging, you can
override this method in your class, something like so:
sub dump_parameters {
my $self = shift;
my $dump = $self->SUPER::dump_parameters();
# Perform fudgery
$dump->{incoming} = $self->{_private};
delete $dump->{superfluous};
return $dump;
}
SEE ALSO
Params::Validate
AUTHOR
Originally by Ken Williams <ken@mathforum.org> and Dave Rolsky
<autarch@urth.org> for the HTML::Mason project. Important feedback
contributed by Jonathan Swartz <swartz@pobox.com>. Extended by Ken
Williams for the AI::Categorizer project.
Currently maintained by Ken Williams.
COPYRIGHT
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.18.2 2014-05-13 Class::Container(3)
