Class::Observable(3) User Contributed Perl Documentation Class::Observable(3)NAMEClass::Observable - Allow other classes and objects to respond to
events in yours
SYNOPSIS
# Define an observable class
package My::Object;
use base qw( Class::Observable );
# Tell all classes/objects observing this object that a state-change
# has occurred
sub create {
my ( $self ) = @_;
eval { $self->_perform_create() };
if ( $@ ) {
My::Exception->throw( "Error saving: $@" );
}
$self->notify_observers();
}
# Same thing, except make the type of change explicit and pass
# arguments.
sub edit {
my ( $self ) = @_;
my %old_values = $self->extract_values;
eval { $self->_perform_edit() };
if ( $@ ) {
My::Exception->throw( "Error saving: $@" );
}
$self->notify_observers( 'edit', old_values => \%old_values );
}
# Define an observer
package My::Observer;
sub update {
my ( $class, $object, $action ) = @_;
unless ( $action ) {
warn "Cannot operation on [", $object->id, "] without action";
return;
}
$class->_on_save( $object ) if ( $action eq 'save' );
$class->_on_update( $object ) if ( $action eq 'update' );
}
# Register the observer class with all instances of the observable
# class
My::Object->add_observer( 'My::Observer' );
# Register the observer class with a single instance of the
# observable class
my $object = My::Object->new( 'foo' );
$object->add_observer( 'My::Observer' );
# Register an observer object the same way
my $observer = My::Observer->new( 'bar' );
My::Object->add_observer( $observer );
my $object = My::Object->new( 'foo' );
$object->add_observer( $observer );
# Register an observer using a subroutine
sub catch_observation { ... }
My::Object->add_observer( \&catch_observation );
my $object = My::Object->new( 'foo' );
$object->add_observer( \&catch_observation );
# Define the observable class as a parent and allow the observers to
# be used by the child
package My::Parent;
use strict;
use base qw( Class::Observable );
sub prepare_for_bed {
my ( $self ) = @_;
$self->notify_observers( 'prepare_for_bed' );
}
sub brush_teeth {
my ( $self ) = @_;
$self->_brush_teeth( time => 45 );
$self->_floss_teeth( time => 30 );
$self->_gargle( time => 30 );
}
sub wash_face { ... }
package My::Child;
use strict;
use base qw( My::Parent );
sub brush_teeth {
my ( $self ) = @_;
$self->_wet_toothbrush();
}
sub wash_face { return }
# Create a class-based observer
package My::ParentRules;
sub update {
my ( $item, $action ) = @_;
if ( $action eq 'prepare_for_bed' ) {
$item->brush_teeth;
$item->wash_face;
}
}
My::Parent->add_observer( __PACKAGE__ );
$parent->prepare_for_bed # brush, floss, gargle, and wash face
$child->prepare_for_bed # pretend to brush, pretend to wash face
DESCRIPTION
If you have ever used Java, you may have run across the
"java.util.Observable" class and the "java.util.Observer" interface.
With them you can decouple an object from the one or more objects that
wish to be notified whenever particular events occur.
These events occur based on a contract with the observed item. They may
occur at the beginning, in the middle or end of a method. In addition,
the object knows that it is being observed. It just does not know how
many or what types of objects are doing the observing. It can therefore
control when the messages get sent to the obsevers.
The behavior of the observers is up to you. However, be aware that we
do not do any error handling from calls to the observers. If an
observer throws a "die", it will bubble up to the observed item and
require handling there. So be careful.
Throughout this documentation we refer to an 'observed item' or
'observable item'. This ambiguity refers to the fact that both a class
and an object can be observed. The behavior when notifying observers is
identical. The only difference comes in which observers are notified.
(See "Observable Classes and Objects" for more information.)
Observable Classes and Objects
The observable item does not need to implement any extra methods or
variables. Whenever it wants to let observers know about a state-change
or occurrence in the object, it just needs to call
"notify_observers()".
As noted above, whether the observed item is a class or object does not
matter -- the behavior is the same. The difference comes in determining
which observers are to be notified:
· If the observed item is a class, all objects instantiated from that
class will use these observers. In addition, all subclasses and
objects instantiated from the subclasses will use these observers.
· If the observed item is an object, only that particular object will
use its observers. Once it falls out of scope then the observers
will no longer be available. (See "Observable Objects and DESTROY"
below.)
Whichever you chose, your documentation should make clear which type of
observed item observers can expect.
So given the following example:
BEGIN {
package Foo;
use base qw( Class::Observable );
sub new { return bless( {}, $_[0] ) }
sub yodel { $_[0]->notify_observers }
package Baz;
use base qw( Foo );
sub yell { $_[0]->notify_observers }
}
sub observer_a { print "Observation A from [$_[0]]\n" }
sub observer_b { print "Observation B from [$_[0]]\n" }
sub observer_c { print "Observation C from [$_[0]]\n" }
Foo->add_observer( \&observer_a );
Baz->add_observer( \&observer_b );
my $foo = Foo->new;
print "Yodeling...\n";
$foo->yodel;
my $baz_a = Baz->new;
print "Yelling A...\n";
$baz_a->yell;
my $baz_b = Baz->new;
$baz_b->add_observer( \&observer_c );
print "Yelling B...\n";
$baz_b->yell;
You would see something like
Yodeling...
Observation A from [Foo=HASH(0x80f7acc)]
Yelling A...
Observation B from [Baz=HASH(0x815c2b4)]
Observation A from [Baz=HASH(0x815c2b4)]
Yelling B...
Observation C from [Baz=HASH(0x815c344)]
Observation B from [Baz=HASH(0x815c344)]
Observation A from [Baz=HASH(0x815c344)]
And since "Bar" is a child of "Foo" and each has one class-level
observer, running either:
my @observers = Baz->get_observers();
my @observers = $baz_a->get_observers();
would return a two-item list. The first item would be the "observer_b"
code reference, the second the "observer_a" code reference. Running:
my @observers = $baz_b->get_observers();
would return a three-item list, including the observer for that
specific object ("observer_c" coderef) as well as from its class (Baz)
and the parent (Foo) of its class.
Observers
There are three types of observers: classes, objects, and subroutines.
All three respond to events when "notify_observers()" is called from an
observable item. The differences among the three are are:
· A class or object observer must implement a method "update()" which
is called when a state-change occurs. The name of the subroutine
observer is irrelevant.
· A class or object observer must take at least two arguments: itself
and the observed item. The subroutine observer is obligated to take
only one argument, the observed item.
Both types of observers may also take an action name and a hashref
of parameters as optional arguments. Whether these are used depends
on the observed item.
· Object observers can maintain state between responding to
observations.
Examples:
Subroutine observer:
sub respond {
my ( $item, $action, $params ) = @_;
return unless ( $action eq 'update' );
# ...
}
$observable->add_observer( \&respond );
Class observer:
package My::ObserverC;
sub update {
my ( $class, $item, $action, $params ) = @_;
return unless ( $action eq 'update' );
# ...
}
Object observer:
package My::ObserverO;
sub new {
my ( $class, $type ) = @_;
return bless ( { type => $type }, $class );
}
sub update {
my ( $self, $item, $action, $params ) = @_;
return unless ( $action eq $self->{type} );
# ...
}
Observable Objects and DESTROY
Previous versions of this module had a problem with maintaining
references to observable objects/coderefs. As a result they'd never be
destroyed. As of 1.04 we're using weak references with "weaken" in
Scalar::Util so this shouldn't be a problem any longer.
METHODS
Observed Item Methods
notify_observers( [ $action, @params ] )
Called from the observed item, this method sends a message to all
observers that a state-change has occurred. The observed item can
optionally include additional information about the type of change that
has occurred and any additional parameters @params which get passed
along to each observer. The observed item should indicate in its API
what information will be passed along to the observers in $action and
@params.
Returns: Nothing
Example:
sub remove {
my ( $self ) = @_;
eval { $self->_remove_item_from_datastore };
if ( $@ ) {
$self->notify_observers( 'remove-fail', error_message => $@ );
}
else {
$self->notify_observers( 'remove' );
}
}
add_observer( @observers )
Adds the one or more observers (@observer) to the observed item. Each
observer can be a class name, object or subroutine -- see "Types of
Observers".
Returns: The number of observers now observing the item.
Example:
# Add a salary check (as a subroutine observer) for a particular
# person
my $person = Person->fetch( 3843857 );
$person->add_observer( \&salary_check );
# Add a salary check (as a class observer) for all people
Person->add_observer( 'Validate::Salary' );
# Add a salary check (as an object observer) for all people
my $salary_policy = Company::Policy::Salary->new( 'pretax' );
Person->add_observer( $salary_policy );
delete_observer( @observers )
Removes the one or more observers (@observer) from the observed item.
Each observer can be a class name, object or subroutine -- see "Types
of Observers".
Note that this only deletes each observer from the observed item
itself. It does not remove observer from any parent classes. Therefore,
if an observer is not registered directly with the observed item
nothing will be removed.
Returns: The number of observers now observing the item.
Examples:
# Remove a class observer from an object
$person->delete_observer( 'Lech::Ogler' );
# Remove an object observer from a class
Person->delete_observer( $salary_policy );
delete_all_observers()
Removes all observers from the observed item.
Note that this only deletes observers registered directly with the
observed item. It does not clear out observers from any parent classes.
WARNING: This method was renamed from "delete_observers". The
"delete_observers" call still works but is deprecated and will
eventually be removed.
Returns: The number of observers removed.
Example:
Person->delete_all_observers();
get_observers()
Returns all observers for an observed item, as well as the observers
for its class and parents as applicable. See "Observable Classes and
Objects" for more information.
Returns: list of observers.
Example:
my @observers = Person->get_observers;
foreach my $o ( @observers ) {
print "Observer is a: ";
print "Class" unless ( ref $o );
print "Subroutine" if ( ref $o eq 'CODE' );
print "Object" if ( ref $o and ref $o ne 'CODE' );
print "\n";
}
copy_observers( $copy_to_observable )
Copies all observers from one observed item to another. We get all
observers from the source, including the observers of parents. (Behind
the scenes we just use "get_observers()", so read that for what we
copy.)
We make no effort to ensure we don't copy an observer that's already
watching the object we're copying to. If this happens you will appear
to get duplicate observations. (But it shouldn't happen often, if
ever.)
Returns: number of observers copied
Example:
# Copy all observers of the 'Person' class to also observe the
# 'Address' class
Person->copy_observers( Address );
# Copy all observers of a $person to also observe a particular
# $address
$person->copy_observers( $address )
count_observers()
Counts the number of observers for an observed item, including ones
inherited from its class and/or parent classes. See "Observable Classes
and Objects" for more information.
Debugging Methods
Note that the debugging messages will try to get information about the
observed item if called from an object. If you have an "id()" method in
the object its value will be used in the message, otherwise it will be
described as "an instance of class Foo".
SET_DEBUG( $bool )
Turn debugging on or off. If set the built-in implementation of
"observer_log()" will issue a warn at appropriate times during the
process.
observer_log( @message )
Issues a "warn" if "SET_DEBUG" hsa been called with a true value. This
gets called multiple times during the registration and notification
process.
To catch the "warn" calls just override this method.
observer_error( @message )
Issues a "die" if we catch an exception when notifying observers. To
catch the "die" and do something else with it just override this
method.
RESOURCES
APIs for "java.util.Observable" and "java.util.Observer". (Docs below
are included with JDK 1.4 but have been consistent for some time.)
<http://java.sun.com/j2se/1.4/docs/api/java/util/Observable.html>
<http://java.sun.com/j2se/1.4/docs/api/java/util/Observer.html>
"Observer and Observable", Todd Sundsted,
http://www.javaworld.com/javaworld/jw-10-1996/jw-10-howto_p.html
<http://www.javaworld.com/javaworld/jw-10-1996/jw-10-howto_p.html>
"Java Tip 29: How to decouple the Observer/Observable object model",
Albert Lopez, http://www.javaworld.com/javatips/jw-javatip29_p.html
<http://www.javaworld.com/javatips/jw-javatip29_p.html>
SEE ALSO
Class::ISA
Class::Trigger
Aspect
COPYRIGHT
Copyright (c) 2002-2004 Chris Winters. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
AUTHOR
Chris Winters <chris@cwinters.com>
perl v5.14.0 2004-10-16 Class::Observable(3)
