NAME

Mixin::Event::Dispatch - mixin methods for simple event/message dispatch framework

VERSION

version 2.000

SYNOPSIS

 # Add a handler then invoke it
 package Some::Class;
 use parent qw(Mixin::Event::Dispatch);
 sub new { bless {}, shift }

 my $obj = Some::Class->new;

 # Subscribe to events - subscribers will be called with an event object,
 # and any event parameters, each time the event is raised.
 $obj->subscribe_to_event(another_event => (my $code = sub {
   my $ev = shift;
   warn "[] @_";
 }));
 $obj->invoke_event(another_event => 'like this');
 # should get output 'Event data: like this'
 $obj->unsubscribe_from_event(another_event => $code);

 # Note that handlers will be called for each instance of an event until they return false,
 # at which point the handler will be removed, so for a permanent handler, make sure to return 1.
 $obj->add_handler_for_event(some_event => sub { my $self = shift; warn "had some_event: @_"; 1; });
 $obj->invoke_event(some_event => 'message here');

 # Attach event handler for all on_XXX named parameters
 package Event::User;
 sub configure {
        my $self = shift;
        my %args = @_;
        $self->add_handler_for_event(
                map { (/^on_(.*)$/) ? ($1 => $args{$_}) : () } keys %args
        );
        return $self;
 }

DESCRIPTION

Add this in as a parent to your class, and it'll provide some methods for defining event handlers ("subscribe_to_event" or "add_handler_for_event") and calling them ("invoke_event").

Note that handlers should return 0 for a one-off handler, and 1 if it should be called again on the next event.

SPECIAL EVENTS

A single event has been reserved for cases where a callback dies:

• event_error - if a handler is available, this will be called instead of dying whenever any other handler dies. If an event_error handler also fails, then this error will be re-thrown. As with the other handlers, you can have more than one event_error handler.

METHODS

invoke_event

Takes an event parameter, and optional additional parameters that are passed to any callbacks.

 $self->invoke_event('new_message', from => 'fred', subject => 'test message');

Returns $self if a handler was found, undef if not.

subscribe_to_event

Subscribe the given coderef to the named event.

Called with a list of event name and handler pairs. An event name can be any string value. The handler is one of the following:

• a coderef will be used directly as a handler, and will be passed the Mixin::Event::Dispatch::Event object representing this event.

• a plain string will be used as a method name

• a subclass of Mixin::Event::Dispatch will be used to delegate the event - use this if you have an object hierarchy and want the parent object to handle events on the current object

If you have an overloaded object which is both a Mixin::Event::Dispatch subclass and provides a coderef overload, it will default to event delegation behaviour. To ensure the overloaded coderef is used instead, pass \&$obj instead.

All handlers will be given an event (a Mixin::Event::Dispatch::Event object) as the first parameter, and any passed event parameters as the remainder of @_.

Example usage:

 my $parent = $obj->parent;
 $obj->subscribe_to_event(
   connect => sub { warn shift->name }, # warns 'connect'
   connect => $parent, # $parent->invoke_event(connect => @_)
   connect => \&$parent, # $parent's overloaded &{}
   joined  => 'on_joined', # the on_joined method in $obj
 );

Note that multiple handlers can be assigned to the same event name.

unsubscribe_from_event

Removes the given coderef from the list of handlers for this event.

Expects pairs of (event name, coderef) entries for the events to unsubscribe from.

Example usage:

 $obj->subscribe_to_event(
   some_event => (my $code = sub { }),
 );
 $obj->unsubscribe_from_event(
   some_event => $code,
 );

If you need to unsubscribe from the event currently being handled, try the "unsubscribe" in Mixin::Event::Dispatch::Event method.

Returns $self.

add_handler_for_event

Adds handlers to the stack for the given events.

 $self->add_handler_for_event(
   new_message => sub { warn @_; 1 },
   login => sub { warn @_; 1 },
   logout => sub { warn @_; 1 },
 );

event_handlers

Accessor for the event stack itself - should return a hashref which maps event names to arrayrefs for the currently defined handlers.

clear_event_handlers

Removes all queued event handlers.

Will also be called when defining the first handler to create the initial "event_handlers" entry, should be overridden by subclass if something other than $self->{event_handlers} should be used.

API HISTORY

Version 2.000 (will) implement the Mixin::Event::Dispatch::Methods class.

Version 1.000 implemented "subscribe_to_event" and Mixin::Event::Dispatch::Event.

Version 0.002 changed to use "event_handlers" instead of event_stack for storing the available handlers (normally only invoke_event and add_handler_for_event are expected to be called directly).

ROLE vs. MIXIN

Role systems might work using the Mixin::Event::Dispatch::Methods module, which allows import of the relevant methods. Try combing this with a thin wrapper using Role::Tiny / Moo::Role / Moose for that. The t/moo-role.t and t/role-tiny.t tests may provide some inspiration.

Alternatively, you could perhaps use this as a component via Class::C3::Componentised.

(I haven't really used any of the above options myself, please let me know if I'm spreading disinformation here)

SEE ALSO

There are at least a dozen similar modules already on CPAN, here's a small sample:

• Event::Distributor - uses Future to sequence callbacks, implementing the concepts discussed in Event-Reflexive programming

• Object::Event - event callback interface used in several AnyEvent modules.

• Ambrosia::Event - part of the Ambrosia web application framework

• Net::MessageBus - event subscription via TCP-based message bus

• Event::Wrappable - wrapping for event listeners

• MooseX::Event - node.js-inspired events, for Moose users

• Beam::Emitter - a Moo::Role for event handling

Note that some frameworks such as Reflex, POE and Mojolicious already have comprehensive message-passing and callback interfaces.

If you're looking for usage examples, try the following:

• Adapter::Async

• Net::Async::AMQP

• EntityModel - uses this as the underlying event-passing mechanism, with some support in EntityModel::Class for indicating event usage metadata

• Protocol::PostgreSQL - mostly an adapter converting PostgreSQL database messages to/from events using this class

• Protocol::IMAP - the same, but for the IMAPv4bis protocol

• Protocol::XMPP - and again for Jabber/XMPP

AUTHOR

Tom Molesworth <cpan@perlsite.co.uk>

with thanks to various helpful people on freenode #perl who suggested making "event_handlers" into an accessor (to support non-hashref objects) and who patiently tried to explain about roles.

Mixin::Event::Dispatch::Methods suggested by mst, primarily for better integration with object systems such as Moo(se).

LICENSE

Copyright Tom Molesworth 2011-2015, based on code originally part of EntityModel. Licensed under the same terms as Perl itself.

cpanm Mixin::Event::Dispatch

perl -MCPAN -e shell
install Mixin::Event::Dispatch