NAME

Beam::Emitter - Role for event emitting classes

VERSION

version 1.002

SYNOPSIS

    package My::Emitter;

    use Moo;
    with 'Beam::Emitter';

    sub do_something {
        my ( $self ) = @_;

        # Give event listeners a chance to prevent something
        my $event = $self->emit( "before_something" );
        return if $event->is_default_stopped;

        # ... do something

        # Notify listeners we're done with something
        $self->emit( 'after_something' );
    }

    sub custom_something {
        my ( $self ) = @_;

        # Send arbitrary arguments to our event listener
        $self->emit_args( 'custom_something', "foo", "bar" );
    }

DESCRIPTION

This role is used by classes that want to emit events to subscribers. A subscriber registers interest in an event using the subscribe or on methods. Then, the class can emit events to be handled by any listening subscribers.

Using the Beam::Event class, subscribers can stop an event from being processed, or prevent the default action from happening.

Using Beam::Event

Beam::Event is an event object with some simple methods to allow subscribers to influence the handling of the event. By calling the stop method, subscribers can stop all futher handling of the event. By calling the the stop_default method, subscribers can allow other subscribers to be notified about the event, but let the emitter know that it shouldn't continue with what it was going to do.

For example, let's build a door that notifies when someone tries to open it. Different instances of a door should allow different checks before the door opens, so we'll emit an event before we decide to open.

    package Door;
    use Moo;
    with 'Beam::Emitter';

    sub open {
        my ( $self, $who ) = @_;
        my $event = $self->emit( 'before_open' );
        return if $event->is_default_stopped;
        $self->open_the_door;
    }

    package main;
    my $door = Door->new;
    $door->open;

Currently, our door will open for anybody. But let's build a door that only open opens after noon (to keep us from having to wake up in the morning).

    use Time::Piece;
    my $restful_door = Door->new;

    $restful_door->on( before_open => sub {
        my ( $event ) = @_;

        my $time = Time::Piece->now;
        if ( $time->hour < 12 ) {
            $event->stop_default;
        }

    } );

    $restful_door->open;

By calling stop_default, we set the is_default_stopped flag, which the door sees and decides not to open.

Using Custom Events

The default Beam::Event is really only useful for notifications. If you want to give your subscribers some data, you need to create a custom event class. This allows you to add attributes and methods to your events (with all the type constraints and coersions you want).

Let's build a door that can keep certain people out. Right now, our door doesn't care who is trying to open it, and our subscribers do not get enough information to deny entry to certain people.

So first we need to build an event object that can let our subscribers know who is knocking on the door.

    package Door::Knock;
    use Moo;
    extends 'Beam::Event';

    has who => (
        is => 'ro',
        required => 1,
    );

Now that we can represent who is knocking, let's notify our subscribers.

    package Door;
    use Moo;
    use Door::Knock; # Our emitter must load the class, Beam::Emitter will not
    with 'Beam::Emitter';

    sub open {
        my ( $self, $who ) = @_;
        my $event = $self->emit( 'before_open', class => 'Door::Knock', who => $who );
        return if $event->is_default_stopped;
        $self->open_the_door;
    }

Finally, let's build a listener that knows who is allowed in the door.

    my $private_door = Door->new;
    $private_door->on( before_open => sub {
        my ( $event ) = @_;

        if ( $event->who ne 'preaction' ) {
            $event->stop_default;
        }

    } );

    $private_door->open;

Without Beam::Event

Although checking is_default_stopped is completely optional, if you do not wish to use the Beam::Event object, you can instead call emit_args instead of emit to give arbitrary arguments to your listeners.

    package Door;
    use Moo;
    with 'Beam::Emitter';

    sub open {
        my ( $self, $who ) = @_;
        $self->emit_args( 'open', $who );
        $self->open_the_door;
    }

There's no way to stop the door being opened, but you can at least notify someone before it does.

METHODS

subscribe ( event_name, subref )

Subscribe to an event from this object. event_name is the name of the event. subref is a subroutine reference that will get either a Beam::Event object (if using the emit method) or something else (if using the emit_args method).

Returns a coderef that, when called, unsubscribes the new subscriber.

    my $unsubscribe = $emitter->subscribe( open_door => sub {
        warn "ding!";
    } );
    $emitter->emit( 'open_door' );  # ding!
    $unsubscribe->();
    $emitter->emit( 'open_door' );  # no ding

This unsubscribe subref makes it easier to stop our subscription in a safe, non-leaking way:

    my $unsub;
    $unsub = $emitter->subscribe( open_door => sub {
        $unsub->(); # Only handle one event
    } );
    $emitter->emit( 'open_door' );

The above code does not leak memory, but the following code does:

    # Create a memory cycle which must be broken manually
    my $cb;
    $cb = sub {
        my ( $event ) = @_;
        $event->emitter->unsubscribe( open_door => $cb ); # Only handle one event
        # Because the callback sub ($cb) closes over a reference to itself
        # ($cb), it can never be cleaned up unless something breaks the
        # cycle explicitly.
    };
    $emitter->subscribe( open_door => $cb );
    $emitter->emit( 'open_door' );

The way to fix this second example is to explicitly undef $cb inside the callback sub. Forgetting to do that will result in a leak. The returned unsubscribe coderef does not have this issue.

on ( event_name, subref )

Alias for "subscribe".

unsubscribe ( event_name [, subref ] )

Unsubscribe from an event. event_name is the name of the event. subref is the single listener subref to be removed. If no subref is given, will remove all listeners for this event.

un ( event_name [, subref ] )

An alias for "unsubscribe"

emit ( name, event_args )

Emit a Beam::Event with the given name. event_args is a list of name => value pairs to give to the Beam::Event constructor.

Use the class key in event_args to specify a different Event class.

emit_args ( name, callback_args )

Emit an event with the given name. callback_args is a list that will be given directly to each subscribed callback.

Use this if you want to avoid using Beam::Event, though you miss out on the control features like stop and stop default.

SEE ALSO

Beam::Event

http://perladvent.org/2013/2013-12-16.html

Coordinating Christmas Dinner with Beam::Emitter by Yanick Champoux.

AUTHOR

Doug Bell <preaction@cpan.org>

CONTRIBUTORS

• Joel Berger <joel.a.berger@gmail.com>

• Yanick Champoux <yanick@babyl.dyndns.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2016 by Doug Bell.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

cpanm Beam::Emitter

perl -MCPAN -e shell
install Beam::Emitter