NAME
   `Event::Distributor' - a simple in-process pub/sub mechanism

SYNOPSIS
    use Event::Distributor;

    my $dist = Event::Distributor->new;

    $dist->declare_signal( "announce" );

    $dist->subscribe_sync( announce => sub {
       my ( $message ) = @_;
       say $message;
    });

    $dist->subscribe_async( announce => sub {
       my ( $message ) = @_;
       return $async_http->POST( "http://server/message", $message );
    });

    $dist->fire_sync( announce => "Hello, world!" );

DESCRIPTION
   Instances of this class provide a simple publish/subscribe mechanism
   within a single process, for either synchronous or Future-based
   asynchronous use.

   A given instance has a set of named events. Subscribers are `CODE'
   references attached to a named event. Publishers can declare the
   existence of a named event, and then later invoke it by passing in
   arguments, which are distributed to all of the subscribers of that named
   event.

   It is specifically *not* an error to request to subscribe an event that
   has not yet been declared, in order to allow multiple modules of code to
   be loaded and subscribe events the others publish, without introducing
   loading order dependencies. An event only needs to be declared by the
   time it is fired.

   Natively all of the events provided by the distributor are
   fully-asynchronous in nature. Each subscriber is expected to return a
   Future instance which will indicate its completion; the results of these
   are merged into a single future returned by the fire method itself.
   However, to support synchronous or semi-synchronous programs using it,
   both the observe and invoke methods also have a synchronous variant.
   Note however, that this module does not provide any kind of asynchronous
   detachment of synchronous functions; using the subscribe_sync method to
   subscribe a long-running blocking function will cause the `fire_*'
   methods to block until that method returns. To achieve a
   truely-asynchronous experience the attached code will need to use some
   kind of asynchronous event system.

   This module is very-much a work-in-progress, and many ideas may still be
   added or changed about it. It is the start of a concrete implementaion
   of some of the ideas in my "Event-Reflexive Programming" series of blog
   posts. See the TODO and SEE ALSO sections for more detail.

EVENTS
   Each of the events known by a distributor has a name. Conceptually each
   also has a type. Currently there are three types of event, a "signal",
   an "action", and a "query".

   * A signal event simply informs subscribers that some event or condition
     has occurred. Additional arguments can be passed from the invoker to
     the subscribers, but subscriptions are not expected to return a
     meaningful value, nor does firing this event return a value. All
     subscriber functions are invoked sequentually and synchronously by a
     `fire_*' method (though, of course, asynchronous subscribers
     synchronously return a future instance, which allows them to continue
     working asynchronously).

   * An action event requires a single subscriber, and represents a request
     from the invoker to the subscriber to perform some activity. This
     behaves much like a regular (Future-returning) method call, except
     that the indirection mechanism of the distributor allows a more
     flexible method of connection between the two sides.

   * A query event invokes subscriber code expecting a successful return,
     returning the first result that is successful. If a synchronous
     subscriber returns a result, or if an asynchronous one returns a
     successful immediate Future, then no further subscribers are invoked,
     and that result is taken immediately. Any other pending Futures are
     then cancelled.

METHODS
 declare_signal
      $distributor->declare_signal( $name )

   Declares a new "signal" event of the given name.

 declare_action
      $distributor->declare_action( $name )

   *Since version 0.04.*

   Declares a new "action" event of the given name.

 declare_query
      $distributor->declare_query( $name )

   *Since version 0.02.*

   Declares a new "query" event of the given name.

 subscribe_async
      $distributor->subscribe_async( $name, $code )

   Adds a new `CODE' reference to the list of subscribers for the named
   event. This subscriber is expected to return a Future that will
   eventually yield its result.

   When invoked the code will be passed the distributor object itself and
   the list of arguments, and is expected to return a Future.

    $f = $code->( $distributor, @args )

 subscribe_sync
      $distributor->subscribe_sync( $name, $code )

   Adds a new `CODE' reference to the list of subscribers for the named
   event. This subscriber is expected to perform its work synchronously and
   return its result immediately.

   In non-blocking or asynchronous applications, this method should only be
   used for simple subscribers which can immediately return having
   completed their work. If the work is likely to take some time by
   blocking on external factors, consider instead using the subscribe_async
   method.

   When invoked the code will be passed the distributor object itself and
   the list of arguments.

    $code->( $distributor, @args )

 fire_async
      $f = $distributor->fire_async( $name, @args )

   Invokes the named event, passing the arguments to the subscriber
   functions. This function returns as soon as all the subscriber functions
   have been invoked, returning a Future that will eventually complete when
   all the futures returned by the subscriber functions have completed.

 fire_sync
      $distributor->fire_sync( $name, @args )

   Invokes the named event, passing the arguments to the subscriber
   functions. This function synchronously waits until all the subscriber
   futures have completed, and will return once they have all done so.

   Note that since this method calls the `get' method on the Future
   instance returned by fire_async, it is required that this either be an
   immediate, or be some subclass that can actually perform the await
   operation. This should be the case if it is provided by an event
   framework or similar, or custom application logic.

TODO
   Some of these ideas appear in the "Event-Reflexive Progamming" series of
   blog posts, and may be suitable for implementation here. All of these
   ideas are simply for consideration; there is no explicit promise that
   any of these will actually be implemented.

   *   Unsubscription from events.

   *   Define (or document the lack of) ordering between subscriptions of a
       given event.

   *   Refine the failure-handling semantics of signals.

   *   Ability to invoke signals after the current one is finished, by
       deferring the `fire' method. Should this be a new `fire_*' method,
       or a property of the signal itself?

   *   More control over the semantics of value-returning events -
       scatter/map/gather pattern.

   *   Sub-heirarchies of events.

   *   Subclasses for specific event frameworks (IO::Async).

   *   Subclasses (or other behaviours) for out-of-process event
       serialisation and subscribers.

   *   Event parameter filtering mechanics - allows parametric heirarchies,
       instrumentation logging, efficient out-of-process subscribers.

SEE ALSO
   Event-Reflexive Programming

AUTHOR
   Paul Evans <[email protected]>

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