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Signals is a new approach for AS3 events, inspired by [http://en.wikipedia.org/wiki/C_Sharp_syntax#Events C# events] and [http://en.wikipedia.org/wiki/Signals_and_slots signals/slots] in Qt.

It’s not finished and I am looking for your impressions, critiques and suggestions.
[http://robertpenner.com/flashblog/2009/09/my-new-as3-event-system-signals.html Please post feedback on my blog].

• [http://code.google.com/p/as3-signals/source/browse/#svn/trunk/src/com/robertpenner/signals Browse source code]
• [http://code.google.com/p/as3-signals/source/browse/#svn/trunk/tests/com/robertpenner/signals Browse unit tests]
• [http://code.google.com/p/as3-signals/source/checkout Checkout from SVN]

== Concept ==

• A Signal is essentially a mini-dispatcher specific to one event, with its own array of listeners.
• A Signal gives an event a concrete membership in a class.
• Listeners subscribe to real objects, not to string-based channels.
• Event string constants are no longer needed.

== Syntax ==

• comparison:
  {{{
  // with EventDispatcher
  button.addEventListener(MouseEvent.CLICK, onClick);

// Signal equivalent
button.click.add(onClick);
}}}

• Create a Signal for a class:
  {{{
  ////// in Item class:
  // Recommended: make the signal read-only.
  protected var _ready:Signal;
  public function get ready():Signal { return _ready; }

public function Item()
{
_ready = new Signal(this, ReadyEvent);
}

protected function sendReady():void
{
_ready.dispatch(new ReadyEvent());
}

////// in another class:
var item:Item = new Item();
protected function onItemReady(e:ReadyEvent):void { … }

item.ready.add(onItemReady);
}}}

== Features ==

• Remove all event listeners:
  {{{
  signal.removeAll();
  }}}

• Retrieve the number of listeners:
  {{{
  var numListeners:uint = signal.length;
  }}}

• Listeners can be added for a one-time call and removed automatically on dispatch:
  {{{
  signal.addOnce(theListener); // result: signal has one listener
  signal.dispatch(theEvent); // result: theListener is called, signal now has no listeners
  }}}

• Any object type can be dispatched to listeners. But using IEvent will enable full functionality.
• A Signal can be initialized with an event class that will validate event objects on dispatch (optional):
  {{{
  progress = new Signal(this, ProgressEvent);
  //later:
  progress.dispatch(new NotProgressEvent()); // will throw ArgumentError
  }}}

• If the Signal’s event class is specified, each listener is checked on `add()` to ensure it declares at least one argument.
• Signals can be placed in interfaces to indicate the events dispatched by a class.
• Events can bubble recursively through `.parent` independent of the display list (experimental).
• Code was developed test-first.

== Background on AS3 Events ==

• [http://robertpenner.com/flashblog/2009/08/my-critique-of-as3-events-part-1.html My Critique of AS3 Events – Part 1]
• [http://robertpenner.com/flashblog/2009/09/as3-events-7-things-ive-learned-from.html AS3 Events – 7 things I’ve learned from community]
• [http://robertpenner.com/flashblog/2009/09/my-critique-of-as3-events-part-2.html My Critique of AS3 Events – Part 2]