Question

Using Closure Library, you can give any object the ability to dispatch events extending goog.events.EventTarget. Is this currently possible using Dart libraries?

I imagine it would look like this:

#import('dart:html');

class Foo implements EventTarget {
  Events get on() {
    // ???
  }
}

main() {
  Foo foo = new Foo();

  // Subscribe to the event.
  foo.on['bar'].add((Event event) => print('bar!'));

  // Dispatch the event.
  foo.on['bar'].dispatch(new Event('bar'));
}

Am I on the right track?

Edit Thanks to Lars Tackmann, a working draft is here: http://try.dartlang.org/s/f6wk

Answer 1

You can do this in multiple ways, one method could be to use typedef‘s to define a generic handler function:

class Event {
   final String type;
   Event(this.type);
}

typedef EventHandler(Event event);

interface EventTarget {
  Map<String,EventHandler> get on();
  void dispatch(Event event);
}

class Foo implements EventTarget {
  Map<String, EventHandler> _handlers;
  Foo() {
    _handlers = new Map();
  } 

  Map<String, EventHandler> get on() {
    return _handlers;
  }

  dispatch(Event event) {
     EventHandler handler = _handlers[event.type];
     handler(event);
  }
}

main() {
  Foo foo = new Foo();

  foo.on['bar'] = (Event event) => print('handling event ${event.type}');

  foo.dispatch(new Event('bar'));
}

I made a DartBoard snippet for you here to play around with.

You might want to experiment with making EventTarget into a base class, unless of cause it messes up your inheritance strategy (in which case a event bus you can inject with a factory might be a better fit).