Unified event handling model

[DartBot]

This issue was originally filed by @seaneagan

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Dart needs a unified approach to handling asynchronous events including:

ui events (dart:html)
async operation completion (Future/Completer in dart:core)
Message sending and receiving (dart:isolates)

Strawman proposal at:

https://docs.google.com/document/d/17crOxOmR5AYwultXbNcfdbvD4ACBw5a9QbvO90FVgtk/edit

[iposva-google]

cc @sigmundch.
cc @madsager.
cc @vsmenon.
cc @kasperl.
Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[sigmundch]

Added Isolates label.

[sethladd]

Comments from a recent Code Lab:

"Contrast: WebSocketConnection "ws.onClosed=(int, String){}" syntax is very different from the WebSocket "ws.on.close.add((e){})" syntax. That's weird and surprising."

[nex3]

The events in dart:io should also be unified in this way.

It would be good if whatever event model we ended up using supported multiple listeners per event. See also issue #4202.

[DartBot]

This comment was originally written by kaisellgren@gmail.com

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I just had a need for firing events in my classes to let other code know about "events". I also ended up writing an ObservableList class that utilizes my event library, but I'd really like to see native support for events Dart so that any class can just fire an event, etc. This could be implemented on a very base class or be a mixin called Observable that you just mix into whatever classes you want to.

[DartBot]

This comment was originally written by rtimon...@gmail.com

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http://ricardo.cc/2012/08/09/Its-time-for-a-native-EventEmitter.html

Explains nicely in what kind of mess you can end up with if you don't provide native language event support, or a canonical complete implementation in the base library.

[sigmundch]

Added Library-Isolates label.

[sigmundch]

Removed Isolates label.

[alan-knight]

Changed the title to: "Unified event handling model".

[DartBot]

This comment was originally written by prujo...@gmail.com

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Since events, messages, async operations, network I/O, iterators, etc. can all be considered either terminating or non-terminating sequences of data, I propose that they all be unified under the same Observable model.

Examples:

Observable
  .fromList([1,2,3,4])
  .observe((next) => print('$next'), () => print('finished!'));

Observable
  .fromEvent(element.on.click)
  .observe((next) => print('you clicked me'), (){}, (e) => print('error!'));

// composability
Observable
  .fromEvent(element.on.click)
  .throttle(50) // only pass data if 50ms has passed
  .transform((event) => 'you clicked something')
  .observe((next) => print(next));

// combinators
Observable
  .merge([observer1, observer2, observer3])
  .observe((next) => print('merged data from list of observers: $next'));

Benefits:

• unified
• familiar
• composable
• extensible (combinators, parallel operators later...)

See http://pub.dartlang.org/packages/reactive as a reference project.

[dgrove]

Added Library-Isolate label.

[dgrove]

Removed Library-Isolates label.

[DartBot]

This comment was originally written by jon...@gmail.com

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I agree with this change request. I want my class to be able to alert other classes that some state has changed.

[DartBot]

This comment was originally written by er...@codesmithtools.com

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The new Stream API in Dart is going to be their unified eventing pattern. It still has some rough edges, but it is completely composable and gives all the desired features listed in this issue. I just created a package that makes exposing custom events as streams really easy. I am guessing that the Dart team will have similar functionality built into the SDK soon. Here is my pub package:

http://pub.dartlang.org/packages/event_stream

Here is an example of how to create a custom event:

class ClassWithEvents implements NotifyPropertyChanged {
  String _someProperty;

  final EventStream<PropertyChangedEventArgs> _onPropertyChangedEvent = new EventStream<PropertyChangedEventArgs>();
  Stream<PropertyChangedEventArgs> get onPropertyChanged => _onPropertyChangedEvent.stream;

  final EventStream _onClosedEvent = new EventStream();
  Stream get onClosed => _onClosedEvent.stream;

  String get someProperty => _someProperty;
  set someProperty(String value) {
    _onPropertyChangedEvent.signal(new PropertyChangedEventArgs('someProperty', value));
    _someProperty = value;
  }

  close() {
    _onClosedEvent.signal();
  }
}

main() {
  var c = new ClassWithEvents();
  c.onPropertyChanged.listen((PropertyChangedEventArgs<String> args) => print('changed: name=${args.propertyName} value=${args.value}'));
  c.onClosed.listen((_) => print('closed'));
  c.someProperty = "test";
  c.close();
}

[DartBot]

This comment was originally written by jonapu...@gmail.com

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Just checked out the event_stream package. It definitely helps simplify things.