fire and asyncFire need return values

[esprehn]

You need a way to detect if an event was cancelled just like the platform does. Normally dispatchEvent() has a return value, and the event has a way to check if the default was prevented. Polymer should expose this value:

if (this.fire("some-event")) {
// do something, the default was not prevented.
}

this.asyncFire("some-event").then(function() {
// do something, the default was not prevented.
});

There's no way to fire an event today and let consumers cancel it which is causing issues in some of my apps where I want to do things like trap all key presses, dispatch a custom event to instances like polymer-signals does, and then only allow the native keypress action to happen if the default is not prevented.

[sjmiles]

Please review: http://jsbin.com/wasaguwu/7/edit

[esprehn]

Amazing, what about asyncFire?

[sjmiles]

It would be better if asyncFire returned a Promise, but we are reluctant to include the Promise polyfill unless we really need it. For now, I would suggest just unrolling it like so:

this.async(function() {
  console.log('async foo prevented: ' + this.fire('foo').defaultPrevented);
});

[sjmiles]

Above, I should have said outright that fire returns the event object.