Summary:
While adeveloper requests the emitter to emit an event, the emitter
may not emit the event immediately instead of putting the request
into a queue and process it later.
This diff allows the developer to provide a callback which will be called
when the event has been emitted.
For instance:
```
class NavigationContext {
push(nextRoute) {
var nextStack = this._stack.push(nextRoute);
this.emit(
'change',
{
reason: 'push',
nextStack: nextStack,
nextRoute: nextRoute,
},
this._onPush
);
}
_onPush(event){
if (event.defaultPrevented) {
return;
}
this._stack = event.nextStack;
this.emit('change');
}
}
```
of the navigator component.
Summary:
Per offline discussion with @evv, we'd like to deprecate the `onDidFocus` and `onWillFocus`
API that makes it really hard for the descendent children of a navigator to observe its focus
change events.
@public
Since for now the descendent children do have access to the navigator via `this.props.navigator`,
this diff makes it easy to observe the focus change event by doing:
```
this.props.navigator.addListener('willfocus', this._onFocus);
```
The goal is to make the event system in navigator more useful and maintainable.
Test Plan:
Test Video: https://www.facebook.com/pxlcld/mrzS
1. jest: ./Libraries/FBReactKit/js/runTests.js NavigationEventEmitter
2. Load UI Explorer: <Navigator />, see console logs that shows the focus change events fires.
Martin Konicek
Hedger Wang