Module provides core (low level) API for working with events in the SDK. This API is mainly for implementing higher level event APIs.

Event listener may be registered on any (event target) object using provided on function:

var { on, once, off, emit } = require('api-utils/event/core');
var target = { name: 'target' };
on(target, 'message', function listener(event) {
  console.log('hello ' + event);
on(target, 'data', console.log);

Event of specific type may be emitted on any event target object using emit function. This will call all registered listeners for the given type on the given event target in the same order they were registered.

emit(target, 'message', 'event');
// info: 'hello event'
emit(target, 'data', { type: 'data' }, 'second arg');
// info: [Object object] 'second arg'

Registered event listeners may be removed using off function:

off(target, 'message');
emit(target, 'message', 'bye');
// info: 'hello bye'

Sometimes listener only cares about first event of specific type. To avoid hassles of removing such listeners there is convenient once function:

once(target, 'load', function() {
emit(target, 'load')
// info: 'ready'
emit(target, 'load')

There are also convenient ways to remove registered listeners. All listeners of the specific type can be easily removed (only two argument must be passed):

off(target, 'message');

Also, removing all registered listeners is possible (only one argument must be passed):