Add once method to Event Dispatcher

src/event-dispatcher.js

@@ -123,6 +123,24 @@ class EventDispatcher {
     event.register(callback);
   }
 
+  /**
+   * Add a function to an event that will be called only once when the label is dispatched.
+   * Uses the `EventDispatcher.on` method with a function wrapped to call off on use.
+   * @param {String} label - The human readable identifier of the event.
+   * @param {Function} callback - Function to be called when the event is fired.
+   * @example
+   * bus.once('one-time-process', callback);
+   * @memberof EventDispatcher
+   */
+  once(label, callback) {
+    debug('once:', label, callback);
+    const cb = (...args) => {
+      this.off(label, cb);
+      callback.apply(this, args);
+    };
+    this.on(label, cb);
+  }
+
   /**
    * Remove a function from an event.
    * @param {String} label - The human readable identifier of the event.

test/event-dispatcher.test.js

@@ -183,6 +183,17 @@ test('#on(label, callback): adds callbacks to the given event', (t) => {
   t.is(Object.keys(ed.events).length, 1);
 });
 
+test('#once(label, callback): adds callbacks to the given event', async (t) => {
+  const ed = new EventDispatcher();
+  t.is(Object.keys(ed.events).length, 0);
+  ed.once('test', a);
+  t.is(Object.keys(ed.events).length, 1);
+  ed.once('test', b);
+  t.is(Object.keys(ed.events).length, 1);
+  await ed.dispatch('test', b);
+  t.is(Object.keys(ed.events).length, 0);
+});
+
 test('#on(label, callback): throws an error with invalid label or callback', (t) => {
   const ed = new EventDispatcher();
   t.throws(() => { ed.on('', a); });