Return a promise in "once" if no listener provided

debug/video-export.html

@@ -37,15 +37,15 @@
         easing: t => t
     });
     // wait for animation to finish
-    await untilMapEvent('moveend');
+    await map.once('moveend');
 }
 
 map.on('load', async () => {
     map.addSource('dem', {type: 'raster-dem', url: 'mapbox://mapbox.mapbox-terrain-dem-v1'});
     map.setTerrain({source: 'dem', exaggeration: 1.5});
 
     // wait until the map settles
-    await untilMapEvent('idle');
+    await map.once('idle');
 
     // uncomment to fine-tune animation without recording:
     // animate(); return;
@@ -102,10 +102,6 @@
     // make sure to run `ffmpeg -i mapbox-gl.mp4 mapbox-gl-optimized.mp4` to compress the video
 });
 
-function untilMapEvent(type) {
-    return new Promise(resolve => map.once(type, resolve));
-}
-
 </script>
 </body>
 </html>

src/util/evented.js

@@ -90,10 +90,15 @@ export class Evented {
      * The listener will be called first time the event fires after the listener is registered.
      *
      * @param {string} type The event type to listen for.
-     * @param {Function} listener The function to be called when the event is fired the first time.
-     * @returns {Object} `this`
+     * @param {Function} listener (optional) The function to be called when the event is fired the first time.
+     *   If not provided, returns a Promise that will be resolved at the same time.
+     * @returns {Object} `this` | Promise
      */
-    once(type: *, listener: Listener) {
+    once(type: *, listener?: Listener): this | Promise<Event> {
+        if (!listener) {
+            return new Promise(resolve => this.once(type, resolve));
+        }
+
         this._oneTimeListeners = this._oneTimeListeners || {};
         _addEventListener(type, listener, this._oneTimeListeners);
 

test/unit/util/evented.test.js

@@ -24,6 +24,12 @@ test('Evented', (t) => {
         t.end();
     });
 
+    t.test('"once" returns a promise if no listener provided', (t) => {
+        const evented = new Evented();
+        evented.once('a').then(() => t.end());
+        evented.fire(new Event('a'));
+    });
+
     t.test('passes data to listeners', (t) => {
         const evented = new Evented();
         evented.on('a', (data) => {