Function EventWatcher

test/fixtures/wpt/resources/testharness.js:956–1071

* Allow DOM events to be handled using Promises. * * This can make it a lot easier to test a very specific series of events, * including ensuring that unexpected events are not fired at any point. * * `EventWatcher` will assert if an event occurs while there is no `wait_for`

(test, watchedNode, eventTypes, timeoutPromise)

     *
     */
    function EventWatcher(test, watchedNode, eventTypes, timeoutPromise)
    {
        if (typeof eventTypes === 'string') {
            eventTypes = [eventTypes];
        }

        var waitingFor = null;

        // This is null unless we are recording all events, in which case it
        // will be an Array object.
        var recordedEvents = null;

        var eventHandler = test.step_func(function(evt) {
            assert_true(!!waitingFor,
                        'Not expecting event, but got ' + evt.type + ' event');
            assert_equals(evt.type, waitingFor.types[0],
                          'Expected ' + waitingFor.types[0] + ' event, but got ' +
                          evt.type + ' event instead');

            if (Array.isArray(recordedEvents)) {
                recordedEvents.push(evt);
            }

            if (waitingFor.types.length > 1) {
                // Pop first event from array
                waitingFor.types.shift();
                return;
            }
            // We need to null out waitingFor before calling the resolve function
            // since the Promise's resolve handlers may call wait_for() which will
            // need to set waitingFor.
            var resolveFunc = waitingFor.resolve;
            waitingFor = null;
            // Likewise, we should reset the state of recordedEvents.
            var result = recordedEvents || evt;
            recordedEvents = null;
            resolveFunc(result);
        });

        for (var i = 0; i < eventTypes.length; i++) {
            watchedNode.addEventListener(eventTypes[i], eventHandler, false);
        }

        /**
         * Returns a Promise that will resolve after the specified event or
         * series of events has occurred.
         *
         * @param {Object} options An optional options object. If the 'record' property
         *                 on this object has the value 'all', when the Promise
         *                 returned by this function is resolved,  *all* Event
         *                 objects that were waited for will be returned as an
         *                 array.
         *
         * @example
         * const watcher = new EventWatcher(t, div, [ 'animationstart',
         *                                            'animationiteration',
         *                                            'animationend' ]);
         * return watcher.wait_for([ 'animationstart', 'animationend' ],