chromium/chrome/browser/resources/chromeos/accessibility/chromevox/testing/fake_objects.js

// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/**
 * Fakes a Chrome event that supports one listener.
 */
FakeChromeEvent = class {
  constructor() {
    /**
     * @private
     * @type {Function}
     */
    this.listener_ = null;
  }

  /**
   * Fakes the corresponding call on a Chrome event.  Sets the listener and
   * fails the test if it is already set.
   * @param {Function} listener The new listener.
   */
  addListener(listener) {
    this.assertNoListener();
    this.listener_ = listener;
  }

  /**
   * Gets the listener of the event, failing the test if there's none.
   * @return {Function} The event's listener.
   */
  getListener() {
    assertNotEquals(null, this.listener_);
    return this.listener_;
  }

  /**
   * Asserts that this object doesn't have any listener added.
   */
  assertNoListener() {
    assertEquals(null, this.listener_);
  }
};