Refactor event handler classes to make it easier to add new event type listeners.

chrome/browser/resources/chromeos/chromevox/cvox2/background/automation_object_constructor_installer.js

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @fileoverview Provides bindings to instantiate objects in the automation API.
+ *
+ * Due to restrictions in the extension system, it is not ordinarily possible to
+ * construct an object defined by the extension API. However, given an instance
+ * of that object, we can save its constructor for future use.
+ */
+
+goog.provide('AutomationObjectConstructorInstaller');
+
+/**
+ * Installs the AutomationNode and AutomationEvent classes based on an
+ * AutomationNode instance.
+ * @param {chrome.automation.AutomationNode} node
+ * @param {function()} callback Called when installation finishes.
+ */
+AutomationObjectConstructorInstaller.init = function(node, callback) {
+  chrome.automation.AutomationNode =
+      /** @type {function (new:chrome.automation.AutomationNode)} */(
+          node.constructor);
+  node.addEventListener(chrome.automation.EventType.childrenChanged,
+      function installAutomationEvent(e) {
+        chrome.automation.AutomationEvent =
+            /** @type {function (new:chrome.automation.AutomationEvent)} */(
+                e.constructor);
+        node.removeEventListener(
+            chrome.automation.EventType.childrenChanged,
+            installAutomationEvent,
+            true);
+        callback();
+      },
+      true);
+};