event-helper-listen.js

/**
 * Copyright 2017 The AMP HTML Authors. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS-IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
  * Whether addEventListener supports options or only takes capture as a boolean
  * @type {boolean|undefined}
  * @visibleForTesting
  */
let optsSupported;

/**
 * Listens for the specified event on the element.
 *
 * Do not use this directly. This method is implemented as a shared
 * dependency. Use `listen()` in either `event-helper` or `3p-frame-messaging`,
 * depending on your use case.
 *
 * @param {!EventTarget} element
 * @param {string} eventType
 * @param {function(!Event)} listener
 * @param {Object=} opt_evtListenerOpts
 * @return {!UnlistenDef}
 */
export function internalListenImplementation(element, eventType, listener,
  opt_evtListenerOpts) {
  let localElement = element;
  let localListener = listener;
  /**
   * @type {?Function}
   */
  let wrapped;

  wrapped = event => {
    try {
      return localListener(event);
    } catch (e) {
      // reportError is installed globally per window in the entry point.
      self.reportError(e);
      throw e;
    }
  };
  const optsSupported = detectEvtListenerOptsSupport();
  let capture = false;
  if (opt_evtListenerOpts) {
    capture = opt_evtListenerOpts.capture;
  }
  localElement.addEventListener(
      eventType,
      wrapped,
      optsSupported ? opt_evtListenerOpts : capture
  );
  return () => {
    if (localElement) {
      localElement.removeEventListener(
          eventType,
          wrapped,
          optsSupported ? opt_evtListenerOpts : capture
      );
    }
    // Ensure these are GC'd
    localListener = null;
    localElement = null;
    wrapped = null;
  };
}

/**
 * Tests whether the browser supports options as an argument of addEventListener
 * or not.
 *
 * @return {boolean}
 */
export function detectEvtListenerOptsSupport() {
  // Only run the test once
  if (optsSupported !== undefined) {
    return optsSupported;
  }

  optsSupported = false;
  try {
    // Test whether browser supports EventListenerOptions or not
    const options = {
      get capture() {
        optsSupported = true;
      },
    };
    self.addEventListener('test-options', null, options);
    self.removeEventListener('test-options', null, options);
  } catch (err) {
    // EventListenerOptions are not supported
  }
  return optsSupported;
}

/**
  * Resets the test for whether addEventListener supports options or not.
  */
export function resetEvtListenerOptsSupportForTesting() {
  optsSupported = undefined;
}