Listener to provide up-to-date Shadow DOM focus events

shadowlisten.js

/**
 * Copyright 2017 Google Inc.
 * 
 * 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
 *
 * https://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.
 */

/**
 * @fileoverview Listener to provide Shadow DOM focus events, even inside shadow roots
 *
 * Normally, listening for focus with Shadow DOM will just return the top-most 'host' with a shadow
 * root. That may not be a problem! But if you want to find the 'real' focused element...
 *
 * To use this library, add this after the script is included-
 *   document.addEventListener('focus', shadowFocusHandler, true);
 *
 * You can then listen for `-shadow-focus` events, which will return the furthest focused element-
 *   document.addEventListener('-shadow-focus', function(ev) {
 *     console.info('got focused element', ev.detail, '..inside outer-most shadow root', ev.target);
 *   });
 */

const shadowFocusHandler = (function() {
  const eventName = '-shadow-focus';
  const dispatch = function(target) {
    const args = {composed: true, bubbles: true, detail: target};
    const customEvent = new CustomEvent(eventName, args);
    target.dispatchEvent(customEvent);
  };

  if (!window.WeakSet || !window.ShadowRoot) {
    return event => dispatch(event.target);
  }

  const focusHandlerSet = new WeakSet();

  /**
   * @param {Node} target to work on
   * @param {function(!FocusEvent)} callback to invoke on focus change
   */
  function _internal(target, callback) {
    let currentFocus = target;  // save real focus

    // #1: get the nearest ShadowRoot
    while (!(target instanceof ShadowRoot)) {
      if (!target) { return; }
      target = target.parentNode;
    }

    // #2: are we already handling it?
    if (focusHandlerSet.has(target)) { return; }
    focusHandlerSet.add(target);

    // #3: setup focus/blur handlers
    const hostEl = target.host;
    const focusinHandler = function(ev) {
      if (ev.target !== currentFocus) {  // prevent dup calls for same focus
        currentFocus = ev.target;
        callback(ev);
      }
    };
    const blurHandler = function(ev) {
      hostEl.removeEventListener('blur', blurHandler, false);
      target.removeEventListener('focusin', focusinHandler, true);
      focusHandlerSet.delete(target);
    };

    // #3: add blur handler to host element
    hostEl.addEventListener('blur', blurHandler, false);

    // #4: add focus handler within shadow root, to observe changes
    target.addEventListener('focusin', focusinHandler, true);

    // #5: find next parent SR, do it again
    _internal(target.host, callback);
  }

  /**
   * @param {!FocusEvent} event to process
   */
  function shadowFocusHandler(event) {
    const target = (event.composedPath ? event.composedPath()[0] : null) || event.target;
    _internal(target, shadowFocusHandler);
    dispatch(target);
  }

  return shadowFocusHandler;
}());