pkg/custom_element/lib/polyfill.dart

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/** Dart APIs for interacting with the JavaScript Custom Elements polyfill. */
library custom_element.polyfill;

import 'dart:async';
import 'dart:html';
import 'dart:js' as js;

/**
 * A future that completes once all custom elements in the initial HTML page
 * have been upgraded.
 *
 * This is needed because the native implementation can update the elements
 * while parsing the HTML document, but the custom element polyfill cannot,
 * so it completes this future once all elements are upgraded.
 */
Future customElementsReady = () {
  if (_isReady) return new Future.value();

  // Not upgraded. Wait for the polyfill to fire the WebComponentsReady event.
  // Note: we listen on document (not on document.body) to allow this polyfill
  // to be loaded in the HEAD element.
  return document.on['WebComponentsReady'].first;
}();

// Return true if we are using the polyfill and upgrade is complete, or if we
// have native document.register and therefore the browser took care of it.
// Otherwise return false, including the case where we can't find the polyfill.
bool get _isReady {
  // If we don't have dart:js, assume things are ready
  if (js.context == null) return true;

  var customElements = js.context['CustomElements'];
  if (customElements == null) {
    // Return true if native document.register, otherwise false.
    // (Maybe the polyfill isn't loaded yet. Wait for it.)
    return document.supportsRegister;
  }

  return customElements['ready'] == true;
}

/**
 * Loads `custom-elements.debug.js` or `custom-elements.min.js` by adding the
 * script tag to the page. Returns a future that completes when custom elements
 * are ready (equivalent to [customElementsReady]).
 *
 * Normally you should add this to your HTML file
 * (the Polymer package will do this automatically), but loading dynamically
 * can be useful for scenarios such as tests.
 */
Future loadCustomElementPolyfill() {
  if (!document.supportsRegister && !js.context.hasProperty('CustomElements')) {
    if (!MutationObserver.supported) {
      var script = new ScriptElement()
          ..src = '/packages/mutation_observer/mutation_observer.js';
      document.head.append(script);
    }

    var script = new ScriptElement()
        ..src = '/packages/custom_element/custom-elements.debug.js';
    document.head.append(script);
    return document.on['WebComponentsReady'].first;
  }
  return new Future.value();
}