<!--
@license
Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
-->

<link rel="import" href="boot.html">

<script>

(function() {

  'use strict';

  // run a callback when HTMLImports are ready or immediately if
  // this api is not available.
  function whenImportsReady(cb) {
    if (window.HTMLImports) {
      HTMLImports.whenReady(cb);
    } else {
      cb();
    }
  }

  /**
   * Convenience method for importing an HTML document imperatively.
   *
   * This method creates a new `<link rel="import">` element with
   * the provided URL and appends it to the document to start loading.
   * In the `onload` callback, the `import` property of the `link`
   * element will contain the imported document contents.
   *
   * @memberof Polymer
   * @param {string} href URL to document to load.
   * @param {?function(!Event):void=} onload Callback to notify when an import successfully
   *   loaded.
   * @param {?function(!ErrorEvent):void=} onerror Callback to notify when an import
   *   unsuccessfully loaded.
   * @param {boolean=} optAsync True if the import should be loaded `async`.
   *   Defaults to `false`.
   * @return {!HTMLLinkElement} The link element for the URL to be loaded.
   */
  Polymer.importHref = function(href, onload, onerror, optAsync) {
    let link = /** @type {HTMLLinkElement} */
      (document.head.querySelector('link[href="' + href + '"][import-href]'));
    if (!link) {
      link = /** @type {HTMLLinkElement} */ (document.createElement('link'));
      link.rel = 'import';
      link.href = href;
      link.setAttribute('import-href', '');
    }
    // always ensure link has `async` attribute if user specified one,
    // even if it was previously not async. This is considered less confusing.
    if (optAsync) {
      link.setAttribute('async', '');
    }
    // NOTE: the link may now be in 3 states: (1) pending insertion,
    // (2) inflight, (3) already loaded. In each case, we need to add
    // event listeners to process callbacks.
    let cleanup = function() {
      link.removeEventListener('load', loadListener);
      link.removeEventListener('error', errorListener);
    };
    let loadListener = function(event) {
      cleanup();
      // In case of a successful load, cache the load event on the link so
      // that it can be used to short-circuit this method in the future when
      // it is called with the same href param.
      link.__dynamicImportLoaded = true;
      if (onload) {
        whenImportsReady(() => {
          onload(event);
        });
      }
    };
    let errorListener = function(event) {
      cleanup();
      // In case of an error, remove the link from the document so that it
      // will be automatically created again the next time `importHref` is
      // called.
      if (link.parentNode) {
        link.parentNode.removeChild(link);
      }
      if (onerror) {
        whenImportsReady(() => {
          onerror(event);
        });
      }
    };
    link.addEventListener('load', loadListener);
    link.addEventListener('error', errorListener);
    if (link.parentNode == null) {
      document.head.appendChild(link);
    // if the link already loaded, dispatch a fake load event
    // so that listeners are called and get a proper event argument.
    } else if (link.__dynamicImportLoaded) {
      link.dispatchEvent(new Event('load'));
    }
    return link;
  };

})();
</script>