Function getElementServicePromiseOrNull

src/element-service.js:175–210

* Returns the promise for service with `id` on the given window if available. * Otherwise, resolves with null (service was not registered). * @param {!Window} win * @param {string} id * @param {string} extension * @param {string} version * @param {boolean=} opt_element * @return {!Promise<Obj

(
  win,
  id,
  extension,
  version,
  opt_element
)

 * @private
 */
function getElementServicePromiseOrNull(
  win,
  id,
  extension,
  version,
  opt_element
) {
  return dom
    .waitForBodyOpenPromise(win.document)
    .then(() => {
      // If there is an extension script wait for it to load before trying
      // to get the service. Prevents a race condition when everything but
      // the extensions is in cache. If there is no script then it's either
      // not present, or the service was defined by a test. In those cases
      // we don't wait around for an extension that does not exist.
      const extensions = getService(win, 'extensions');

      // TODO(jpettitt) investigate registerExtension to short circuit
      // the dom call in extensionScriptsInNode()
      if (!extensionScriptInNode(extensions.win, extension, version)) {
        return null;
      }
      return extensions.waitForExtension(extension, version);
    })
    .then((ext) => {
      if (!ext) {
        return null;
      }
      // If this service is provided by an element, then we can't depend on
      // the service (they may not use the element).
      if (opt_element) {
        return getServicePromiseOrNull(win, id);
      }
      return getServicePromise(win, id);
    });
}