Method coreServicesAvailable

src/service/performance-impl.js:293–365

* Listens to viewer and resource events. * @return {!Promise}

()

   * @return {!Promise}
   */
  coreServicesAvailable() {
    const {documentElement} = this.win.document;
    this.ampdoc_ = Services.ampdoc(documentElement);
    this.viewer_ = Services.viewerForDoc(documentElement);
    this.resources_ = Services.resourcesForDoc(documentElement);
    this.documentInfo_ = Services.documentInfoForDoc(this.ampdoc_);

    this.isPerformanceTrackingOn_ =
      this.viewer_.isEmbedded() && this.viewer_.getParam('csi') === '1';

    // This is for redundancy. Call flush on any visibility change.
    this.ampdoc_.onVisibilityChanged(this.flush.bind(this));

    // Does not need to wait for messaging ready since it will be queued
    // if it isn't ready.
    this.measureUserPerceivedVisualCompletenessTime_();

    // Can be null which would mean this AMP page is not embedded
    // and has no messaging channel.
    const channelPromise = this.viewer_.whenMessagingReady();

    this.ampdoc_.whenFirstVisible().then(() => {
      this.tick(TickLabel_Enum.ON_FIRST_VISIBLE);
      this.flush();
    });

    const registerVisibilityChangeListener =
      this.supportsLargestContentfulPaint_ || this.supportsLayoutShift_;
    // Register a handler to record metrics when the page enters the hidden
    // lifecycle state.
    if (registerVisibilityChangeListener) {
      this.ampdoc_.onVisibilityChanged(this.onAmpDocVisibilityChange_);
    }

    // We don't check `isPerformanceTrackingOn` here since there are some
    // events that we call on the viewer even though performance tracking
    // is off we only need to know if the AMP page has a messaging
    // channel or not.
    if (!channelPromise) {
      return Promise.resolve();
    }

    return channelPromise
      .then(() => {
        // Tick the "messaging ready" signal.
        this.tickDelta(
          TickLabel_Enum.MESSAGING_READY,
          this.win.performance.now()
        );

        // Tick timeOrigin so that epoch time can be calculated by consumers.
        this.tick(TickLabel_Enum.TIME_ORIGIN, undefined, this.timeOrigin_);

        const usqp = this.ampdoc_.getMetaByName('amp-usqp');
        if (usqp) {
          usqp.split(',').forEach((exp) => {
            this.addEnabledExperiment('ssr-' + exp);
          });