Method observe

ads/inabox/position-observer.js:47–67

* Start to observe the target element's position change and trigger callback. * TODO: maybe take DOM mutation into consideration * @param {!Element} element * @param {function(!PositionEntryDef)} callback * @return {!UnlistenDef}

(element, callback)

   * @return {!UnlistenDef}
   */
  observe(element, callback) {
    if (!this.positionObservable_) {
      this.positionObservable_ = new Observable();
      const listener = throttle(
        this.win_,
        () => {
          this.update_();
          this.positionObservable_.fire();
        },
        MIN_EVENT_INTERVAL_IN_MS
      );
      this.update_();
      this.win_.addEventListener('scroll', listener, true);
      this.win_.addEventListener('resize', listener, true);
    }
    // Send the 1st ping immediately
    callback(this.getPositionEntry_(element));
    return this.positionObservable_.add(() => {
      callback(this.getPositionEntry_(element));
    });
  }

  /**
   * Updates viewport rect.