Method registerContainer

src/utils/display-observer.js:178–202

* @param {!Element} container * @param {?Element=} opt_root

(container, opt_root)

   * @param {?Element=} opt_root
   */
  registerContainer(container, opt_root) {
    const existing = findObserverByContainer(this.observers_, container);
    if (existing != -1) {
      return;
    }

    /** @type {!ObserverDef} */
    const observer = {
      container,
      root: opt_root || container,
      contains: (target) => containsNotSelf(container, target),
      // Start with null as IntersectionObserver. Will be initialized when
      // the container itself becomes displayed.
      io: null,
    };
    const index = this.observers_.length;
    this.observers_.push(observer);
    this.observe(container, this.containerObserved_);

    this.targetObserverCallbacks_.forEach((_, target) => {
      // Reset observation to `null` and wait for the actual measurement.
      const value = observer.contains(target) ? null : false;
      this.setObservation_(target, index, value, /* callbacks */ null);
    });
  }

  /**
   * @param {!Element} container