Method whenWithinViewport

src/service/resource.js:691–713

* @param {number|boolean} viewport derived from renderOutsideViewport. * @return {!Promise} resolves when underlying element is built and within the * viewport range given.

(viewport)

   *    viewport range given.
   */
  whenWithinViewport(viewport) {
    // TODO(#30620): remove this method once IntersectionObserver{root:doc} is
    // polyfilled.
    devAssert(viewport !== false);
    // Resolve is already laid out or viewport is true.
    if (!this.isLayoutPending() || viewport === true) {
      return Promise.resolve();
    }
    // See if pre-existing promise.
    const viewportNum = dev().assertNumber(viewport);
    const key = String(viewportNum);
    if (this.withViewportDeferreds_ && this.withViewportDeferreds_[key]) {
      return this.withViewportDeferreds_[key].promise;
    }
    // See if already within viewport multiplier.
    if (this.isWithinViewportRatio(viewportNum)) {
      return Promise.resolve();
    }
    // return promise that will trigger when within viewport multiple.
    this.withViewportDeferreds_ = this.withViewportDeferreds_ || {};
    this.withViewportDeferreds_[key] = new Deferred();
    return this.withViewportDeferreds_[key].promise;
  }

  /** @private resolves promises populated via whenWithinViewport. */
  resolveDeferredsWhenWithinViewports_() {