Method discoverWork_

src/service/resources-impl.js:1048–1259

* Discovers work that needs to be done since the last pass. If viewport * has changed, it will try to build new elements, measure changed elements, * and schedule layouts and preloads within a reasonable distance of the * current viewport. Finally, this process also updates inViewport state

()

   * @private
   */
  discoverWork_() {
    // TODO(dvoytenko): vsync separation may be needed for different phases

    const now = this.win.Date.now();

    // Ensure all resources layout phase complete; when relayoutAll is requested
    // force re-layout.
    const {
      elementsThatScrolled_: elementsThatScrolled,
      relayoutAll_: relayoutAll,
      relayoutTop_: relayoutTop,
    } = this;
    this.relayoutAll_ = false;
    this.relayoutTop_ = -1;

    // Phase 1: Build and relayout as needed. All mutations happen here.
    let relayoutCount = 0;
    let remeasureCount = 0;
    for (let i = 0; i < this.resources_.length; i++) {
      const r = this.resources_[i];
      if (
        r.getState() == ResourceState_Enum.NOT_BUILT &&
        !r.isBuilding() &&
        !r.element.R1()
      ) {
        this.buildOrScheduleBuildForResource_(r, /* checkForDupes */ true);
      }

      if (
        relayoutAll ||
        !r.hasBeenMeasured() ||
        // NOT_LAID_OUT is the state after build() but before measure().
        r.getState() == ResourceState_Enum.NOT_LAID_OUT
      ) {
        relayoutCount++;
      }
      if (r.isMeasureRequested()) {
        remeasureCount++;
      }
    }

    // Phase 2: Remeasure if there were any relayouts. Unfortunately, currently
    // there's no way to optimize this. All reads happen here.
    let toUnload;
    if (
      relayoutCount > 0 ||
      remeasureCount > 0 ||
      relayoutAll ||
      relayoutTop != -1 ||
      elementsThatScrolled.length > 0
    ) {
      for (let i = 0; i < this.resources_.length; i++) {
        const r = this.resources_[i];
        if ((r.hasOwner() && !r.isMeasureRequested()) || r.element.R1()) {
          // If element has owner, and measure is not requested, do nothing.
          continue;
        }
        let needsMeasure =