Method update

src/service/fixed-layer.js:396–556

* Performs fixed/sticky actions. * 1. Updates `top` styling if necessary. * 2. On iOS/Iframe moves elements between fixed layer and BODY depending on * whether they are currently visible and fixed * @return {!Promise}

()

   * @return {!Promise}
   */
  update() {
    // Some of the elements may no longer be in DOM.
    /** @type {!Array<!ElementDef>} */
    const toRemove = this.elements_.filter(
      (fe) => !this.ampdoc.contains(fe.element)
    );
    toRemove.forEach((fe) => this.tearDownElement_(fe.element));

    if (this.elements_.length == 0) {
      return Promise.resolve();
    }

    // Clear out the update pass since we're doing the work now.
    this.updatePass_.cancel();

    // Next, the positioning-related properties will be measured. If a
    // potentially fixed/sticky element turns out to be actually fixed/sticky,
    // it will be decorated and possibly moved to a separate layer.
    let hasTransferables = false;
    return this.vsync_
      .runPromise(
        {
          measure: (state) => {
            const elements = this.elements_;
            const autoTops = [];
            const {win} = this.ampdoc;

            // Notice that this code intentionally breaks vsync contract.
            // Unfortunately, there's no way to reliably test whether or not
            // `top` has been set to a non-auto value on all platforms. To work
            // this around, this code compares `style.top` values with a new
            // `style.bottom` value.
            // 1. Unset top from previous mutates and set bottom to an extremely
            // large value (to catch cases where sticky-tops are in a long way
            // down inside a scroller).
            for (let i = 0; i < elements.length; i++) {
              setImportantStyles(elements[i].element, {
                top: '',
                bottom: '-9999vh',
                transition: 'none',
              });
            }
            // 2. Capture the `style.top` with this new `style.bottom` value. If
            // this element has a non-auto top, this value will remain constant
            // regardless of bottom.
            for (let i = 0; i < elements.length; i++) {
              autoTops.push(computedStyle(win, elements[i].element).top);
            }
            // 3. Cleanup the `style.bottom`.
            for (let i = 0; i < elements.length; i++) {
              setStyle(elements[i].element, 'bottom', '');
            }

            for (let i = 0; i < elements.length; i++) {
              const fe = elements[i];
              const {element, forceTransfer} = fe;
              const style = computedStyle(win, element);