Method tryUpgrade_

src/custom-element.js:1303–1345

* Try to upgrade the element with the provided implementation. * @return {!Promise|undefined} * @private @final

()

     * @private @final
     */
    tryUpgrade_() {
      if (this.isInTemplate_) {
        return;
      }
      if (this.upgradeState_ != UpgradeState_Enum.NOT_UPGRADED) {
        // Already upgraded or in progress or failed.
        return;
      }

      const Ctor = devAssert(
        this.implClass_,
        'Implementation must not be a stub'
      );

      const impl = new Ctor(this);

      // The `upgradeCallback` only allows redirect once for the top-level
      // non-stub class. We may allow nested upgrades later, but they will
      // certainly be bad for performance.
      this.upgradeState_ = UpgradeState_Enum.UPGRADE_IN_PROGRESS;
      const startTime = win.Date.now();
      const res = impl.upgradeCallback();
      if (!res) {
        // Nothing returned: the current object is the upgraded version.
        this.completeUpgrade_(impl, startTime);
      } else if (typeof res.then == 'function') {
        // It's a promise: wait until it's done.
        return res
          .then((upgrade) => {
            this.completeUpgrade_(upgrade || impl, startTime);
          })
          .catch((reason) => {
            this.upgradeState_ = UpgradeState_Enum.UPGRADE_FAILED;
            rethrowAsync(reason);
          });
      } else {
        // It's an actual instance: upgrade immediately.
        this.completeUpgrade_(
          /** @type {!./base-element.BaseElement} */ (res),
          startTime
        );
      }
    }

    /**
     * Called when the element is disconnected from the DOM.