Method subscribe

src/core/context/values.js:187–200

* Adds a subscriber for the specified property. If the property has not * yet been tracked, the tracking is started. If the used value is * already available, the handler is called immediately. The handler is * only called if a valid used value is available and only if this value * has c

(prop, handler)

   * @template T
   */
  subscribe(prop, handler) {
    const used = this.startUsed_(prop);

    if (!pushIfNotExist(used.subscribers, handler)) {
      // Already a subscriber.
      return;
    }

    // The handler is notified right away if the value is available.
    const existingValue = used.value;
    if (isDefined(existingValue) && this.isConnected_()) {
      handler(existingValue);
    }
  }

  /**
   * Unsubscribes a previously added handler. If there are no other subscribers