code: @ObjectiveCName("subscribeWithListener:notify:")
      public void subscribe(ValueChangedListener<T> listener, boolean notify) {
          // im.actor.runtime.Runtime.checkMainThread();
  
          if (listeners.contains(listener)) {
              return;
          }
          listeners.add(listener);
          if (notify) {
              listener.onChanged(get(), this);
          }
      }
docs: Subscribe to value updates
  
  @param listener update listener
  @param notify   perform notify about current value