code: public boolean addOnScaledListener(OnScaledListener listener) {
          final boolean added = mOnScaledListeners.add(listener);
          if (added) {
              listener.onScaled(mSceneRootObject.getTransform().getScaleX());
          }
          return added;
      }
docs: Register a listener for {@link OnScaledListener#onScaled(float)
  onScaled()} notifications.
  <p>
  Newly registered listeners are immediately called with the current
  scaling.
  
  @param listener An implementation of {@link OnScaledListener}.
  @return {@code True} if the listener is not already added, {@code false}
  if it has already been registered.