private void attachController() {
    if (mIsControllerAttached) {
      return;
    }
    mEventTracker.recordEvent(Event.ON_ATTACH_CONTROLLER);
    mIsControllerAttached = true;
    if (mController != null && mController.getHierarchy() != null) {
      mController.onAttach();
    }
  }
Returns whether currently set controller is valid: not null and attached to the hierarchy that
is held by the holder
drawee/src/main/java/com/facebook/drawee/view/DraweeHolder.java