@Override
  public String toString() {
    return Objects.toStringHelper(this)
        .add("controllerAttached", mIsControllerAttached)
        .add("holderAttached", mIsHolderAttached)
        .add("drawableVisible", mIsVisible)
        .add("events", mEventTracker.toString())
        .toString();
  }
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