public boolean verifyDrawable(Drawable who) {
    for (int i = 0; i < mHolders.size(); ++i) {
      if (who == get(i).getTopLevelDrawable()) {
        return true;
      }
    }
    return false;
  }
Returns true if the argument is a top-level Drawable in this holder.
drawee/src/main/java/com/facebook/drawee/view/MultiDraweeHolder.java