public void onAttach() {
    if (mIsAttached) {
      return;
    }
    mIsAttached = true;
    for (int i = 0; i < mHolders.size(); ++i) {
      mHolders.get(i).onAttach();
    }
  }
Gets the controller ready to display the images.

<p>The containing view must call this method from both {@link View#onFinishTemporaryDetach()}
and {@link View#onAttachedToWindow()}.
drawee/src/main/java/com/facebook/drawee/view/MultiDraweeHolder.java