ViewPager.OnPageChangeListener setInternalPageChangeListener(ViewPager.OnPageChangeListener listener) {
        ViewPager.OnPageChangeListener oldListener = mInternalPageChangeListener;
        mInternalPageChangeListener = listener;
        return oldListener;
    }
Set a separate OnPageChangeListener for internal use by the support library.

@param listener Listener to set
@return The old listener that was set, if any.
dkplayer-sample/src/main/java/xyz/doikki/dkplayer/widget/VerticalViewPager.java