ItemInfo infoForChild(View child) {
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (mAdapter.isViewFromObject(child, ii.object)) {
                return ii;
            }
        }
        return null;
    }
This is the persistent state that is saved by ViewPager.  Only needed
if you are creating a sublass of ViewPager that must save its own
state, in which case it should implement a subclass of this which
contains that state.
dkplayer-sample/src/main/java/xyz/doikki/dkplayer/widget/VerticalViewPager.java