@Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.position = mCurItem;
        if (mAdapter != null) {
            ss.adapterState = mAdapter.saveState();
        }
        return ss;
    }
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