@Override
        public String toString() {
            return "FragmentPager.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " position=" + position + "}";
        }
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