@Override
    public void addView(View child, int index, ViewGroup.LayoutParams params) {
        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params);
        }
        final LayoutParams lp = (LayoutParams) params;
        lp.isDecor |= child instanceof Decor;
        if (mInLayout) {
            if (lp != null && lp.isDecor) {
                throw new IllegalStateException("Cannot add pager decor view during layout");
            }
            lp.needsMeasure = true;
            addViewInLayout(child, index, params);
        } else {
            super.addView(child, index, params);
        }

        if (USE_CACHE) {
            if (child.getVisibility() != GONE) {
                child.setDrawingCacheEnabled(mScrollingCacheEnabled);
            } else {
                child.setDrawingCacheEnabled(false);
            }
        }
    }
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