private void setScrollState(int newState) {
        if (mScrollState == newState) {
            return;
        }

        mScrollState = newState;
        if (mPageTransformer != null) {
            // PageTransformers can do complex things that benefit from hardware layers.
            enableLayers(newState != SCROLL_STATE_IDLE);
        }
        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrollStateChanged(newState);
        }
    }
Used internally to tag special types of child views that should be added as
pager decorations by default.
dkplayer-sample/src/main/java/xyz/doikki/dkplayer/widget/VerticalViewPager.java