@Override
    public void addView(View child) {
        if (getChildCount() > 0) {
            throw new IllegalStateException("ScrollView can host only one direct child");
        }

        super.addView(child);
    }
@return The maximum amount this scroll view will scroll in response to
  an arrow event.
app/src/main/java/home/smart/fly/animations/widget/NestedScrollView.java