@Override
    protected float getTopFadingEdgeStrength() {
        if (getChildCount() == 0) {
            return 0.0f;
        }

        final int length = getVerticalFadingEdgeLength();
        final int scrollY = getScrollY();
        if (scrollY < length) {
            return scrollY / (float) length;
        }

        return 1.0f;
    }
Sentinel value for no current active pointer.
Used by {@link #mActivePointerId}.
app/src/main/java/home/smart/fly/animations/widget/NestedScrollView.java