2. View::onDrawScrollBars
scrollBar.setParameters(computeVerticalScrollRange(),
computeVerticalScrollOffset(),
computeVerticalScrollExtent(), true);
3. View::onDrawVerticalScrollBar
4. ScrollBarDrawable::draw
drawTrack(canvas, r, vertical);
int size = vertical ? r.height() : r.width();
int thickness = vertical ? r.width() : r.height();
int length = Math.round((float) size * extent / range);
int offset = Math.round((float) (size - length) * mOffset / (range - extent));
int minLength = thickness * 2;
if (length < minLength) {
// avoid the too-big thumb
if (offset + length > size) {
drawThumb(canvas, r, offset, length, vertical);