2012-09-22 Android的滚动条实现细节

1. View::Draw
2. View::onDrawScrollBars
scrollBar.setParameters(computeVerticalScrollRange(),
            computeVerticalScrollOffset(),
            computeVerticalScrollExtent(), true);
​
3. View::onDrawVerticalScrollBar
    scrollBar.draw
​
4. ScrollBarDrawable::draw
​
Rect r = getBounds();
​
if (drawTrack) {
    drawTrack(canvas, r, vertical);
}
​
if (drawThumb) {
    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));
​
    // avoid the tiny thumb
    int minLength = thickness * 2;
    if (length < minLength) {
        length = minLength;
    }
    // avoid the too-big thumb
    if (offset + length > size) {
        offset = size - length;
    }
​
    drawThumb(canvas, r, offset, length, vertical);
}