android Canvas 向左上、右上、左下 方向绘制矩形不能跟随手指移动问题

是因为朝左上、右上、左下 三个方向，相对于起始点坐标系有所变化，所以在Draw方法中需按情况讨论：

if ((startPoint.getX() <= endPoint.getX() && startPoint.getY() <= endPoint.getY())) {    rect = new RectF(startPoint.getX(), startPoint.getY(), endPoint.getX(), endPoint.getY());} else if ((startPoint.getX() <= endPoint.getX() && startPoint.getY() >= endPoint.getY())) {    rect = new RectF(startPoint.getX(), endPoint.getY(), endPoint.getX(), startPoint.getY());} else if ((startPoint.getX() > endPoint.getX() && startPoint.getY() > endPoint.getY())) {    rect = new RectF(endPoint.getX(), endPoint.getY(), startPoint.getX(), startPoint.getY());} else if ((startPoint.getX() > endPoint.getX() && startPoint.getY() < endPoint.getY())) {    rect = new RectF(endPoint.getX(), startPoint.getY(), startPoint.getX(), endPoint.getY());}