surfaceview画矩形只能画一个，每次画第二个矩形第一个就消失了！求大神赐教

public void draw() { Canvas canvas = getHolder().lockCanvas(); canvas.drawColor(Color.WHITE); canvas.drawRect(startX,startY,stopX,stopY, p); canvas.drawPath(path, p); getHolder().unlockCanvasAndPost(canvas); } @Override public boolean onTouch(View v, MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: startX = event.getX(); startY = event.getY(); draw(); break; case MotionEvent.ACTION_MOVE: stopX = event.getX(); stopY = event.getY(); draw(); break; } return true; }

这是画矩形的方法，早屏幕上触摸画图