android Canvas清屏只需三句话

public void drawLine(Point p1, Point p2, Point p3)  
    {  
              
        Canvas canvas = sh.lockCanvas();  
        canvas.drawColor(Color.TRANSPARENT);  
        Paint p = new Paint();  
        //清屏  
        p.setXfermode(new PorterDuffXfermode(Mode.CLEAR));  
        canvas.drawPaint(p);  
        p.setXfermode(new PorterDuffXfermode(Mode.SRC));  
          
        p.setAntiAlias(true);  
        p.setColor(Color.RED);  
        p.setStyle(Paint.Style.FILL);  
        canvas.drawLine(0,0, 500, 500, p);  
        sh.unlockCanvasAndPost(canvas);  
          
    }