Android 通过触摸动态地在屏幕上画矩形效果

public class GameView extends View {
  //  声明Paint对象
  private Paint mPaint = null;
  private int StrokeWidth = 5;
  private Rect rect = new Rect(0,0,0,0);//手动绘制矩形
  public GameView(Context context){
    super(context);
    //构建对象
    mPaint = new Paint();
    mPaint.setColor(Color.RED);
    //开启线程
    // new Thread(this).start();
  }
  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    //设置无锯齿
    mPaint.setAntiAlias(true);
    canvas.drawARGB(50,255,227,0);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeWidth(StrokeWidth);
    mPaint.setColor(Color.GREEN);
    mPaint.setAlpha(100);
    // 绘制绿色实心矩形
    canvas.drawRect(100, 200, 400, 200 + 400, mPaint);
    mPaint.setColor(Color.RED);
    canvas.drawRect(rect,mPaint);
  }
  @Override
  public boolean onTouchEvent(MotionEvent event) {
    int x = (int)event.getX();
    int y = (int)event.getY();
    switch (event.getAction()){
      case MotionEvent.ACTION_DOWN:
        rect.right+=StrokeWidth;
        rect.bottom+=StrokeWidth;
        invalidate(rect);
        rect.left = x;
        rect.top = y;
        rect.right =rect.left;
        rect.bottom = rect.top;
      case MotionEvent.ACTION_MOVE:
        Rect old =
        new Rect(rect.left,rect.top,rect.right+StrokeWidth,rect.bottom+StrokeWidth);
        rect.right = x;
        rect.bottom = y;
        old.union(x,y);
        invalidate(old);
        break;
      case MotionEvent.ACTION_UP:
        break;
      default:
        break;
    }
    return true;//处理了触摸信息，消息不再传递
  }
}

super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    gameView = new GameView(this);
    addContentView(gameView); 

Rect old = new Rect(rect.left,rect.top,rect.right+StrokeWidth,rect.bottom+StrokeWidth);