1） 小球在窗口内的移动不能垂直向下，可以以某个角度斜向下运动。小球最初在窗口顶部出现的位置可以固定，以可以随机在某个位置出现

（2） 运动方向可以通过每次在x方向和y方向上增量不同来得到。

能检测窗口边界，碰到边界后会发生碰撞并且沿合理的方向运动。

在每次绘制小球时候，判断绘制该小球的位置是否已经到了窗口的上下左右边界。如果到了，就应该通过改变x和y方向的增量。来改变方向。 -1) The ball is not moving in the window vertically downward at an angle oblique to the next campaign. The ball first to appear in the top of the window can be fixed to a location can appear random (2) the direction of movement through time in the x direction and y direction to get the incremental difference. Can detect the window border, the border will encounter a collision, and along the rational direction. When drawing the ball every time, draw the ball to determine whether the location of the window and down to the left and right borders. If it arrives, it should be by changing the x and y directions of the increments. To change direction.