Iphone 弹跳球程序希望球重新出现在顶部并再次落下

现在在我的程序中，球在Y轴上上上下反弹。我想要的是球只能向下移动。当它到达屏幕底部时，我希望它在顶部重新出现，然后再次向下开始。不断地从上到下移动

@implementation BounceViewController
@synthesize ball;

-(void) bounce{

    ball.center = CGPointMake(ball.center.x+position.x, ball.center.y+position.y); {

        if(ball.center.y>450 ||ball.center.y <0)
            position.y = -position.y; }
}

- (void)viewDidLoad {
    position = CGPointMake(0, 10);

    [NSTimer scheduledTimerWithTimeInterval:.05 target:self selector:@selector(bounce) userInfo:nil repeats:YES];
    [super viewDidLoad];
}
- (void)viewDidUnload
{
        [self setBall:nil];
        [super viewDidUnload];
}


@end

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在反弹方法中，替换线

if(ball.center.y>450 ||ball.center.y <0)
            position.y = -position.y;

与

if(ball.center.y>450 ||ball.center.y <0)
      ball.center = CGPointMake(ball.center.x+position.x, position.y);