Ios 碰撞行为不起作用

我在UIKit Dynamics尝试我的手，并尝试创建一个简单的动画，其中一个球从屏幕边界反弹

如下面的代码所示，当我添加重力行为时，球似乎像预期的那样离开屏幕。但是当我添加碰撞行为时，什么也没有发生。球保持在屏幕的中心，我最初在那里画它

我还尝试使用连续推力选项添加推力行为，但球仍然不移动

@implementation TAMViewController

UIDynamicAnimator* animator;
UIGravityBehavior* gravity;
UICollisionBehavior* collision;
UIPushBehavior* push;


- (void)viewDidLoad
{
    [super viewDidLoad];

    TAMBouncyView *ballView = [[TAMBouncyView alloc] initWithFrame:CGRectMake(0,0, 
                                self.view.frame.size.width,  
                                self.view.frame.size.height)];

    [self.view addSubview:ballView];

    animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
    gravity = [[UIGravityBehavior alloc] initWithItems:@[ballView]];
    [animator addBehavior:gravity];

    collision = [[UICollisionBehavior alloc] initWithItems:@[ballView]];
    [collision setTranslatesReferenceBoundsIntoBoundary:YES];
    [animator addBehavior:collision];

    push = [[UIPushBehavior alloc] initWithItems:@[ballView]    
                                     mode:UIPushBehaviorModeContinuous];
    [animator addBehavior:push];

    // Do any additional setup after loading the view, typically from a nib.
}