ios – 为什么UIKit Dynamics不起作用？

我正在研究iOS开发.当我添加一个rect视图并希望它受重力影响,构建并运行我的测试应用程序时,我发现它确实没有引力.
这是我的viewController.m中的viewDidLoad代码：

[super viewDidLoad];

// Do any additional setup after loading the view,typically from a nib.
UIView *squre = [[UIView alloc] initWithFrame:CGRectMake(100,100,100)];
squre.BACkgroundColor = [UIColor grayColor];
[self.view addSubview:squre];

UIDynamicAnimator *_animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
UIGravityBehavior *_gravity = [[UIGravityBehavior alloc] initWithItems:@[squre]];
[_animator addBehavior:_gravity];@H_616_4@

@property (nonatomiC) UIDynamicAnimator *animator;@H_616_4@ 
 
并设置它：
 
  
 
self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];@H_616_4@