iOS 7 动画UIDynamicAnimator

- (void)viewDidLoad
{
    [super viewDidLoad]; 
    [self initUI];
    [self performSelector:@selector(startGravity:) withObject:Nil afterDelay:1];// Do any additional setup after loading the view, typically from a nib.
}

// 开始重力行为
- (IBAction)startGravity:(id)sender
{
　//　   设为全局才会有效，不知什么原因
    animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];//动画方法都根据这个调用
    UIGravityBehavior* gravityBeahvior = [[UIGravityBehavior alloc] initWithItems:@[HUDView]];
    [animator addBehavior:gravityBeahvior];
    UIAttachmentBehavior *attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:HUDView attachedToAnchor:CGPointMake(100, 0)];
        [attachmentBehavior setLength:200];
        [attachmentBehavior setDamping:0.1];
        [attachmentBehavior setFrequency:5];
        [animator  addBehavior:attachmentBehavior];
}
-(void)initUI{
    HUDView=[[UIView alloc]initWithFrame:(CGRectMake(0, 0, 100, 100))];
    HUDView.backgroundColor=[UIColor orangeColor];
    HUDView.backgroundColor=[UIColor redColor];
    HUDView.center = CGPointMake(100, 100);
    HUDView.layer.cornerRadius=50;
    [self.view addSubview:HUDView];  
}