ios7 uigravitybehavior

@interface  ()

@property (nonatomic, strong) UIView *squareView;
//声明动画师
@property (nonatomic, strong) UIDynamicAnimator *animator;
@end

@implementation 

- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    
	//创建方形view，and添加的view，长宽100
    self.squareView = [[UIView alloc] initWithFrame:
                       CGRectMake(0.0f, 0.0f, 100.0f, 100.0f)];
    //添加背景颜色绿色
    self.squareView.backgroundColor = [UIColor greenColor];
    //初始化center
    self.squareView.center = self.view.center;
    //方形添加到view
    [self.view addSubview:self.squareView];
    
    //创建动画师和重力
    self.animator = [[UIDynamicAnimator alloc]
                     initWithReferenceView:self.view];
    //创建重力行为，为方形
    UIGravityBehavior *gravity = [[UIGravityBehavior alloc]
                                  initWithItems:@[self.squareView]];
    //动画师添加行为：gravity
    [self.animator addBehavior:gravity];
    
}

@end