ios动力特效，最重要的一点 属性保持（写了动力特效但是没效果的原因就在这里）

@property (nonatomic, strong) UIDynamicItemBehavior *square1PropertiesBehavior;

@property (nonatomic, strong) UIDynamicItemBehavior *square2PropertiesBehavior;

@property (nonatomic, strong) UIDynamicAnimator *animator;

@property(nonatomic,strong)UIDynamicAnimator*dy;

@end

@implementation ViewController

- (void)viewDidLoad {

[super viewDidLoad];

UIView * apple = [[UIView alloc] initWithFrame:CGRectMake(40,40, 40, 40)];

apple.backgroundColor = [UIColor redColor];

[self.view addSubview:apple];

UIDynamicAnimator *animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

UIGravityBehavior *gravityBeahvior = [[UIGravityBehavior alloc] initWithItems:@[apple]];

UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc] initWithItems:@[apple]];

UIDynamicItemBehavior*behavior = [[UIDynamicItemBehavior alloc] initWithItems:@[apple]];

//behavior.elasticity = ;

collisionBehavior.translatesReferenceBoundsIntoBoundary = YES;

[animator addBehavior:gravityBeahvior];

[animator addBehavior:collisionBehavior];

[animator addBehavior:behavior];

self.dy = animator; //属性保持才能有动画效果

}

- (void)didReceiveMemoryWarning {

[super didReceiveMemoryWarning];

// Dispose of any resources that can be recreated.

}