UIDynamic是苹果IOS7才开始使用的一种技术，是一种物理引擎，实现例如重力，碰撞等现象。

Dynamic初试：实现简单的重力，弹性碰撞效果
UIDynamicAnimator：物理引擎，可以向里面添加重力，弹性碰撞效果
UIGravityBehavior：重力行为
UICollisionBehavior: 弹性碰撞行为
UIDynamicItemBehavior :自定义属性（弹性，密度，摩擦力，旋转，阻力）

简单代码实现

@implementation ViewController
{
  UIDynamicAnimator *_animator;
  UIGravityBehavior *_gravity;
    UICollisionBehavior *_collision;
    UIDynamicItemBehavior *_itemDynamic;
}

- (void)viewDidLoad {

    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    UIView * square = [[UIView alloc] initWithFrame:CGRectMake(180, 0, 150, 150)];
    square.backgroundColor = [UIColor redColor];
    [self.view addSubview:square];
    UIView * line = [[UIView alloc] initWithFrame:CGRectMake(0, 400, 200, 30)];
    line.backgroundColor = [UIColor blueColor];
    [self.view addSubview:line];
    _animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
    //创建重力行为
    _gravity = [[UIGravityBehavior alloc] initWithItems:@[square]];
    //创建弹性碰撞
    _collision = [[UICollisionBehavior alloc] initWithItems:@[square]];
    //默认边界是否被使用
    _collision.translatesReferenceBoundsIntoBoundary = YES;

 CGPoint rightPoint = CGPointMake(line.frame.origin.x + line.frame.size.width, line.frame.origin.y);
    NSLog(@"x = %f y = %f", rightPoint.x, rightPoint.y);
    //添加弹性碰撞范围
    [_collision addBoundaryWithIdentifier:@"line" fromPoint:line.frame.origin toPoint:rightPoint];
    _itemDynamic = [[UIDynamicItemBehavior alloc] initWithItems:@[square]];
 _itemDynamic.elasticity = 0.9; //设置弹性
   _itemDynamic.density = 0.5;//设置密度
  _itemDynamic.friction = 0;//设置摩擦力
   _itemDynamic.resistance = 0.9;//设置阻力(相对于滑行来说)
    _itemDynamic.allowsRotation = YES;//是否允许旋转
   _itemDynamic.angularResistance = 0.9;
    [_animator addBehavior:_gravity];
    [_animator addBehavior:_collision];

}