-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event{

UIGravityBehavior *gravity = [[UIGravityBehavior alloc]initWithItems:@[self.redView]];

gravity.gravityDirection = CGVectorMake(1.0, 1.0);

---

UICollisionBehavior *collision = [[UICollisionBehavior alloc]initWithItems:@[self.redView,self.greenView ]];

collision.translatesReferenceBoundsIntoBoundary = YES;

// 给碰撞添加边界

CGPoint startPoint = CGPointMake(0, self.view.frame.size.height-100);

CGPoint endPoint = CGPointMake(self.view.frame.size.width, self.view.frame.size.height-100);

//    [collision addBoundaryWithIdentifier:@"123" fromPoint:startPoint toPoint:endPoint];

UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(100, 200, 200 , 300)];

[collision addBoundaryWithIdentifier:@"321" forPath:path];

// 移除边界用的标示 @“123”

//  将仿真行为添加到仿真器中

[self.animator addBehavior:gravity];

[self.animator addBehavior:collision];

}

-(UIDynamicAnimator*)animator{

if (_animator == nil) {

_animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];

}

return  _animator;

}

1.2 捕捉行为

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event{

// 移除里面的行为

     [self.animator removeAllBehaviors];

CGPoint point = [[ touches anyObject] locationInView:self.view];

//    创建捕捉行为

UISnapBehavior *snap = [[UISnapBehavior alloc]initWithItem:self.redView snapToPoint:point];

// 设置阻力系数(0-1.0)

snap.damping = 0;

//添加行为

[self.animator addBehavior:snap];

}