UIAttachmentBehavior－动画效果：依附，关联

1。关联动画项，使其中一个运动后，相关联的对象也随之一起运动

2。初始化

            - (instancetype)initWithItem:(id<UIDynamicItem>)item attachedToAnchor:(CGPoint)point

     - (instancetype)initWithItem:(id<UIDynamicItem>)item offsetFromCenter:(UIOffset)p1 attachedToAnchor:(CGPoint)point

e.g.

- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    
    [self createGestureRecognizer]; //创建拖动
    [self createSmallSquareView];
    [self createAnchorView];  //创建锚点，基准视图，
    [self createAnimatorAndBehaviors];  //创建关联动画    
}

- (void) createSmallSquareView{
    self.squareView = [[UIView alloc] initWithFrame:  CGRectMake(0.0f, 0.0f, 80.0f, 80.0f)];   
    self.squareView.backgroundColor = [UIColor greenColor];
    self.squareView.center = self.view.center;
    
    self.squareViewAnchorView = [[UIView alloc] initWithFrame: CGRectMake(60.0f, 0.0f, 20.0f, 20.0f)];
    self.squareViewAnchorView.backgroundColor = [UIColor brownColor];
    [self.squareView addSubview:self.squareViewAnchorView];
    
    [self.view addSubview:self.squareView];
}
- (void) createAnchorView{
    
    self.anchorView = [[UIView alloc] initWithFrame: CGRectMake(120.0f, 120.0f, 20.0f, 20.0f)];
    self.anchorView.backgroundColor = [UIColor redColor];
    [self.view addSubview:self.anchorView]; 
}
- (void) createGestureRecognizer{
    UIPanGestureRecognizer *panGestureRecognizer =
    [[UIPanGestureRecognizer alloc] initWithTarget:self
                                            action:@selector(handlePan:)];
    [self.view addGestureRecognizer:panGestureRecognizer];
}

- (void) handlePan:(UIPanGestureRecognizer *)paramPan{   //拖动的点作为锚点
    CGPoint tapPoint = [paramPan locationInView:self.view];
    [self.attachmentBehavior setAnchorPoint:tapPoint];
    self.anchorView.center = tapPoint;   
}
- (void) createAnimatorAndBehaviors{
    
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
    
    /* Create collision detection */
    UICollisionBehavior *collision = [[UICollisionBehavior alloc]
                                      initWithItems:@[self.squareView]];
    collision.translatesReferenceBoundsIntoBoundary = YES;
//    self.attachmentBehavior = [ [UIAttachmentBehavior alloc] initWithItem:self.squareView
//                               attachedToAnchor:self.anchorView.center];
 

//    self.attachmentBehavior = [ [UIAttachmentBehavior alloc] initWithItem:self.squareViewAnchorView
//                               attachedToAnchor:self.anchorView.center];

   

    //squareView视图右上方的点会跟着锚点一起动，带动了squareView整个视图跟着锚点一起动
    UIOffset offset = UIOffsetMake(self.squareViewAnchorView.center.x, self.squareViewAnchorView.center.y);
    self.attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:self.squareView
                                                        offsetFromCenter:offset    //偏移
                                                        attachedToAnchor:self.anchorView.center];

    [self.animator addBehavior:collision];
    [self.animator addBehavior:self.attachmentBehavior];
}