UIKit Dynamic 动力特效----吸附

本文是吸附特效的一个小Demo:

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UIAttachmentBehavior

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iOS7开始的技术。 提供了一个模拟真实世界中力学相关的动画和交互系统，比如，重力、碰撞、吸附等

效果：

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鼠标拖拽吸附点，释放后，滑块摆动…….

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实现思路：
1.添加拖拽UIPanGestureRecognizer手势

- (IBAction)tap:(UIPanGestureRecognizer *)sender {
    CGPoint location = [sender locationInView:self.view];
    //将手势滑动到的点作为吸附行为的锚点
    self.attachment.anchorPoint = location;
}

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2.重写场景、重力、吸附的getter方法

@property(nonatomic,strong)UIDynamicAnimator *animator;
@property(nonatomic,strong)UIGravityBehavior *gravity;
@property(nonatomic,strong)UIAttachmentBehavior *attachment;

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- (UIDynamicAnimator *)animator{
    if (!_animator) {
        _animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];
    }
    return _animator;
}

- (UIGravityBehavior *)gravity{
    if (!_gravity) {
        _gravity = [[UIGravityBehavior alloc]initWithItems:@[self.imageView]];
    }
    return _gravity;
}
- (UIAttachmentBehavior *)attachment{
    CGPoint attachmentAnchor = CGPointMake(self.imageView.center.x, self.imageView.center.y - 100);
    if (!_attachment) {
        _attachment = [[UIAttachmentBehavior alloc]initWithItem:self.imageView attachedToAnchor:attachmentAnchor];
    }
    return _attachment;
}

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3.利用贝塞尔曲线绘制锚点到中心点的悬挂的线，并注意将特效添加到场景中。

- (void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    [self.animator addBehavior:self.gravity];

    self.attachment.action = ^{
        //绘制锚点到中心点的悬挂的线
        UIBezierPath *path = [UIBezierPath bezierPath];
        [path moveToPoint:self.attachment.anchorPoint];
        [path addLineToPoint:self.imageView.center];
        BackgroundView *bgView = (BackgroundView *)self.view;
        bgView.path = path;
        path.lineWidth = 5;
        [bgView setNeedsDisplay];
    };
    [self.animator addBehavior:self.attachment];
}

谢谢大家！