qq的app中要是有新的联系人发消息过来，相应联系人的cell右边会有一个红色的圆圈表示消息条数。如果去触碰那个圆圈，可以发现它竟然会跟着手指的移动而移动。

在一定范围内，手指离开屏幕，会发现红色圆圈会自动弹性的回到原来的位置。而如果超出一定距离，这个圆圈会做一个销毁的动画，从而从view上移除掉。 

产品要求公司的App也要有效果，并花了些时间去学习它的实现过程，发现其实原理还是比较简单的。

（由于mac制作gif图片实在过于麻烦，所以效果只能是看看图片。）

Demo的github地址：https://github.com/wzpziyi1/QQ-Goo

这是实现过程中的一些效果图片：

经过分析，可以发现，是两个圆和一个不规则矩形位置、大小的变化。一开始，小的圆圈和大的圆圈的center是相同的，当移动大圆的时候，小圆的半径随着大圆离小圆的距离变远而变小，当大圆距离小圆一定距离时，将小圆隐藏掉，中间的不规则矩形remove掉。

那么，不规则矩形怎么表示呢？可以利用Core Graphics在drawRect方法里面绘制不规则矩形的path，然后利用颜色fill就行。不规则矩形是随着大圆的移动而不断变化的，如果在drawRect方法里面绘制，那么在移动过程中不断调用setNeedsDisplay方法进行重绘。这是种可行的方案，我所用的也大致是这种思路。

不过，我没有在drawRect方法里面绘制，而是利用了CAShapeLayer，将不规则矩形的path绘制在shapeLayer里面，这样在移动大圆的过程中不断更新CAShapeLayer的path即可。

当然，难点并在在这里。而是不规则矩形的各个点的位置。要绘制这个不规则矩形，需要知道六个点的位置：

有了这些点的坐标，那么就可以用UIBezierPath来绘制相应的路径，代码如下：

- (UIBezierPath *)pathWithBigCircleView:(UIView *)bigCircleView smallCircleView:(UIView *)smallCircleView
{
    
    CGPoint bigCenter = bigCircleView.center;
    CGFloat x2 = bigCenter.x;
    CGFloat y2 = bigCenter.y;
    CGFloat r2 = bigCircleView.bounds.size.width / 2;
    
    CGPoint smallCenter = smallCircleView.center;
    CGFloat x1 = smallCenter.x;
    CGFloat y1 = smallCenter.y;
    CGFloat r1 = smallCircleView.bounds.size.width / 2;
    
    
    
    // 获取圆心距离
    CGFloat d = [self distanceWithPointA:bigCenter pointB:smallCenter];
    
    //Θ:(xita)
    CGFloat sinθ = (x2 - x1) / d;
    
    CGFloat cosθ = (y2 - y1) / d;
    
    // 坐标系基于父控件
    CGPoint pointA = CGPointMake(x1 - r1 * cosθ , y1 + r1 * sinθ);
    CGPoint pointB = CGPointMake(x1 + r1 * cosθ , y1 - r1 * sinθ);
    CGPoint pointC = CGPointMake(x2 + r2 * cosθ , y2 - r2 * sinθ);
    CGPoint pointD = CGPointMake(x2 - r2 * cosθ , y2 + r2 * sinθ);
    CGPoint pointO = CGPointMake(pointA.x + d / 2 * sinθ , pointA.y + d / 2 * cosθ);
    CGPoint pointP =  CGPointMake(pointB.x + d / 2 * sinθ , pointB.y + d / 2 * cosθ);
    
    UIBezierPath *path = [UIBezierPath bezierPath];
    
    // A
    [path moveToPoint:pointA];
    
    // AB
    [path addLineToPoint:pointB];
    
    // 绘制BC曲线
    [path addQuadCurveToPoint:pointC controlPoint:pointP];
    
    
    // CD
    [path addLineToPoint:pointD];
    
    // 绘制DA曲线
    [path addQuadCurveToPoint:pointA controlPoint:pointO];
    
    return path;
}

在实现过程中，我是自定义UIButton的，需要注意的是，在监听button的拖动时，最好是给它添加UIPanGestureRecognizer手势，而不要在touchesBegin方法里面去判断它的移动位置，因为Touches系列方法会屏蔽button的点击。

自定义的这个button默认就是大圆，包含一个小圆(UIView)属性，但是这个小圆并不是添加在自定义的这个button（也就是大圆）里面，而是在button的superView上。因为小圆并不需要随着大圆位置的改变而改变位置，相应的，shapeLayer也是添加在button（大圆）的父控件上。

给大圆添加了pan手势，在pan:方法里面随之改变小圆的大小和绘制shapeLayer的path。

当pan手势状态为End的时候，需要判断大圆与小圆的距离有没有超出最大距离，如果超过，那么添加一个gif图片，播放销毁大圆的过程。如果没有被销毁，那么大圆需要复位，相应代码： 

#import "ZYGooView.h"

#define kMaxDistance 100

@interface ZYGooView ()
@property (nonatomic, weak) UIView *smallCircleView;

@property (nonatomic, assign) CGFloat smallCircleR;

@property (nonatomic, weak) CAShapeLayer *shapeLayer;
@end

@implementation ZYGooView

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        [self commitInit];
        
    }
    return self;
}

- (void)awakeFromNib
{
    [self commitInit];
}

- (void)commitInit
{
    self.layer.cornerRadius = self.frame.size.width * 0.5;
    self.layer.masksToBounds = YES;
    
    self.smallCircleR = self.frame.size.width * 0.5;
    self.smallCircleView.bounds = self.bounds;
    self.smallCircleView.center = self.center;
    self.smallCircleView.layer.cornerRadius = self.smallCircleView.frame.size.width * 0.5;
    
    [self addGesture];
}

#pragma mark ----懒加载方法

- (UIView *)smallCircleView
{
    if (_smallCircleView == nil) {
        UIView *view = [[UIView alloc] init];
        
        view.backgroundColor = self.backgroundColor;
        
        [self.superview addSubview:view];
        
        [self.superview insertSubview:view atIndex:0];
        
        _smallCircleView = view;
        
    }
    return _smallCircleView;
}

- (CAShapeLayer *)shapeLayer
{
    if (_shapeLayer == nil) {
        CAShapeLayer *shapeLayer = [CAShapeLayer layer];
        shapeLayer.path = [self pathWithBigCircleView:self smallCircleView:self.smallCircleView].CGPath;
        shapeLayer.fillColor = self.backgroundColor.CGColor;
        
        [self.superview.layer addSublayer:shapeLayer];
        
        [self.superview.layer insertSublayer:shapeLayer atIndex:0];
        
        _shapeLayer = shapeLayer;
    }
    return _shapeLayer;
}

#pragma mark ----其他方法

- (void)addGesture
{
    UIPanGestureRecognizer *recognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(pan:)];
    [self addGestureRecognizer:recognizer];
}



- (void)pan:(UIPanGestureRecognizer *)recognizer
{
    CGPoint point = [recognizer translationInView:self.superview];
    
    CGPoint center = self.center;
    center.x += point.x;
    center.y += point.y;
    self.center = center;
    //复位
    [recognizer setTranslation:CGPointZero inView:self];
    
    CGFloat distance = [self distanceWithPointA:self.smallCircleView.center pointB:self.center];
    
    if (distance == 0) return;
    
    CGFloat newR = self.smallCircleR - distance / 15.0;
    NSLog(@"%f", newR);
    self.smallCircleView.bounds = CGRectMake(0, 0, newR * 2, newR * 2);
    self.smallCircleView.layer.cornerRadius = newR;
    
    if (distance > kMaxDistance || newR <= 0) {
        self.smallCircleView.hidden = YES;
        [self.shapeLayer removeFromSuperlayer];
        self.shapeLayer = nil;
    }
    
    if (distance <= kMaxDistance && self.smallCircleView.hidden == NO) {
        self.shapeLayer.path = [self pathWithBigCircleView:self smallCircleView:self.smallCircleView].CGPath;
    }
    
    if (recognizer.state == UIGestureRecognizerStateEnded) {
        if (distance <= kMaxDistance) {
            
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.03 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                [self.shapeLayer removeFromSuperlayer];
                self.shapeLayer = nil;
            });
            
            [UIView animateWithDuration:0.4 delay:0 usingSpringWithDamping:0.6 initialSpringVelocity:0 options:UIViewAnimationOptionCurveLinear animations:^{
                self.center = self.smallCircleView.center;
                
            } completion:^(BOOL finished) {
                self.smallCircleView.hidden = NO;
            }];
        }
        else {
            UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.bounds];
            [self addSubview:imageView];
            
            NSMutableArray *images = [NSMutableArray array];
            
            for (int i = 1; i <= 8; i++) {
                NSString *imageName = [NSString stringWithFormat:@"%d", i];
                UIImage *image = [UIImage imageNamed:imageName];
                [images addObject:image];
            }
            
            imageView.animationImages = images;
            imageView.animationDuration = 0.6;
            imageView.animationRepeatCount = 1;
            [imageView startAnimating];
            
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                [self removeFromSuperview];
            });
        }
    }
}

- (CGFloat)distanceWithPointA:(CGPoint)pointA pointB:(CGPoint)pointB
{
    CGFloat dx = pointB.x - pointA.x;
    CGFloat dy = pointB.y - pointA.y;
    
    return sqrt(dx * dx + dy * dy);
}

- (UIBezierPath *)pathWithBigCircleView:(UIView *)bigCircleView smallCircleView:(UIView *)smallCircleView
{
    
    CGPoint bigCenter = bigCircleView.center;
    CGFloat x2 = bigCenter.x;
    CGFloat y2 = bigCenter.y;
    CGFloat r2 = bigCircleView.bounds.size.width / 2;
    
    CGPoint smallCenter = smallCircleView.center;
    CGFloat x1 = smallCenter.x;
    CGFloat y1 = smallCenter.y;
    CGFloat r1 = smallCircleView.bounds.size.width / 2;
    
    
    
    // 获取圆心距离
    CGFloat d = [self distanceWithPointA:bigCenter pointB:smallCenter];
    
    //Θ:(xita)
    CGFloat sinθ = (x2 - x1) / d;
    
    CGFloat cosθ = (y2 - y1) / d;
    
    // 坐标系基于父控件
    CGPoint pointA = CGPointMake(x1 - r1 * cosθ , y1 + r1 * sinθ);
    CGPoint pointB = CGPointMake(x1 + r1 * cosθ , y1 - r1 * sinθ);
    CGPoint pointC = CGPointMake(x2 + r2 * cosθ , y2 - r2 * sinθ);
    CGPoint pointD = CGPointMake(x2 - r2 * cosθ , y2 + r2 * sinθ);
    CGPoint pointO = CGPointMake(pointA.x + d / 2 * sinθ , pointA.y + d / 2 * cosθ);
    CGPoint pointP =  CGPointMake(pointB.x + d / 2 * sinθ , pointB.y + d / 2 * cosθ);
    
    UIBezierPath *path = [UIBezierPath bezierPath];
    
    // A
    [path moveToPoint:pointA];
    
    // AB
    [path addLineToPoint:pointB];
    
    // 绘制BC曲线
    [path addQuadCurveToPoint:pointC controlPoint:pointP];
    
    
    // CD
    [path addLineToPoint:pointD];
    
    // 绘制DA曲线
    [path addQuadCurveToPoint:pointA controlPoint:pointO];
    
    return path;
}

@end