iOS 水波纹（中心向外扩散）

这个比较简单只需重写父类方法-(void)drawRect:(CGRect)rect；在该方法里写动画即可：

-(void)drawRect:(CGRect)rect {

    [super drawRect:rect];

    [[self backgroundColor] setFill]; //

    UIRectFill(rect);

    NSInteger pulsingCount = 3; // 波纹数

    double animationDuration = 6; // 波纹扩散速度

    CALayer * animationLayer = [CALayer layer];

    for (int i = 0; i < pulsingCount; i++) {

        // 创建波纹

        CALayer * pulsingLayer = [CALayer layer];

        pulsingLayer.frame = CGRectMake(0, 0, rect.size.width, rect.size.height);

        pulsingLayer.cornerRadius = rect.size.height / 2.0;

        pulsingLayer.borderColor = [UIColor whiteColor].CGColor;

        pulsingLayer.borderWidth = 1;

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