iOS 转盘效果

//转盘效果，如果使用autolayout创建的view，会出问题，建议使用纯代码//开始- (void)startRotating{    if (self.link) return;        // 1秒内刷新60次    CADisplayLink *link = [CADisplayLink displayLinkWithTarget:self selector:@selector(update)];    [link addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];    self.link = link;}//停止- (void)stopRotating{    [self.link invalidate];    self.link = nil;}//刷新- (void)update{    self.centerWheel.transform = CGAffineTransformRotate(self.centerWheel.transform, M_PI / 500);}