用CADisplayLink做雪花飘飞的效果

//方法每秒钟调用60次
    /*
     CADisplayLink用来重绘，绘图
     NSTimer用于计时，重复调用
     
     */
    self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(handleAction:)];
    //
//    self.displayLink.frameInterval = 0.5;

    [self.displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

- (void)handleAction:(CADisplayLink *)displayLink{
    
    UIImage *image = [UIImage imageNamed:@"雪花"];
    UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
    CGFloat scale = arc4random_uniform(60) / 100.0;
    imageView.transform = CGAffineTransformMakeScale(scale, scale);
    CGSize winSize = self.view.bounds.size;
    CGFloat x = arc4random_uniform(winSize.width);
    CGFloat y = - imageView.frame.size.height;
    imageView.center = CGPointMake(x, y);
    
    [self.view addSubview:imageView];
    [UIView animateWithDuration:arc4random_uniform(10) animations:^{
        CGFloat toX = arc4random_uniform(winSize.width);
        CGFloat toY = imageView.frame.size.height * 0.5 + winSize.height;
        
        imageView.center = CGPointMake(toX, toY);

        /**雪花旋转*/

        imageView.transform = CGAffineTransformRotate(imageView.transform, arc4random_uniform(M_PI * 2));
        
        imageView.alpha = 0.5;
    } completion:^(BOOL finished) {

        /**动画结束见图片移除*/

        [imageView removeFromSuperview];
    }];
    
//    NSLog(@"%ld",self.displayLink.frameInterval);
//    NSLog(@"%lf",self.displayLink.duration);
//    
}