方法一：利用NSMutableArray來實現

NSMutable *array;全局變量，在.m文件中用定時器調用

-(void)down{
    static int jianGe = 0;
    jianGe++;
    if (jianGe > 2) {
        jianGe = 0;
        [self snowState];
    }
    [self snowDown];
}

-(void)snowState{
    for (int i  = 0; i < [array count]; i++) {
        MyImageView *img = [array objectAtIndex:i];
        if (img.tag == 0) {
            img.tag = 1;
            [self.view addSubview:img];
            break;//提取雪花單片
        }
        
    }
}

-(void)snowDown{
    for (int i = 0; i < [array count]; i++) {
        MyImageView *img = [array objectAtIndex:i];
        if (img.tag == 1) {
            img.center = CGPointMake(img.center.x, img.center.y+10);
            if (img.center.y > 480+25) {
                img.tag = 0;
                img.center = CGPointMake(arc4random()%370 - 50, -50);
            }
        }
    }
}

方法二：利用幀動畫

用定時器調用

//ViewController.m
-(void)snowDown{//動畫效果
    UIImageView *iv = [[UIImageView alloc]init];
    int a = arc4random()%51;//雪花大小隨機（0~50）
    iv.frame = CGRectMake(arc4random()%370 - 50, -50, a, a);
    iv.image = [UIImage imageNamed:@"flake"];
    [self.view addSubview:iv];
    
    [UIView beginAnimations:nil context:(__bridge void *)(iv)];
    [UIView setAnimationDuration:5];
    [UIView setAnimationDelegate:self];
    [UIView setAnimationDidStopSelector:@selector(animationDidStop:finished:context:)];
    iv.frame = CGRectMake(arc4random()%370 - 50, 440, a, a);
    [UIView commitAnimations];
}

-(void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context//到底部消除
{
    UIImageView *image = (__bridge UIImageView *)context;
    [image removeFromSuperview];
}

幀動畫比用NSMutableArray要簡潔，易懂