雪花飘落-动画制作（UIImage+NSTimer定时器）

本文主要使用UIImage和定时器制作一个简单的雪花飘落Demo.

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效果：

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效果解释：随机生成雪花数目，飘到屏幕底部指定位置，设置时间，底部的雪花“消融”。

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源代码：

#define FPS 30.0
#define MAX_SIZE 10
#define MAX_DURATION 10


- (void)viewDidLoad
{
    [super viewDidLoad];
//启动定时器，创建雪花
    [NSTimer scheduledTimerWithTimeInterval:1/FPS target:self selector:@selector(animate:) userInfo:nil repeats:YES];
}

-(void)animate:(NSTimer *)timer{
//1.创建一个雪花
UIImageView *snow = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"snow.png"]];
int viewWidth = self.view.bounds.size.width;
int viewHeight = self.view.bounds.size.height;
CGFloat size = MAX_SIZE+arc4random()%MAX_SIZE;
    snow.frame = CGRectMake(arc4random()%viewWidth, -20, size, size);
    [self.view addSubview:snow];
//2.创建动画
    [UIView animateWithDuration:arc4random()%MAX_DURATION+2  delay:0 options:UIViewAnimationOptionCurveEaseIn animations:^{
//3. 设置动画结束时雪花的位置信息
int offset = arc4random()%100 - 50;
        snow.center = CGPointMake(snow.center.x+offset, viewHeight-30);
    }completion:^(BOOL finished) {
//4.落地动画结束时，开始融雪
        [UIView animateWithDuration:arc4random()%MAX_DURATION delay:0 options:UIViewAnimationOptionCurveEaseIn  animations:^{
            snow.alpha = 0;
        } completion:^(BOOL finished) {
//5.融雪动画结束时，将图片移出父视图
            [snow removeFromSuperview];
        }];
    }];
}

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谢谢大家！