一个生成雪花的定时器，随机大小和位置，下落和消失动画。
//定时器,生成雪花***
[NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(flakeDown) userInfo:nil repeats:YES];
}
//生成雪花的方法
- (void)flakeDown {
//注意:定时器调用的方法中,创建对象不用全局变量
UIImageView *imageView = [[UIImageView alloc] init];
imageView.image= [UIImage imageNamed:@"flake.png"];
//随机生成一个雪花宽度10 - 40
//获取随机数:先获取随机范围内的数字个数
int width =arc4random() % 31 + 10;
//随机生成雪花的横坐标
int x1 = arc4random() % (321-width);
imageView.frame=CGRectMake(x1, -width, width, width);
[self.view addSubview:imageView];
//让雪花下落
[UIView beginAnimations:nil context:(__bridgevoid*)(imageView)];
//随机一个动画时间,控制下落速度2.0 - 5.0
float time = (arc4random()%31 + 20)/10.0;
NSLog(@"%.1f",time);
[UIView setAnimationDuration:time];
//下落曲线为加速
[UIView setAnimationCurve:UIViewAnimationCurveEaseIn];
//动画结束调方法
[UIView setAnimationDelegate:self];
[UIViewset AnimationDidStopSelector:@selector(animationDidStop:finished:context:)];
//随机一个雪花落地时的横坐标
int x2 =arc4random() % (321-width);
imageView.frame=CGRectMake(x2, 480-width, width, width);
[UIView commitAnimations];
}
//动画结束之后调用的方法。使雪花消失
- (void)animationDidStop:(NSString*)animationID finished:(NSNumber*)finished context:(void*)context {
UIImageView *flake = (__bridgeUIImageView*)context;
[UIView beginAnimations:nil context:nil];
//[UIView setAnimationDelay:1];
[UIView setAnimationDuration:1];
flake.alpha= 0;
[UIView commitAnimations];
}
