雪花飘落效果

- (void)viewDidLoad {
    [super viewDidLoad];
	flakeImage = [UIImage imageNamed:@"flake.png"];
	[NSTimer scheduledTimerWithTimeInterval:(0.05) target:self selector:@selector(play) userInfo:nil repeats:YES];
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation {
	return (interfaceOrientation == UIInterfaceOrientationLandscapeLeft);
}

- (void)play {
	UIImageView *flakeView = [[UIImageView alloc] initWithImage:flakeImage];
	int startX = round(random() % 1024);
	int endX = round(random() % 1024);
	double scale = 1 / round(random() % 100) + 1.0;
	double speed = 1 / round(random() % 100) + 1.0;
	flakeView.frame = CGRectMake(startX, -100.0, 25.0 * scale, 25.0 * scale);
	flakeView.alpha = 0.55;
	[self.view addSubview:flakeView];	
	[UIView beginAnimations:nil context:flakeView];
	[UIView setAnimationDuration:5 * speed];
	flakeView.frame = CGRectMake(endX, 768.0, 25.0 * scale, 25.0 * scale);
	[UIView setAnimationDidStopSelector:@selector(onAnimationComplete:finished:context:)];
	[UIView setAnimationDelegate:self];
	[UIView commitAnimations];
}

- (void)onAnimationComplete:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context {	
	UIImageView *flakeView = context;
	[flakeView removeFromSuperview];
	[flakeView release];
}

 

示例图：