IOS 下雪动画

#define SNOW_IMAGENAME         @"snow"

#define IMAGE_X                arc4random()%(int)Main_Screen_Width
#define IMAGE_ALPHA            ((float)(arc4random()%10))/10
#define IMAGE_WIDTH            arc4random()%20 + 10
#define PLUS_HEIGHT            Main_Screen_Height/25

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    _imagesArray = [[NSMutableArray alloc] init];
    for (int i = 0; i < 20; ++ i) {
        UIImageView *imageView = [[UIImageView alloc] initWithImage:IMAGENAMED(SNOW_IMAGENAME)];
        float x = IMAGE_WIDTH;
        imageView.frame = CGRectMake(IMAGE_X, -30, x, x);
        imageView.alpha = IMAGE_ALPHA;
        [self.view addSubview:imageView];
        [_imagesArray addObject:imageView];
    }
    [NSTimer scheduledTimerWithTimeInterval:.3 target:self selector:@selector(makeSnow) userInfo:nil repeats:YES];
}

static int i = 0;
- (void)makeSnow
{
    i = i + 1;
    if ([_imagesArray count] > 0) {
        UIImageView *imageView = [_imagesArray objectAtIndex:0];
        imageView.tag = i;
        [_imagesArray removeObjectAtIndex:0];
        [self snowFall:imageView];
    }

}

- (void)snowFall:(UIImageView *)aImageView
{
    [UIView beginAnimations:[NSString stringWithFormat:@"%i",aImageView.tag] context:nil];
    [UIView setAnimationDuration:6];
    [UIView setAnimationDelegate:self];
    aImageView.frame = CGRectMake(aImageView.frame.origin.x, Main_Screen_Height, aImageView.frame.size.width, aImageView.frame.size.height);
    NSLog(@"%@",aImageView);
    [UIView commitAnimations];
}

- (void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context
{
    UIImageView *imageView = (UIImageView *)[self.view viewWithTag:[animationID intValue]];
    float x = IMAGE_WIDTH;
    imageView.frame = CGRectMake(IMAGE_X, -30, x, x);
    [_imagesArray addObject:imageView];
}

// 来自Code4App上的一个Demo