ViewController.h

 #import <UIKit/UIKit.h>

 @interface ViewController : UIViewController{

     UIImage* flakeImage;

 }

 - (void)onTimer;

 @end

 ViewController.m

 #import "ViewController.h"

 @interface ViewController ()

 @end

 @implementation ViewController

 - (void)viewDidLoad

 {

     [super viewDidLoad];

     self.view.backgroundColor = [UIColor colorWithRed:0.5 green:0.5 blue:1.0 alpha:1.0];

     flakeImage = [UIImage imageNamed:@"2.png"];

     [NSTimer scheduledTimerWithTimeInterval:(0.05) target:self selector:@selector(onTimer) userInfo:nil repeats:YES];

 }

 - (void)onTimer

 {

     //创建图像视图对象

     UIImageView* flakeView = [[UIImageView alloc] initWithImage:flakeImage];

     int startX = round(random() % );

     int endX = round(random() % );

     double scale =  / round(random() % ) + 1.0;

     double speed =  / round(random() % ) + 1.0;

     flakeView.frame = CGRectMake(startX, -100.0, 25.0 * scale, 25.0 * scale);

     flakeView.alpha = 0.5;

     [self.view addSubview:flakeView];

     [UIView beginAnimations:nil context:(__bridge void *)(flakeView)];

     [UIView setAnimationDuration: * speed];

     flakeView.frame = CGRectMake(endX, 540.0, 25.0 * scale, 25.0 * scale);

     [UIView commitAnimations];

 }

 - (void)didReceiveMemoryWarning

 {

     [super didReceiveMemoryWarning];

     // Dispose of any resources that can be recreated.

 }