178实现满天飞雪效果

效果如下：

ViewController.h

1 #import <UIKit/UIKit.h>
2
3 @interface ViewController : UIViewController
4 @property (strong, nonatomic) UIImage *imgSnowflake;
5
6 @end

ViewController.m

 1 #import "ViewController.h"
 2
 3 @interface ViewController ()
 4 - (void)layoutUI;
 5 - (void)snow;
 6 @end
 7
 8 @implementation ViewController
 9 #define kApplicationFrame [[UIScreen mainScreen] applicationFrame]
10 #define kWidthOfSnowflake 30.0
11
12 - (void)viewDidLoad {
13     [super viewDidLoad];
14
15     [self layoutUI];
16 }
17
18 - (void)didReceiveMemoryWarning {
19     [super didReceiveMemoryWarning];
20     // Dispose of any resources that can be recreated.
21 }
22
23 - (void)layoutUI {
24     self.view.backgroundColor = [UIColor colorWithRed:0.218 green:0.219 blue:0.196 alpha:1.000];
25     _imgSnowflake = [UIImage imageNamed:@"Snowflake"];
26
27     //定时器；每隔0.5秒执行一次
28     [NSTimer scheduledTimerWithTimeInterval:0.5
29                                      target:self
30                                    selector:@selector(snow)
31                                    userInfo:nil
32                                     repeats:YES];
33 }
34
35 - (void)snow {
36     NSString *strWidthOfScene = [NSString stringWithFormat:@"%f", kApplicationFrame.size.width-kWidthOfSnowflake];
37     CGFloat startX = arc4random()%[strWidthOfScene integerValue]; //产生随机数0到strWidthOfScene-1
38     CGFloat endX = arc4random()%([strWidthOfScene integerValue]+1); //产生随机数1到strWidthOfScene
39
40     UIImageView *imgV = [[UIImageView alloc] initWithImage:_imgSnowflake];
41     imgV.frame = CGRectMake(startX, -20.0, kWidthOfSnowflake, kWidthOfSnowflake);
42     imgV.alpha = 0.8;
43     [self.view addSubview:imgV];
44
45     [UIView beginAnimations:nil context:NULL];
46     [UIView setAnimationDuration:5];
47     imgV.frame = CGRectMake(endX,
48                             kApplicationFrame.size.height+20.0-kWidthOfSnowflake,
49                             kWidthOfSnowflake,
50                             kWidthOfSnowflake);
51     [UIView commitAnimations];
52 }
53
54 @end