GCD的顺序下载

 在实际的项目开发中，我们往往需要从网络加载图片资源，为了不影响主线程，我们需要开辟新的线程。同时为了控制不同线程之间的前后顺序，需要引入信号量机制。这里我们来实现一个案例：从网络加载3张图片，当加载完第一张后在加载第二张，然后再加载第三张。

（1）首先也需要引入GCD的源代码，然后在ViewController代码中实现如下：

[objc] view plaincopyprint?

1. #import "ViewController.h"  
2. #import "GCD.h"  
3.   
4. @interface ViewController ()  
5.   
6. @property(strong,nonatomic) UIImageView *view1;  
7. @property(strong,nonatomic) UIImageView *view2;  
8. @property(strong,nonatomic) UIImageView *view3;  
9.   
10. @end  
11.   
12. @implementation ViewController  
13.   
14. - (void)viewDidLoad {  
15.   [super viewDidLoad];  
16.     
17.     
18.   self.view1 = [self setImageViewFrame:CGRectMake(0, 0, 300, 200)];  
19.   self.view2 = [self setImageViewFrame:CGRectMake(0, 200, 300, 200)];  
20.   self.view3 = [self setImageViewFrame:CGRectMake(0, 400, 300, 200)];  
21.     
22.   NSString *url01 = @"http://pic17.nipic.com/20111026/8662498_125925242183_2.jpg";  
23.   NSString *url02 = @"http://b.hiphotos.baidu.com/zhidao/wh%3D450%2C600/sign=53da5874352ac65c67506e77cec29e27/9f2f070828381f30807909bda9014c086e06f046.jpg";  
24.   NSString *url03 = @"http://cimg.163.com/sport/0412/23/wu2305.jpg";  
25.     
26.     
27.   GCDSemaphore *semaphore = [[GCDSemaphore alloc] init];  
28.   GCDSemaphore *semaphore2 = [[GCDSemaphore alloc] init];  
29.     
30.   //开启三个异步线程；  
31.     
32.   [GCDQueue executeInGlobalQueue:^{  
33.       
34.     UIImage *image01 = [self accessDataByNetwork:url01];  
35.       
36.     [GCDQueue executeInMainQueue:^{  
37.         
38.       [UIView animateWithDuration:2.0f  
39.                        animations:^{  
40.                          self.view1.image = image01;  
41.                          [self.view1 setAlpha:1.0f];  
42.                            
43.                        } completion:^(BOOL finished) {  
44.                            
45.                          //通知第1个信号量；  
46.                          [semaphore signal];  
47.                        }];  
48.         
49.     }];  
50.       
51.   }];  
52.     
53.     
54.   [GCDQueue executeInGlobalQueue:^{  
55.       
56.     UIImage *image02 = [self accessDataByNetwork:url02];  
57.       
58.     //第1个信号量等待；  
59.     [semaphore wait];  
60.       
61.     [GCDQueue executeInMainQueue:^{  
62.         
63.       [UIView animateWithDuration:2.0f  
64.                        animations:^{  
65.                          self.view2.image = image02;  
66.                          [self.view2 setAlpha:1.0f];  
67.                            
68.                        } completion:^(BOOL finished) {  
69.                            
70.                          //通知第2个信号量；  
71.                          [semaphore2 signal];  
72.                        }];  
73.         
74.     }];  
75.       
76.   }];  
77.     
78.     
79.     
80.   [GCDQueue executeInGlobalQueue:^{  
81.       
82.     UIImage *image03 = [self accessDataByNetwork:url03];  
83.       
84.     //第2个信号量等待；  
85.     [semaphore2 wait];  
86.       
87.     [GCDQueue executeInMainQueue:^{  
88.         
89.       [UIView animateWithDuration:2.0f  
90.                        animations:^{  
91.                          self.view3.image = image03;  
92.                          [self.view3 setAlpha:1.0f];  
93.                            
94.                        } completion:^(BOOL finished) {  
95.                            
96.                        }];  
97.         
98.     }];  
99.       
100.   }];  
101.     
102. }  
103.   
104.   
105. //设置图片大小位置和透明度；alpha = 0表示完全透明；  
106. - (UIImageView *)setImageViewFrame:(CGRect)frame{  
107.     
108.   UIImageView *imageView = [[UIImageView alloc] initWithFrame:frame];  
109.   imageView.alpha = 0.f;  
110.   [self.view addSubview:imageView];  
111.     
112.   return imageView;  
113.     
114. }  
115.   
116.   
117. //进行网络请求；  
118. - (UIImage *)accessDataByNetwork:(NSString *)string{  
119.     
120.   NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:string]];  
121.   NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil];  
122.     
123.   return [UIImage imageWithData:data];  
124.     
125. }  
126.   
127.   
128.   
129.   
130. @end