GCD多线程编程

什么是GCD

Grand Central Dispatch (GCD)是Apple开发的一个多核编程的解决方法。该方法在Mac OS X 10.6雪豹中首次推出，并随后被引入到了iOS4.0中。GCD是一个替代诸如NSThread, NSOperationQueue, NSInvocationOperation等技术的很高效和强大的技术，它看起来象就其它语言的闭包(Closure)一样，但苹果把它叫做blocks。

应用举例

让我们来看一个编程场景。我们要在iphone上做一个下载网页的功能，该功能非常简单，就是在iphone上放置一个按钮，点击该按钮时，显示一个转动的圆圈，表示正在进行下载，下载完成之后，将内容加载到界面上的一个文本控件中。

不用GCD前

虽然功能简单，但是我们必须把下载过程放到后台线程中，否则会阻塞UI线程显示。所以，如果不用GCD, 我们需要写如下3个方法：

someClick 方法是点击按钮后的代码，可以看到我们用NSInvocationOperation建了一个后台线程，并且放到NSOperationQueue中。后台线程执行download方法。
download 方法处理下载网页的逻辑。下载完成后用performSelectorOnMainThread执行download_completed 方法。
download_completed 进行clear up的工作，并把下载的内容显示到文本控件中。

这3个方法的代码如下。可以看到，虽然开始下载 -> 下载中 -> 下载完成这3个步骤是整个功能的三步。但是它们却被切分成了3块。他们之间因为是3个方法，所以还需要传递数据参数。如果是复杂的应用，数据参数很可能就不象本例子中的NSString那么简单了，另外，下载可能放到Model的类中来做，而界面的控制放到View Controller层来做，这使得本来就分开的代码变得更加散落。代码的可读性大大降低。

 
    
     
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">10</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">11</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">12</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">13</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">14</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">15</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">16</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">17</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">18</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">19</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">20</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">21</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">22</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">23</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">24</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">25</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">26</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">27</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">28</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">29</span>
 
       static NSOperationQueue * queue;  - (IBAction)someClick:(id)sender {  self.indicator.hidden = NO;  [self.indicator startAnimating];  queue = [[NSOperationQueue alloc] init];  NSInvocationOperation * op = [[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download) object:nil] autorelease];  [queue addOperation:op]; }  - (void)download {  NSURL * url = [NSURL URLWithString:@"http://www.youdao.com"];  NSError * error;  NSString * data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];  if (data != nil) {  [self performSelectorOnMainThread:@selector(download_completed:) withObject:data waitUntilDone:NO];  } else {  NSLog(@"error when download:%@", error);  [queue release];  } }  - (void) download_completed:(NSString *) data {  NSLog(@"call back");  [self.indicator stopAnimating];  self.indicator.hidden = YES;  self.content.text = data;  [queue release]; }   
     
 
    
  

使用GCD后

如果使用GCD，以上3个方法都可以放到一起，如下所示：

 
    
     
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">10</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">11</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">12</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">13</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">14</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">15</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">16</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">17</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">18</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">19</span>
 
       // 原代码块一 self.indicator.hidden = NO; [self.indicator startAnimating]; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  // 原代码块二  NSURL * url = [NSURL URLWithString:@"http://www.youdao.com"];  NSError * error;  NSString * data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];  if (data != nil) {  // 原代码块三  dispatch_async(dispatch_get_main_queue(), ^{  [self.indicator stopAnimating];  self.indicator.hidden = YES;  self.content.text = data;  });  } else {  NSLog(@"error when download:%@", error);  } });   
     
 
    
  

首先我们可以看到，代码变短了。因为少了原来3个方法的定义，也少了相互之间需要传递的变量的封装。

另外，代码变清楚了，虽然是异步的代码，但是它们被GCD合理的整合在一起，逻辑非常清晰。如果应用上MVC模式，我们也可以将View Controller层的回调函数用GCD的方式传递给Modal层，这相比以前用@selector的方式，代码的逻辑关系会更加清楚。

GCD的定义

简单GCD的定义有点象函数指针，差别是用 ^ 替代了函数指针的 * 号，如下所示：

 
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
 
        // 申明变量  (void) (^loggerBlock)(void);  // 定义   loggerBlock = ^{  NSLog(@"Hello world");  };  // 调用  loggerBlock();

但是大多数时候，我们通常使用内联的方式来定义它，即将它的程序块写在调用的函数里面，例如这样：

 
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
 
        dispatch_async(dispatch_get_global_queue(0, 0), ^{  // something  });

从上面大家可以看出，block有如下特点：

程序块可以在代码中以内联的方式来定义。
程序块可以访问在创建它的范围内的可用的变量。

系统提供的dispatch方法

为了方便地使用GCD，苹果提供了一些方法方便我们将block放在主线程或后台线程执行，或者延后执行。使用的例子如下：

 
    
     
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">10</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">11</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">12</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">13</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">14</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">15</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">16</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">17</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">18</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">19</span>
 
        // 后台执行：  dispatch_async(dispatch_get_global_queue(0, 0), ^{  // something  });  // 主线程执行：  dispatch_async(dispatch_get_main_queue(), ^{  // something  });  // 一次性执行：  static dispatch_once_t onceToken;  dispatch_once(&onceToken, ^{  // code to be executed once  });  // 延迟2秒执行：  double delayInSeconds = 2.0;  dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);  dispatch_after(popTime, dispatch_get_main_queue(), ^(void){  // code to be executed on the main queue after delay  });   
     
 
    
  

dispatch_queue_t 也可以自己定义，如要要自定义queue，可以用dispatch_queue_create方法，示例如下：

 
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
 
       dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL); dispatch_async(urls_queue, ^{  // your code }); dispatch_release(urls_queue);

另外，GCD还有一些高级用法，例如让后台2个线程并行执行，然后等2个线程都结束后，再汇总执行结果。这个可以用dispatch_group, dispatch_group_async 和 dispatch_group_notify来实现，示例如下：

 
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">10</span>
 
        dispatch_group_t group = dispatch_group_create();  dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{  // 并行执行的线程一  });  dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{  // 并行执行的线程二  });  dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{  // 汇总结果  });

修改block之外的变量

默认情况下，在程序块中访问的外部变量是复制过去的，即写操作不对原变量生效。但是你可以加上 __block来让其写操作生效，示例代码如下：

 
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
 
        __block int a = 0;  void (^foo)(void) = ^{  a = 1;  }  foo();  // 这里，a的值被修改为1

后台运行

GCD的另一个用处是可以让程序在后台较长久的运行。在没有使用GCD时，当app被按home键退出后，app仅有最多5秒钟的时候做一些保存或清理资源的工作。但是在使用GCD后，app最多有10分钟的时间在后台长久运行。这个时间可以用来做清理本地缓存，发送统计数据等工作。

让程序在后台长久运行的示例代码如下：

 
    
     
       <span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">1</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">2</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">3</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">4</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">5</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">6</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">7</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">8</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">9</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">10</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">11</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">12</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">13</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">14</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">15</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">16</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">17</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">18</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">19</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">20</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">21</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">22</span>
<span class="line-number" style="margin:0px; padding:0px; border:0px; font-family:inherit; font-style:inherit; font-variant:inherit; line-height:inherit; vertical-align:baseline; color:rgb(88,110,117)!important">23</span>
 
       // AppDelegate.h文件 @property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundUpdateTask;  // AppDelegate.m文件 - (void)applicationDidEnterBackground:(UIApplication *)application {  [self beingBackgroundUpdateTask];  // 在这里加上你需要长久运行的代码  [self endBackgroundUpdateTask]; }  - (void)beingBackgroundUpdateTask {  self.backgroundUpdateTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{  [self endBackgroundUpdateTask];  }]; }  - (void)endBackgroundUpdateTask {  [[UIApplication sharedApplication] endBackgroundTask: self.backgroundUpdateTask];  self.backgroundUpdateTask = UIBackgroundTaskInvalid; }   
     
 
    
  

总结

总体来说，GCD能够极大地方便开发者进行多线程编程。如果你的app不需要支持iOS4.0以下的系统，那么就应该尽量使用GCD来处理后台线程和UI线程的交互。