IOS学习之路八（GCD与多线程）

 GCD，全称Grand Central Dispath，是苹果开发的一种支持并行操作的机制。它的主要部件是一个FIFO队列和一个线程池，前者用来添加任务，后者用来执行任务。

     GCD中的FIFO队列称为dispatch queue，它可以保证先进来的任务先得到执行（但不保证一定先执行结束）。

     通过与线程池的配合，dispatch queue分为下面两种：

•      Serial Dispatch Queue -- 线程池只提供一个线程用来执行任务，所以后一个任务必须等到前一个任务执行结束才能开始。
•      Concurrent Dispatch Queue -- 线程池提供多个线程来执行任务，所以可以按序启动多个任务并发执行。

  1. Basic Management

       我们可以通过dispatch_queue_cretae来创建队列，然后用dispatch_release释放。比如下面两段代码分别创建串行队列和并行队列：

  dispatch_queue_t serialQ = dispatch_queue_create("eg.gcd.SerialQueue", DISPATCH_QUEUE_SERIAL);
  dispatch_async(serialQ, ^{
      // Code here
  });
  dispatch_release(serialQ);
  
  dispatch_queue_t concurrentQ = dispatch_queue_create("eg.gcd.ConcurrentQueue", DISPATCH_QUEUE_CONCURRENT);
  dispatch_async(concurrentQ, ^{
      // Code here
  });
  dispatch_release(concurrentQ);

   而系统默认就有一个串行队列main_queue和并行队列global_queue：

   

  dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  dispatch_queue_t mainQ = dispatch_get_main_queue();

  
       通常，我们可以在global_queue中做一些long-running的任务，完成后在main_queue中更新UI，避免UI阻塞，无法响应用户操作：

  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      // long-running task
      dispatch_async(dispatch_get_main_queue(), ^{
          // update UI
      });
  });

       上面提到dispatch_async这个接口，用来提交blcok给指定queue进行异步执行。这个接口会在成功提交block后立即返回，然后继续执行下去。由于block是定义在栈上的，所以需要将其复制到堆上，见这里。

       与之相对应的是dispatch_sync接口，提交block以供同步执行。这个接口会等到block执行结束才返回，所以不需要复制block。So，如果在调用该接口在当前queue上指派任务，就会导致deadlock。维基百科上给了段示例代码：

   

  dispatch_queue_t exampleQueue = dispatch_queue_create("com.example.unique.identifier", NULL );
  dispatch_sync( exampleQueue,^{
    dispatch_sync( exampleQueue,^{
      printf("I am now deadlocked...
  ");
    });});
  dispatch_release( exampleQueue );

       如果追求的是并发，那么dispatch_sync有什么用呢？关于dispatch_sync的用途，StackOverFlow是这样讨论的：

  Can anyone explain with really clear use cases what the purpose of dispatch_sync in GCD is for? I can't understand where and why I would have to use this.

  高手回答：

   

  You use it when you want to execute a block and wait for the results.

  One example of this is the pattern where you're using a dispatch queue instead of locks for synchronization. For example, assume you have a shared NSMutableArray a, with access mediated by dispatch queue q. A background thread might be appending to the array (async), while your foreground thread is pulling the first item off (synchronously):

  NSMutableArray*a =[[NSMutableArray alloc] init];// All access to `a` is via this dispatch queue!dispatch_queue_t q = dispatch_queue_create("com.foo.samplequeue", NULL);
  
  dispatch_async(q,^{[a addObject:something];});// append to array, non-blocking
  
  __block Something*first = nil;// "__block" to make results from block available
  dispatch_sync(q,^{// note that these 3 statements...if([a count]>0){// ...are all executed together...
               first =[a objectAtIndex:0];// ...as part of a single block...[a removeObjectAtIndex:0];// ...to ensure consistent results}});

  
  

   

  下面附上一个Demo  :(后台队列下载图片，当下载完后，调用主线程在前台显示)

  在Concoller中:

   

  -(void)viewDidAppear:(BOOL)animated{
  
  
      dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
      dispatch_async(concurrentQueue, ^{
          __block UIImage *image = nil;
          dispatch_sync(concurrentQueue, ^{
              /* 下载图片 */
  
              NSString *urlAsString = @"http://images.apple.com/mac/home/images/promo_lead_macbook_air.jpg";
             NSURL *url = [NSURL URLWithString:urlAsString];
              NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
              NSError *downloadError = nil;
              NSData *imageData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&downloadError];
              if (downloadError == nil && imageData != nil){
                 image = [UIImage imageWithData:imageData];
              } else if(downloadError!=nil){
                  NSLog(@"Error happened = %@", downloadError);
              }else {
                  NSLog(@"No data could get downloaded from the URL.");
              }
          });
          //在主队列中把图片展示给用户
          dispatch_sync(dispatch_get_main_queue(), ^{
  
              if (image != nil){
                  /* 创建一个UIImageView */
                  UIImageView *imageView = [[UIImageView alloc]
                                            initWithFrame:self.view.bounds];
                  /* 设置图片*/
                  [imageView setImage:image];
                  /* 设置图片比例*/
                  [imageView setContentMode:UIViewContentModeScaleAspectFit];
                  /* 添加视图 */
                  [self.view addSubview:imageView];
              } else {
                  NSLog(@"Image isn't downloaded. Nothing to display.");
              } });
       });
  
  }

  
  
  

  运行结果：