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：
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  1. dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
  2. 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：
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  1. dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
  2.     // long-running task  
  3.     dispatch_async(dispatch_get_main_queue(), ^{  
  4.         // update UI  
  5.     });  
  6. });  
       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：

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  1. dispatch_queue_t exampleQueue = dispatch_queue_create("com.example.unique.identifier", NULL );  
  2. dispatch_sync( exampleQueue,^{  
  3.   dispatch_sync( exampleQueue,^{  
  4.     printf("I am now deadlocked...\n");  
  5.   });});  
  6. 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 availabledispatch_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：