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：
1. dispatch_queue_t serialQ = dispatch_queue_create("eg.gcd.SerialQueue", DISPATCH_QUEUE_SERIAL);
2. dispatch_async(serialQ, ^{
3. // Code here
4. });
5. dispatch_release(serialQ);
6. dispatch_queue_t concurrentQ = dispatch_queue_create("eg.gcd.ConcurrentQueue", DISPATCH_QUEUE_CONCURRENT);
7. dispatch_async(concurrentQ, ^{
8. // Code here
9. });
10. 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：
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：
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：
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 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:
1. -(void)viewDidAppear:(BOOL)animated{
2. dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
3. dispatch_async(concurrentQueue, ^{
4. __block UIImage *image = nil;
5. dispatch_sync(concurrentQueue, ^{
6. /* 下载图片 */
7. NSString *urlAsString = @"http://images.apple.com/mac/home/images/promo_lead_macbook_air.jpg";
8. NSURL *url = [NSURL URLWithString:urlAsString];
9. NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
10. NSError *downloadError = nil;
11. NSData *imageData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&downloadError];
12. if (downloadError == nil && imageData != nil){
13. image = [UIImage imageWithData:imageData];
14. } else if(downloadError!=nil){
15. NSLog(@"Error happened = %@", downloadError);
16. }else {
17. NSLog(@"No data could get downloaded from the URL.");
18. }
19. });
20. //在主队列中把图片展示给用户
21. dispatch_sync(dispatch_get_main_queue(), ^{
22. if (image != nil){
23. /* 创建一个UIImageView */
24. UIImageView *imageView = [[UIImageView alloc]
25. initWithFrame:self.view.bounds];
26. /* 设置图片*/
27. [imageView setImage:image];
28. /* 设置图片比例*/
29. [imageView setContentMode:UIViewContentModeScaleAspectFit];
30. /* 添加视图 */
31. [self.view addSubview:imageView];
32. } else {
33. NSLog(@"Image isn't downloaded. Nothing to display.");
34. } });
35. });
36. }
运行结果：

巴特西

GCD与多线程

GCD与多线程

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