GCD与多线程

时间:2024-12-05 21:07:26

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。*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,*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:

        GCD与多线程