1、dispatch_async

为了避免程序在处理耗时的操作时界面卡死，如读取网络数据、IO、数据库读写等，我们会开启一个新的线程处理这些操作，然后通知主线程刷新UI。用GCD实现实现，代码框架如下，

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
    // 耗时的操作  
    dispatch_async(dispatch_get_main_queue(), ^{  
        // 更新界面  
    });  
});  

例如：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
    NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];  
    NSData * data = [[NSData alloc]initWithContentsOfURL:url];  
    UIImage *image = [[UIImage alloc]initWithData:data];  
    if (data != nil) {  
        dispatch_async(dispatch_get_main_queue(), ^{  
            self.imageView.image = image;  
         });  
    }  
}); 

2、dispatch_group_async

dispatch_group_async可以实现监听一组任务是否完成，完成后得到通知执行其他的操作。比如现在有三个下载任务，当三个任务都下载完成后才通知界面刷新UI。代码如下，

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
dispatch_group_t group = dispatch_group_create();  

dispatch_group_async(group, queue, ^{  
    [NSThread sleepForTimeInterval:1];  
    NSLog(@"group1");  
});  

dispatch_group_async(group, queue, ^{  
    [NSThread sleepForTimeInterval:2];  
    NSLog(@"group2");  
});  

dispatch_group_async(group, queue, ^{  
    [NSThread sleepForTimeInterval:3];  
    NSLog(@"group3");  
});  

dispatch_group_notify(group, dispatch_get_main_queue(), ^{  
    NSLog(@"refreshUI");  
});  

dispatch_release(group);  

dispatch_group_async是异步的方法，运行后可以看到打印结果：

2017-03-02 15:23:23.627 GCDTest[32327:18237] group1
2017-03-02 15:23:24.626 GCDTest[32327:19c6b] group2
2017-03-02 15:23:25.626 GCDTest[32327:20236] group3
2017-03-02 15:23:26.626 GCDTest[32327:e347] refreshUI

3、dispatch_barrier_async
dispatch_barrier_async的作用是在前面的任务执行结束后它才执行，而且它后面的任务等它执行完成之后才会执行。barrier，栅栏的意思。如：

dispatch_queue_t queue = dispatch_queue_create("diapatchQueue", DISPATCH_QUEUE_CONCURRENT); 

dispatch_async(queue, ^{  
    [NSThread sleepForTimeInterval:2];  
    NSLog(@"dispatch_async1");  
});  

dispatch_async(queue, ^{  
    [NSThread sleepForTimeInterval:4];  
    NSLog(@"dispatch_async2");  
});  

dispatch_barrier_async(queue, ^{  
    NSLog(@"dispatch_barrier_async");  
    [NSThread sleepForTimeInterval:4];  
});  

dispatch_async(queue, ^{  
    [NSThread sleepForTimeInterval:1];  
    NSLog(@"dispatch_async3");  
});  

打印结果如下：

2017-03-02 15:25:23.627 GCDTest[32327:11391] dispatch_async1

2017-03-02 15:25:25.627 GCDTest[32327:11391] dispatch_async2

2017-03-02 15:25:25.627 GCDTest[32327:11391] dispatch_barrier_async

2017-03-02 15:25:30.627 GCDTest[32327:11391] dispatch_async3