iOS GCD编程

- (void)gcdDownload  
{  
    static dispatch_once_t once;  
    static dispatch_queue_t queue;  
    //create download queue  
    dispatch_once(&once, ^{  
        queue =dispatch_queue_create("com.xxx.download.background",DISPATCH_QUEUE_CONCURRENT);  
    });  
    //__block type  
    __block BOOL downloadFlag = NO;  
    dispatch_async(queue, ^{  
//        downloadFlag = [Download sendRequest:request];  
        NSLog(@"长时间任务，如网络下载");  
    });  
  
    dispatch_barrier_async(queue,^{  
        if (downloadFlag) {  
            NSLog(@"下载成功完成");  
        }  
          
        dispatch_async(dispatch_get_main_queue(), ^{  
            NSLog(@"执行完下载，回掉回主线程，例如刷新UI");  
          
        });  
                       });  
}