dispatch_group

dispatch_group最常见的功能就是，进入一个页面有多个网络请求，我们想在所有的请求完成后再进行UI的刷新。

代码：

- (void)test_dispatch_group {
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("com.jueyingxx.concurrent_queue", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"task 1");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"task 2");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"task 3");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"task 4");
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"main task");
    });
}

输出结果：

2016-11-04 10:19:41.685 GCD_Demo[64555:1619028] task 4
2016-11-04 10:19:41.685 GCD_Demo[64555:1619026] task 2
2016-11-04 10:19:41.685 GCD_Demo[64555:1619025] task 1
2016-11-04 10:19:41.685 GCD_Demo[64555:1619034] task 3
2016-11-04 10:19:41.689 GCD_Demo[64555:1619002] main task

结论：

1、task1234任务是随机执行的。
2、main task总会在task1234全部完成后执行，符合我们的预期
3、这种做法就是前面的任务都是并发执行的，最后我们回调主线程，做一些我们需要做的事，比如tableview的reloaddata。