GCD之Dispatch Group

 

首先，我们来看下面的一段代码 ARC模式下

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, ^{
        NSLog(@"block one");
    });
    dispatch_group_async(group, queue, ^{
        NSLog(@"block two");
    });
    dispatch_group_async(group, queue, ^{
        NSLog(@"block three");
    });
    dispatch_group_async(group, queue, ^{
        NSLog(@"block four");
    });
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"main do");
    });

 执行结果是

013-12-31 23:31:56.677 GCDDemo[449:320b] block two
2013-12-31 23:31:56.677 GCDDemo[449:1303] block one
2013-12-31 23:31:56.677 GCDDemo[449:3603] block four
2013-12-31 23:31:56.677 GCDDemo[449:3503] block three
2013-12-31 23:31:56.683 GCDDemo[449:70b] main do

 解释

首先我们获得了系统的一个queue，这个queue是不等待处理中结束的queue,所以我们看到的执行结果是杂乱无章的。

接着我们create一个group,然后我们向队列中追加block，并加入group。当组内的任务都执行结束的时候，发出通知。