dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_enter(group);
NSLog(@"111");
dispatch_group_leave(group);
dispatch_group_enter(group);
NSLog(@"222");
dispatch_group_leave(group);
dispatch_group_enter(group);
NSLog(@"333");
dispatch_group_leave(group);
dispatch_group_notify(group, queue, ^{
NSLog(@"全部输出完成！");
});
NSLog(@"444");
输出结果：444 111 222 333 全部输出完成！
