小賢的部落格

    // 建立Group    

    dispatch_group_t group = dispatch_group_create();

    // Group 1 Async

    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        [self Process];

    });

    // Group 2 Async

    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        [self Process];

    });

    // 當 Group 1, Group 2都完成時，執行Block

    dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSLog(@"完成了");

    }); 

參考資料:

http://stackoverflow.com/questions/11909629/waiting-until-two-async-blocks-are-executed-before-starting-another-block