dispatch_group_tgroup = dispatch_group_create();
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
//執行1個耗時的異步操作
});
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
//執行1個耗時的異步操作
});
dispatch_group_notify(group,dispatch_get_main_queue(),^{
//等前面的異步操作都執行完畢後,回到主線程...
});