子線程延遲執行

1.默認爲主線程，dispatch_get_main_queue()修改後以下

//子線程延遲執行
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //須要執行的操做
        });