//系統提供的dispatch方法        //后臺執行：    dispatch_async(dispatch_get_global_queue(0, 0), ^{        // something    });        //主線程執行：    dispatch_async(dispatch_get_main_queue(), ^{        // something    });        //一次性執行：    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        //something    });        // 延遲2秒執行：    double delayInSeconds = 2.0;    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){        // 延遲2s在主線程中執行    });        //自定義queue    dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);    dispatch_async(urls_queue, ^{        //something    });        //讓后臺2個線程并行執行，然后等2個線程都結束后，再匯總執行結果    dispatch_group_t group = dispatch_group_create();    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{        // 并行執行的線程一    });    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{        // 并行執行的線程二    });    dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{        // 匯總結果    });