【读书笔记】GCD-系统提供的dispatch方法

  //系统提供的dispatch方法        //后台执行：    dispatch_async(dispatch_get_global_queue(0, 0), ^{        // something    });        //主线程执行：    dispatch_async(dispatch_get_main_queue(), ^{        // something    });        //一次性执行：    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        //something    });        // 延迟2秒执行：    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){        // 延迟2s在主线程中执行    });        //自定义queue    dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);    dispatch_async(urls_queue, ^{        //something    });        //让后台2个线程并行执行，然后等2个线程都结束后，再汇总执行结果    dispatch_group_t group = dispatch_group_create();    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{        // 并行执行的线程一    });    dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{        // 并行执行的线程二    });    dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{        // 汇总结果    });