dispatch_queue_t t = dispatch_queue_create("com.hello", DISPATCH_QUEUE_CONCURRENT);

    dispatch_queue_t gt = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    //设置quene优先级

    dispatch_set_target_queue(t, gt);

    //dispatch_time定义相对时间

    dispatch_time_t time_t = dispatch_time(DISPATCH_TIME_NOW, 3ull * NSEC_PER_SEC);

    dispatch_after(time_t, dispatch_get_main_queue(), ^{

    });

    //通过timespec 定义绝对时间

    NSTimeInterval interval = [[NSDate date] timeIntervalSince1970];

    doublesec, subsec;

    subsec =modf(interval, &sec);

    structtimespectime;

    time.tv_sec= sec;

    time.tv_nsec= subsec *NSEC_PER_SEC;

    dispatch_time_t wtime_t = dispatch_walltime(&time, 0);

    dispatch_group_t group_t = dispatch_group_create();

    dispatch_group_async(group_t, gt, ^{

    });

    dispatch_group_notify(group_t, gt, ^{

    });

    //DISPATCH_QUEUE_CONCURRENT quene 在已经提交到quene中的任务完成后加入，

//    on such a queue, a barrier block

//    * will not run until all blocks submitted to the queue earlier have completed,

//    * and any blocks submitted to the queue after a barrier block will not run

//    * until the barrier block has completed.

    dispatch_barrier_async(gt, ^{

    });

    //等待全部执行完成 done才执行

    dispatch_apply(10, gt, ^(size_titeration) {

    });

    NSLog(@"done");

    //挂起和恢复quene

    dispatch_suspend(t);

    dispatch_resume(t);

    dispatch_semaphore_t sema_t = dispatch_semaphore_create(1);

    dispatch_semaphore_signal(sema_t);

    dispatch_semaphore_wait(sema_t, DISPATCH_TIME_FOREVER);

//    dispatch_source_t?