dispatch_apply and dispatch_barrier_async

 //自动开启线程数目控制，最优不会过多崩溃

    dispatch_apply(100, dispatch_get_global_queue(0, 0), ^(size_t index) {

        NSLog(@"%ld--%@", index,[NSThread currentThread]);

    });

dispatch_barrier_async  不会阻塞当前线程，等队列中任务都运行完在运行自己，比自己后加入的等自己运行完才能运行。

dispatch_barrier_sync 会阻塞当前线程一起等待，知道barr内运行完。

dispatch_queue_t queue = dispatch_queue_create("conQueue", DISPATCH_QUEUE_CONCURRENT);

    dispatch_async(queue, ^{

        [selfdownLoad];

        NSLog(@"1111Finish,%@",[NSThread currentThread]);

    });

    dispatch_async(queue, ^{

        [selfdownLoad];

        NSLog(@"2222Finish,%@",[NSThread currentThread]);

    });

    dispatch_barrier_sync(queue, ^{

        [selfdownLoad];

        NSLog(@"BarrierFinish,%@",[NSThread currentThread]);

    });

    dispatch_async(queue, ^{

        NSLog(@"3333Finish,%@",[NSThread currentThread]);

    });

    NSLog(@"End");