信号量 等待阻塞网络线程

 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, 0), ^{
    dispatch_semaphore_t sema = dispatch_semaphore_create(1);
    [网络请求:^{
        //请求回调
     NSLog(@"执行顺序1");
        dispatch_semaphore_signal(sema);  
    }];
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    NSLog(@"执行顺序2");
});