Monotouch：在 iPhone 上关闭飞行模式后 WebRequest 连接失败

Question

我的单点触控应用程序正在与网络服务进行定期后台同步。它运行完美并正确检测飞行模式。当我关闭 WiFi 时，它会自动开始使用 WWAN（GPRS、3G）连接。到目前为止，我非常满意，但是...关闭飞行模式后，当没有可用的 WiFi 时，我的应用程序将无法重新连接。

它使用 NetworkReachability 对象正确检测 WWAN 是否可用以及是否需要连接。但第一次尝试超时（90 秒后我使用计时器中止正在运行的请求）。当我再次尝试时，只要调用 EndGetRequestStream，就会收到 WebException“错误：ConnectionFailure（没有到主机的路由）”。再次连接的唯一方法是启动另一个应用程序，例如“邮件”，它会建立连接。之后我的应用程序再次完美连接。或者等待几分钟，直到 iPhone 进入睡眠状态。唤醒后，连接再次建立正常。

我做错了什么？

下面的代码是使用 ThreadPool.QueueUserWorkItem(CreateRequest); 启动的

    /// <summary>
    /// Sync step 1: Create the request and start asynchronously sending the data.
    /// </summary>
    private void CreateRequest(object state)
    {
        try
        {
            Console.WriteLine("Phase 1 started...");
            if (!IsNetworkAvailable())
            {
                Ready(SyncState.NoConnection);
                return;
            }
            UIApplication.SharedApplication.NetworkActivityIndicatorVisible = true;
            _request = (HttpWebRequest)WebRequest.Create(SyncUrl);
            _request.Method = HttpMethodPost;
            _request.ContentType = HttpContentTypeJson;
            Console.WriteLine("Phase 2 is starting...");
            _request.BeginGetRequestStream(new AsyncCallback(StartRequest), null);
        }
        catch (WebException e)
        {
            Console.WriteLine("WebException: " + e.Message + "\r\nStatus: " + e.Status);
            Ready(SyncState.ConnectionError);
        }
    }

    /// <summary>
    /// Sync step 2: Read syncdata from database and send to server.
    /// Start getting the response asynchronously.
    /// </summary>
    private void StartRequest(IAsyncResult asyncResult)
    {
        Console.WriteLine("Phase 2 started...");
        try
        {
            using (var stream = _request.EndGetRequestStream(asyncResult))
            {
                using (var textStream = new StreamWriter(stream))
                {
                    Database.Instance.CreateSyncData().Save(textStream);
                }
            }
            Console.WriteLine("Phase 3 is starting...");
            _request.BeginGetResponse(new AsyncCallback(ProcessResponse), null);
        }
        catch (WebException e)
        {
            Console.WriteLine("WebException: " + e.Message + "\r\nStatus: " + e.Status);
            Ready(SyncState.ConnectionError);
        }
    }

    /// <summary>
    /// Sync step 3: Get the response and process.
    /// </summary>
    private void ProcessResponse(IAsyncResult asyncResult)
    {
        Console.WriteLine("Phase 3 started...");
        try
        {
            using (HttpWebResponse response = (HttpWebResponse)_request.EndGetResponse(asyncResult))
            {
                using (var textStream = new StreamReader(response.GetResponseStream()))
                {
                    var data = (JsonObject)JsonObject.Load(textStream);
                    Database.Instance.ProcessSyncReply(data);
                    Console.WriteLine("Success: " + data.ToString());
                    LastSyncTime = DateTime.Now;
                    Ready(SyncState.Synchronized);
                }
            }
        }
        catch (WebException e)
        {
            Console.WriteLine("WebException: " + e.Message + "\r\nStatus: " + e.Status);
            Ready(SyncState.ConnectionError);
        }
    }

    private bool IsNetworkAvailable(out bool connectionRequired, out bool onlyWWAN)
    {
        bool flagsAvailable;
        NetworkReachabilityFlags networkReachabilityFlags = (NetworkReachabilityFlags)0;
        using (var networkReachability = new NetworkReachability(HostName))
        {
            flagsAvailable = networkReachability.TryGetFlags(out networkReachabilityFlags);
        }
        connectionRequired = 0 != (networkReachabilityFlags & NetworkReachabilityFlags.ConnectionRequired);
        onlyWWAN = 0 != (networkReachabilityFlags & NetworkReachabilityFlags.IsWWAN);
        return flagsAvailable && 0 != (networkReachabilityFlags & NetworkReachabilityFlags.Reachable);
    }

    private bool IsNetworkAvailable()
    {
        bool connectionRequired;
        bool onlyWWAN;
        bool available = IsNetworkAvailable(out connectionRequired, out onlyWWAN);
        string status = "Network status: ";
        if (!available)
            status += "Not available";
        else
        {
            status += "Available; ";
            if (onlyWWAN)
                status += "Mobile; ";
            if (connectionRequired)
                status += "Connection required";
        }
        Console.WriteLine(status);
        return available;
    }

Answer 1

MonoTouch 高级对象（ftp、smtp、http）利用 BSD 套接字处理网络事务。 Apple 有一种机制，即使 3G/EDGE 连接处于“活动状态”，它实际上也会进入睡眠状态。唤醒它的唯一方法是使用 CFStream 或 NSStream 资源，没有公开的 API 来唤醒 BSD 套接字的 GPRS 连接。值得庆幸的是，您可以解决这个问题。 MonoTouch 提供了一个 API：

MonoTouch.ObjCRuntime.Runtime.StartWWAN (Uri uri);

该 api 只接受 HTTP/HTTPs uri，并将快速连接到指定的 API，以重新唤醒所有连接的 WWAN，此时 WWAN 将保持活动状态，直到再次飞行或超时。