UWP AppServiceConnection - SendResponseAsync returns AppServiceResponseStatus.Failure

问题

I'm trying to create a UWP service app on the Raspberry Pi3 which provides the access to the on board UART. I'm facing an issue about the AppConnection Request/response.

this is the service method that handles the incoming requests from client apps

internal class Inbound
{
    public static async void OnRequestReceived(AppServiceConnection sender, AppServiceRequestReceivedEventArgs args)
    {
        var messageDeferral = args.GetDeferral();
        var response = new ValueSet();

        bool success = false;
        var msg = args.Request.Message.Keys;
        if (args.Request.Message.TryGetValue(ServiceApiRequests.Keys.Command, out object command))
        {
            try
            {
                switch (command)
                {

                    case ServiceApiRequests.CommandValues.UartWrite:
                        if (args.Request.Message.TryGetValue(ServiceApiRequests.Keys.UartTxBuffer, out object txBuffer))
                        {
                            string rxBuff = "";
                            success = await Pi3.Peripherals.Uart.GerInstance(57600).Write((string)txBuffer);

                            if (success)
                            {
                                Debug.WriteLine("Tx: " + (string)txBuffer);
                                if (args.Request.Message.TryGetValue(ServiceApiRequests.Keys.ReadUartResponse, out object getResponse))
                                {
                                    if ((string)getResponse == ServiceApiRequests.ReadUartResponse.Yes)
                                    {
                                        rxBuff = await Pi3.Peripherals.Uart.GerInstance(57600).Read();
                                        Debug.WriteLine("Rx: " + rxBuff);
                                    }
                                }
                            }
                            response.Add(ServiceApiRequests.Keys.UartRxBuffer, rxBuff);

                        }
                        break;
                }
            }
            catch (Exception ex)
            {
                success = false;
            }
        }

        response.Add(new KeyValuePair<string, object>(ServiceApiRequests.Keys.Result, success ? ServiceApiRequests.ResultValues.Ok : ServiceApiRequests.ResultValues.Ko));
        var result = await args.Request.SendResponseAsync(response);
        if (result == AppServiceResponseStatus.Failure)
        {
            Debug.WriteLine("Failed to send the response");
        }
        messageDeferral.Complete();
    }
}

As you can figure out, the Uart class is get using the Singleton pattern using the method Pi3.Peripherals.Uart.GerInstance(57600).

Following the code i using for send the request from the client app.

    public static class Uart
{
    public static IAsyncOperation<string> SendCommand(this AppServiceConnection DriverControllerConnection, string txBuffer, string awaitResponse = ServiceApiRequests.ReadUartResponse.Yes)
    {
        return _SendCommand(DriverControllerConnection, txBuffer, awaitResponse).AsAsyncOperation();
    }
    private static async Task<string> _SendCommand(AppServiceConnection DriverControllerConnection, string txBuffer, string awaitResponse)
    {
        AppServiceResponse response = null;
        string response_str = "";
        try
        {
            if (DriverControllerConnection != null)
            {
                response = await DriverControllerConnection.SendMessageAsync(new ServiceApiRequests.UartWrite().GetCommand(txBuffer, awaitResponse));
                if (response.Status == AppServiceResponseStatus.Success)
                {
                    if (response.Message.TryGetValue(ServiceApiRequests.Keys.Result, out object result))
                    {
                        if ((string)result == ServiceApiRequests.ResultValues.Ok && awaitResponse == ServiceApiRequests.ReadUartResponse.Yes)
                        {
                            response_str = response.Message[ServiceApiRequests.Keys.UartRxBuffer] as string;
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            // TODO: log
        }
        return response_str;
    }
}

The system works well just for a while, until i have response.Status == AppServiceResponseStatus.Success , then the result of the request changes and it becomes AppServiceResponseStatus.Failure. This way the program counter never steps into the condition if (response.Status == AppServiceResponseStatus.Success).

Any idea about the cause? Thank you so much for the help.

EDIT

Follow the suggestions, i added an handler for the ServiceClosed event. This is the main class.

public sealed class DriverListener : IBackgroundTask
{
    private BackgroundTaskDeferral backgroundTaskDeferral;
    private AppServiceConnection appServiceConnection;
    public void Run(IBackgroundTaskInstance taskInstance)
    {
        backgroundTaskDeferral = taskInstance.GetDeferral();
        // taskInstance.Canceled += OnTaskCanceled;

        var triggerDetails = taskInstance.TriggerDetails as AppServiceTriggerDetails;
        appServiceConnection = triggerDetails.AppServiceConnection;
        appServiceConnection.RequestReceived += Inbound.OnRequestReceived;
        appServiceConnection.ServiceClosed += OnTaskCanceled;
    }

    private void OnTaskCanceled(AppServiceConnection sender, AppServiceClosedEventArgs reason)
    {
        if (this.backgroundTaskDeferral != null)
        {
            Debug.WriteLine("ServiceClosed");
            // Complete the service deferral.
            this.backgroundTaskDeferral.Complete();
        }
    }
}

Placing a breakpoint in this function, i see that it was never triggered. The app connection is opened using the singleton pattern, and putted in a dll that i use in the client app

    public static AppServiceConnection GetDriverConnectionInstance()
    {
        if (_DriverConnectionInstance == null)
        {
            try
            {
                _DriverConnectionInstance = OpenDriverConnection().AsTask().GetAwaiter().GetResult();
            }
            catch
            {

            }
        }
        return _DriverConnectionInstance;   
    }

I also add a Request to the service that toggles a led, and i noticed that the led status changes but the response from the app service is still "Failure" and the message is null.

回答1:

The AppService has a default lifetime of 25sec, unless it is being requested by the foreground experience. When the service shuts down the connection, your client process will receive the ServiceClosed event, so you know you will need to reopen the connection the next time you want to send a request.

来源：https://stackoverflow.com/questions/50907993/uwp-appserviceconnection-sendresponseasync-returns-appserviceresponsestatus-fa