MessagingCenter多次射击的问题

Question

我到处寻找解决问题的办法。我注意到，在我的安卓应用程序中，每当我从<button Clicked="GPSToggle_Clicked">中触发一个事件时，由于某种原因，它会增加调用我的方法的次数。所以，在我编译并在手机上加载这个之后，我点击了我的"GPSToggle_Clicked“按钮，然后停止再按那个按钮。在第一个“停止”中，我将得到以下输出的一个实例：

    ---------------------------------------------------------------Attempting string parsing
---------------------------------------------------------------Sending string to SubmitGeneratedGPX
---------------------------------------------------------------path: /storage/emulated/0/Download/GPX/2022-10-27-02-44-06.gpx
---------------------------------------------------------------GPX File creation success
---------------------------------------------------------------:RawBufferToJsonString: [{"Timestamp":"2022-10-27T18:43:52.849+00:00","Latitude":41.5263818,"Longitude":-81.6507923,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7990270853042603,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:53.696+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:54.526+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:55.374+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:56.21+00:00","Latitude":41.5263811,"Longitude":-81.650792,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7160584926605225,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2}]

每次在应用程序上单击start/stop时，我都会得到输出中的实时数据乘以自上次编译以来启动/停止的次数。

启动的主应用程序页面按钮事件：

private async void GPSToggle_Clicked(object sender, EventArgs e)
    {
        var LocationPermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.LocationAlways>();
        var FileReadPermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.StorageRead>();
        var FileWritePermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.StorageWrite>();
        if(LocationPermissionStatus == Xamarin.Essentials.PermissionStatus.Denied)
        {
            // TODO
            return;
        }
        
        // run if device is android
        if(Device.RuntimePlatform == Device.Android)
        {
            if (!CrossGeolocator.Current.IsGeolocationAvailable || !CrossGeolocator.Current.IsGeolocationEnabled)
            {
                // gps is not enabled, throw alert
                Console.WriteLine("---------------------------------------------------------------GPS is DISABLED");
                await DisplayAlert("Error", "GPS is not enabled. You must enable GPS to use this feature", "Ok");
            }
            else
            {
                // set our IsTracking = true flag
                if (!IsTracking)
                {
                    // start background listening for GPS
                    await StartListening();
                    Console.WriteLine("---------------------------------------------------------------Listening: " + CrossGeolocator.Current.IsListening);
                    StartService();
                    Console.WriteLine("---------------------------------------------------------------Service initiated");
                    IsTracking = true;
                    Console.WriteLine("---------------------------------------------------------------Tracking initiated");
                    GPSToggle.Text = "Stop Tracking";
                }
                else
                {
                    //
                    // verify that the submittal wasn't done in error, before stopping services and submitting data
                    bool DoneInError = await DisplayAlert("Finish?", "Are you sure you want to stop services and submit?", "No", "Yes");
                    if (!DoneInError)
                    {
                        await StopListening();
                        Console.WriteLine("---------------------------------------------------------------listening:" + CrossGeolocator.Current.IsListening);
                        IsTracking = false;
                        Console.WriteLine("---------------------------------------------------------------Tracking ceased");
                        // stop the gps service
                        StopService();
                        Console.WriteLine("---------------------------------------------------------------Service ceased");
                        // stop the background listening for gps
                        Console.WriteLine("---------------------------------------------------------------Attempt GPX parse from buffer obj");

                        GPSToggle.Text = "Start Tracking";
                    }
                }
            }
        }
    }

具体而言，这一行：

StartService();

在同一个类(特别是MessagingCenter.Send<> )中触发此方法，该类启动我的前台服务，将gps数据记录到缓冲区中：

private void StartService()
    {            
        var startServiceMessage = new StartServiceMessage();
        MessagingCenter.Send(startServiceMessage, "ServiceStarted");
        Preferences.Set("LocationServiceRunning", true);
        StatusLabel.Text = "Location service has been started";
        Console.WriteLine("---------------------------------------------------------------location service has been started. preferences saved");
    }

和

StopService(); 

触发此方法以停止服务并从前台检索gps缓冲区数据到主线程：

private void StopService()
    {
        var stopServiceMessage = new StopServiceMessage();
        MessagingCenter.Unsubscribe<App, List<Location>>(this, "GPXBufferData");
        MessagingCenter.Subscribe<App, List<Location>>(this, "GPXBufferData", (sender, args) =>
        {
            RawGPXData = args;
            Generate_CreateGPX_File(RawGPXData);
            RawBufferToJsonString = GPXParse.GenerateJSON_GPXPoints(RawGPXData);
            Console.WriteLine("---------------------------------------------------------------:RawBufferToJsonString: " + RawBufferToJsonString);
            PromptForSubmission_GPXPoints_API();
        });
        Console.WriteLine("--------------------------------------------------------------------------");
        MessagingCenter.Send(stopServiceMessage, "ServiceStopped");
        Preferences.Set("LocationServiceRunning", false);
        Console.WriteLine("---------------------------------------------------------------Location service stopped. preferences saved");
    }

在上面的片段中，在GPSLocationService.cs文件中订阅了这一行：

MessagingCenter.Send(stopServiceMessage, "ServiceStopped");

这是与此相关的GPSLocationService.cs文件的一部分：

public async Task Run(CancellationToken token)
    {
        int ObjCount = 0;
        await Task.Run(async () => {
            // if the task was stopped
            // check the buffer for data, if data, send to GPXGenerator
            MessagingCenter.Subscribe<StopServiceMessage>(this, "ServiceStopped",
            message =>
            {
                if (GPSBufferObj != null)
                {
                    Device.BeginInvokeOnMainThread(() =>
                    {
                        MessagingCenter.Unsubscribe<App, List<Location>>((App)Xamarin.Forms.Application.Current, "GPXBufferData");
                        MessagingCenter.Send<App, List<Location>>((App)Xamarin.Forms.Application.Current, "GPXBufferData", GPSBufferObj);

                    });
                }
            });

            return;
        }, token);
    }

我相信我已经找到了这个问题的起点。在我的StopService()方法中，我有以下一行(只是为了跟踪我在缓冲区中的位置)，它只被发送到输出一次。

Console.WriteLine("--------------------------------------------------------------------------");

但是，如果我将这一行放在我的GPSLocationService.cs文件的粘贴部分中，我将得到递增的输出。我倾向于将嵌套的任务作为问题，我用这个示例repro：https://github.com/jfversluis/XFBackgroundLocationSample编写了这个不符合逻辑的代码

Answer 1

您的代码中没有有MessagingCenter.Unsubscribe<StopServiceMessage> anywhere。StopServiceMessage是您正在累积订阅的内容。

您需要确保Unsubscribe取消订阅以前订阅的实例。在我看来，GPSLocationService有多个实例。在这种情况下，this不再引用原始实例。取消订阅不会做任何事情，除非您有最初订阅的this。

如果是这样，则创建一个GPSLocationService实例一次，并将其存储在一个静态变量中。再用一次。开始/停止它，但不要丢弃它。

或者，如果您只想从每个订阅中获得一次消息，那么在收到每条消息后立即取消订阅：

MessagingCenter.Subscribe<StopServiceMessage>(this, "ServiceStopped",
  message =>
  {
    MessagingCenter.Unsubscribe<StopServiceMessage>(this, "ServiceStopped");
    ... your code ...
  });

在任何有订阅的地方都使用相同的模式(除非像Jason建议的那样，在app开始时只订阅一次)。