Issue with API calls; initially the API is called with blank push id and later it will call with exact push id

Question

I have implemented push notification in my project using Plugin.Firebase package. My problem is on saving the FCM id to database API.

I am generating the FCM token on login page constructor and saving it to preferences like below:

public EmailPMAPage()

public EmailPMAPage()
{
    InitializeComponent();
    GetToken();
}

private async void GetToken()
{
    try
    {
        await CrossFirebaseCloudMessaging.Current.CheckIfValidAsync();
        var fcmToken = await CrossFirebaseCloudMessaging.Current.GetTokenAsync();
        Preferences.Set("PushDeviceToken", fcmToken);
        Console.WriteLine($"FCM token::>> {fcmToken}");
    }
    catch(Exception e)
    {
        Debug.WriteLine("token exception:>>"+e);
    }
}

When the user enter email and password and tap the login button I call an API to save the device details to DB. My problem is initially the API is called with blank push id and later it will call with exact push id. But only one API call is there on the code and only once it is hitting when I add the debuggers. I tried debugging it a lot and can't find from where the blank FCM id API is calling. As a result of this, the database sometimes has no data on FCM id field.

JSON when empty FCM ID (push_id)

{
  "data": {
    "type": "devices",
    "attributes": {
      "mobile_application_key": "ea9248173e.......",
      "technology": "android",
      "push_id": "",
      "app_version": "1.2.4"
    }
  }
}

JSON with exact FCM ID (push_id)

{
  "data": {
    "id": "4662",
    "type": "devices",
    "attributes": {
      "mobile_application_key": "ea9248173e.....",
      "technology": "android",
      "push_id": "fO3uUn39............",
      "app_version": "1.2.4"
    }
  }
}

The both JSON's are different in structures, the empty push id has no id field. Below is my viewmodel:

LoginPMAViewModel.txt

The device details are saving on the UpdateDeviceTokenAsync(string deviceTokenString) function.

Answer 1

Hi @Sreenivasan, Sreejith ,

Thanks for reaching out.

Please note that since I’m working from the demo project you provided, this code is meant as reference, you may need to adapt it to your actual project.

Your GetToken() method is async void and gets called in the page constructor without being awaited. The result is:

1. Page constructor runs GetToken() (but doesn’t wait for it to finish)
2. User enters email/password and taps login
3. UpdateDevice() runs immediately
4. FCM token is still empty -> first API call goes out with push_id: ""
5. A short moment later, GetToken() completes and saves the token
6. Any later API call sends the correct token

Why the JSONs look different:

• First call -> new device record (no id)
• Second call -> updates existing record (has id)

Here's my suggested approach:

1. Ensure the FCM token is ready before login

File: EmailPMAPage.xaml.cs

protected async override void OnAppearing()
{
    base.OnAppearing();
    // Make sure FCM token is ready
    await EnsureFCMTokenIsReadyAsync();
    Task.Run(async delegate { await new DeviceInfoService().CheckServerVersionAsync(); });
    Task.Run(async delegate { await ViewModel.RefreshScreen(); });
    ViewModel.CallBackDisplayEndpoints += HandlerCallBackDisplayEndpoints;
    if(BindingContext is LoginPMAViewModel vm)
    {
        await vm.InitializeAsync();
    }
}

private async Task EnsureFCMTokenIsReadyAsync()
{
    try
    {
        var existingToken = Settings.GetProperty(Settings.PushDevicetoken);
        if (!string.IsNullOrEmpty(existingToken)) return;
        await CrossFirebaseCloudMessaging.Current.CheckIfValidAsync();
        var fcmToken = await CrossFirebaseCloudMessaging.Current.GetTokenAsync();
        if (!string.IsNullOrEmpty(fcmToken))
        {
            Settings.SetProperty(Settings.PushDevicetoken, fcmToken);
            Console.WriteLine($"FCM token retrieved: {fcmToken.Substring(0, 10)}...");
        }
    }
    catch (Exception e)
    {
        Console.WriteLine($"FCM token exception: {e.Message}");
        Debug.WriteLine(e);
    }
}

2. Update the UpdateDevice() method

File: LoginPMAViewModel.cs

public static async Task UpdateDeviceAsync()
{
    try
    {
        var pushDevicetoken = Settings.GetProperty(Settings.PushDevicetoken) ?? string.Empty;
        if (string.IsNullOrEmpty(pushDevicetoken)) return;
        var result = await UpdateDeviceTokenAsync(pushDevicetoken);
        Console.WriteLine($"Device update result: {result}");
    }
    catch (Exception ex)
    {
        Console.WriteLine($"UpdateDevice error: {ex.Message}");
    }
}

And make sure calls in Login() use await UpdateDeviceAsync() instead of UpdateDevice():

// When policies invalid
if (!isPolicyDocumentsValid) await UpdateDeviceAsync();

// Successful login
else
{
    await UpdateTimeZone(result.User);
    await UpdateDeviceAsync();
    Settings.SetProperty(Settings.SleepTime, DateTime.UtcNow.Ticks.ToString());
}

// When result is null
else
{
    await UpdateDeviceAsync();
    var master = MainService.HomePage.Value as DashboardMasterPage;
}

Hope this helps! If my answer was helpful - kindly follow the instructions here so others with the same problem can benefit as well.

Answer 2

Most likely it’s a race condition where the unawaited call to GetToken completes and updates preferences after the api call. you will need to add locking code to prevent the api call before gettoken completes.