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c#wpfxamluser-interfacetemplate10

C# + XAML - UI didn't get updated after changes


I want to update my UI, but it didn't get updated, even when the PropertyChanged event got called. At startup i get the first values, but after changes it won't update.

While Debugging, i can see, that the values getting updated and the PropertyChanged event got fired, but the getter didn't get called.

(BTW: I'm relatively new to C#, respectively to the M-V-VM concept.)

XAML

<Page.DataContext>
    <vm:MainPageViewModel x:Name="ViewModel" />
</Page.DataContext>
...
<TextBlock x:Name="LatitudeVar" Text="{Binding LatitudeVar, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="79" Canvas.Left="189" Canvas.Top="38" />

C# (MainPageViewModel)

public class MainPageViewModel : ViewModelBase, INotifyPropertyChanged
{
    private Session _session;
    public MainPageViewModel()
    {
        _session = Session.Instance;
    }

    public static readonly MainPageViewModel Instance = new MainPageViewModel();

    public override async Task OnNavigatedToAsync(object parameter, NavigationMode mode, IDictionary<string, object> suspensionState)
    {

        // App just started, so we get GPS access and eventually initialize the client
        Session.InitializeClient();
        await StartGpsDataService();
        await Task.CompletedTask;
    }

    ...

    private string _latitude;
    public string LatitudeVar
    {
        get { return _session.Latitude; }
        set { _session.Latitude = value; NotifyPropertyChanged(); }
    }

    ...

    public async Task StartGpsDataService()
    {
        await Session.InitializeDataUpdate();
    }

     public new event PropertyChangedEventHandler PropertyChanged;

    [Annotations.NotifyPropertyChangedInvocator]
    protected virtual void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

C# (Session)

public class Session : INotifyPropertyChanged
{
    public static void InitializeClient()
    {
        MainPageViewModel mpvm = new MainPageViewModel();
        _mpvm = MainPageViewModel.Instance;
    }

    private static Session _instance;
    public static Session Instance
    {
        get
        {
            if (_instance == null)
            {
                _instance = new Session();
            }
            return _instance;
        }
    }


    private static Session _sc;
    internal static Session Sc
    {
        get { return _sc; }
        set { _sc = value; }
    }


    private static MainPageViewModel _mpvm;

    private string _latitude;
    public string Latitude
    {
        get { return _latitude; }
        set
        {
            if (_latitude == value) return; _latitude = value; RaiseOnPropertyChanged("Latitude"); }
    }

    ...

     public void UpdateGpsData(Geopoint point, Geopoint geopointOld)
    {
        _mpvm.LatitudeVar = point.Position.Latitude.ToString();
    }

    public static async Task InitializeDataUpdate()
    {
       Sc = Session.Instance;
       Sc.StartTime = DateTime.Now;
       Sc.GetPosition(Geoposition.Coordinate.Point);
    }

    public new event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void RaiseOnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

Some ways i tried before:

C# (MainPageViewModel)

private string _latitude;
public string LatitudeVar
{
    get { return _latitude; }
    set { _latitude = value; NotifyPropertyChanged("LatitudeVar"); }
}

Result: Values were not displayed.

C# (Session)

public void UpdateGpsData(Geopoint point, Geopoint geopointOld)
{
    Latitude = point.Position.Latitude.ToString();
}

Result: Values were diplayed at startup, but not updated.

EDIT: Found a solution: Some code sorting, thanks for the hint ;) Now my Session.cs is a Model. All relavant methods are now in the ViewModel. And i took care of, that only one instance exists:

public MainPageViewModel()
{
   _session = Session.Instance;
   _instance = this;
}

Solution

  • Woah, that's quite a contrived singleton pattern implementation.

    Firstly, the MainPageViewModel instance created in Session.InitializeClient is not the instance that is being used by the view. The XAML

    <Page.DataContext>
        <vm:MainPageViewModel x:Name="ViewModel" />
    </Page.DataContext>
    

    Will instantiate a new instance of MainPageViewModel so the code

    public void UpdateGpsData(Geopoint point, Geopoint geopointOld)
    {
        _mpvm.LatitudeVar = point.Position.Latitude.ToString();
    }
    

    Will have no effect on the view.

    While I would strongly encourage you to remove the attempt at a singleton for the Session and consider using a messaging pattern to communicate between system components, you should be able to get your code working by exposing the Session instance singleton as a readonly property on the MainPageViewModel and binding to that directly as follows:

    <TextBlock Text="{Binding Path=Session.Latitude, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    

    Of course, this then means your UI is writing directly to a singleton component in your system in which it's probably not desirable to implement INotifyPropertyChanged.

    As suggested, I'd strongly recommend rearchitecting and removing the singleton and/or embracing messaging between system components.