C#•3y ago

WPF MVVM PropertyChangedEventHandler always null - Not updating Binding [Answered]

I have a MainWindow which loads a view (UserControl) in a ContentControl as Content per Binding to an Property in my MainViewModel. The initial Load works fine but changes made afterwards don't get published to the MainView. OnPropertyChanged is being called- and INotifyPropertyChanged is implemented via an "ObservableObject" Wrapper-Class. The PropertyChangedEventHandler is always null the View never attaches to it. My MainWindow

<Window x:Class="_505_GUI_Battleships.MainWindow"
       [...]>

    <Window.DataContext>
        <viewModel:MainViewModel/>
    </Window.DataContext>

    <Grid Margin="0 2 0 0">
    [...]
        <ContentControl Grid.Row="1" Content="{Binding Path=CurrentView, UpdateSourceTrigger=PropertyChanged}" />
    </Grid>
</Window>

<Window x:Class="_505_GUI_Battleships.MainWindow"
       [...]>

    <Window.DataContext>
        <viewModel:MainViewModel/>
    </Window.DataContext>

    <Grid Margin="0 2 0 0">
    [...]
        <ContentControl Grid.Row="1" Content="{Binding Path=CurrentView, UpdateSourceTrigger=PropertyChanged}" />
    </Grid>
</Window>

My ViewModel

public class MainViewModel : ObservableObject
{
    private readonly MainModel _mainModel;

    internal readonly DummyViewModel? DummyVm;
    internal readonly StartViewModel? StartVm;

    private object _currentView = null!;

    public object CurrentView
    {
        get => _currentView;
        set => Update(ref _currentView, value);
    }

    public ICommand StartViewModelCommand => new RelayCommand(_ => _mainModel.ChangeView(ViewChanged, StartVm));
    [...]

    public MainViewModel()
    {
        _mainModel = new MainModel();

        StartVm = new StartViewModel();
        DummyVm = new DummyViewModel();

        CurrentView = StartVm;
    }

    private void ViewChanged(object view)
    {
        CurrentView = view;
    }
}

public class MainViewModel : ObservableObject
{
    private readonly MainModel _mainModel;

    internal readonly DummyViewModel? DummyVm;
    internal readonly StartViewModel? StartVm;

    private object _currentView = null!;

    public object CurrentView
    {
        get => _currentView;
        set => Update(ref _currentView, value);
    }

    public ICommand StartViewModelCommand => new RelayCommand(_ => _mainModel.ChangeView(ViewChanged, StartVm));
    [...]

    public MainViewModel()
    {
        _mainModel = new MainModel();

        StartVm = new StartViewModel();
        DummyVm = new DummyViewModel();

        CurrentView = StartVm;
    }

    private void ViewChanged(object view)
    {
        CurrentView = view;
    }
}

public sealed class MainModel
{
    public void ChangeView(Action<object> action, object? param)
    {
        if ( param != null )
            action(param);
    }
}

public sealed class MainModel
{
    public void ChangeView(Action<object> action, object? param)
    {
        if ( param != null )
            action(param);
    }
}

21 Replies

AfterLifeOP•3y ago

RelayCommand Class

public class RelayCommand : ICommand
{
    private readonly Action<object?> _action;

    public RelayCommand(Action<object?> action)
    {
        _action = action;
    }

    public void Execute(object? parameter)
    {
        _action(parameter);
    }

    public bool CanExecute(object? parameter)
    {
        return true;
    }

    public event EventHandler? CanExecuteChanged
    {
        add { }
        remove { }
    }
}

public class RelayCommand : ICommand
{
    private readonly Action<object?> _action;

    public RelayCommand(Action<object?> action)
    {
        _action = action;
    }

    public void Execute(object? parameter)
    {
        _action(parameter);
    }

    public bool CanExecute(object? parameter)
    {
        return true;
    }

    public event EventHandler? CanExecuteChanged
    {
        add { }
        remove { }
    }
}

ObservableObject Class

public class ObservableObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;

    protected void Update<T>(ref T field, T value, [CallerMemberName] string? propertyName = null)
    {
        if (EqualityComparer<T>.Default.Equals(field, value))
            return;
        field = value;
        OnPropertyChanged(propertyName);
    }

    private void OnPropertyChanged([CallerMemberName] string? propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class ObservableObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;

    protected void Update<T>(ref T field, T value, [CallerMemberName] string? propertyName = null)
    {
        if (EqualityComparer<T>.Default.Equals(field, value))
            return;
        field = value;
        OnPropertyChanged(propertyName);
    }

    private void OnPropertyChanged([CallerMemberName] string? propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

By setting the

   public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainViewModel();
    }

   public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new MainViewModel();
    }

in code instead of xaml the EventHandler isn't null anymore. But the Content is still not updated even thou the Value of the Binded Property changed

canton7•3y ago

UpdateSourceTrigger is wrong -- that's about when the ContentControl writes back to the CurrentView property (the "source"), which it should never do. I don't think that's your issue though

But the Content is still not updated even thou the Value of the Binded Property changed

Are you sure the binding isn't updating? Note that you're binding a ViewModel, but the ContentControl only knows how to display UI elements (i.e. views), so it isn't going to show anything when you give it a ViewModel to display

AfterLifeOP•3y ago

Almost forgot, ViewModels and Views are linked via App.xaml DataTemplates

<Application x:Class="_505_GUI_Battleships.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:view="clr-namespace:_505_GUI_Battleships.MVVM.View"
             xmlns:viewModel="clr-namespace:_505_GUI_Battleships.MVVM.ViewModel"
             xmlns:mainView="clr-namespace:_505_GUI_Battleships"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Style/MenuButtonStyle.xaml" />
                <ResourceDictionary Source="Style/TaskButtonStyle.xaml" />
            </ResourceDictionary.MergedDictionaries>
            <DataTemplate DataType="{x:Type viewModel:MainViewModel}">
                <mainView:MainWindow />
            </DataTemplate>
            <DataTemplate DataType="{x:Type viewModel:StartViewModel}">
                <view:StartView />
            </DataTemplate>
            <DataTemplate DataType="{x:Type viewModel:DummyViewModel}">
                <view:DummyView />
            </DataTemplate>
        </ResourceDictionary>
    </Application.Resources>
</Application>

<Application x:Class="_505_GUI_Battleships.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:view="clr-namespace:_505_GUI_Battleships.MVVM.View"
             xmlns:viewModel="clr-namespace:_505_GUI_Battleships.MVVM.ViewModel"
             xmlns:mainView="clr-namespace:_505_GUI_Battleships"
             StartupUri="MainWindow.xaml">

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Style/MenuButtonStyle.xaml" />
                <ResourceDictionary Source="Style/TaskButtonStyle.xaml" />
            </ResourceDictionary.MergedDictionaries>
            <DataTemplate DataType="{x:Type viewModel:MainViewModel}">
                <mainView:MainWindow />
            </DataTemplate>
            <DataTemplate DataType="{x:Type viewModel:StartViewModel}">
                <view:StartView />
            </DataTemplate>
            <DataTemplate DataType="{x:Type viewModel:DummyViewModel}">
                <view:DummyView />
            </DataTemplate>
        </ResourceDictionary>
    </Application.Resources>
</Application>

I can only see the Value change in the Property

canton7•3y ago

Where's that? Ah, you said App.xaml I still stand by my question -- do you know whether the binding's getting updated? It might be the ViewModel->View stuff that's breaking You can turn up the binding debug level, put a converter on it, etc Also, https://github.com/canton7/Stylet 🙂

AfterLifeOP•3y ago

I thought i know but the issues i have gets me questioning myself right now. I'm confused about the Fact that the ContentControl loads the StartVm Viewmodel as the View I've assigned in the App.xaml correctly. Same happens when i use DummyVm as the initial Value in MainViewModel's Constructor. Only after changing the Value the issue persists, which is why i can't wrap my head around it. I can verify that the Value of the Binded Property Changed to the Correct Value; and the PropertyChanged event is being fired with "something" attached to it. If the initial Load works with the viewmodels as the values for "CurrentView"; shouldn't the same work via an Command updating the Binding? Thank you, I'll take a look later today

canton7•3y ago

I'm confused about the Fact that the ContentControl loads the StartVm Viewmodel as the View I've assigned in the App.xaml correctly.

Good point.

If the initial Load works with the viewmodels as the values for "CurrentView"; shouldn't the same work via an Command updating the Binding?

Yeah, it should do Next step it to turn up the binding debug level I think Debug -> Options -> Debugging -> Output Window -> Data Binding, set to something high

canton7•3y ago

canton7•3y ago

(then check the Output window)

AfterLifeOP•3y ago

The Clicked event get's through and the Value is changed.. Except of the initial loading i don't get any logs about Bindings doing anything. LogLevel is set to All

AfterLifeOP•3y ago

Alternatively tried using the PresentationTraceSources.TraceLevel=High - Still the same no single Log message

<ContentControl Grid.Row="1" Content="{Binding CurrentView, PresentationTraceSources.TraceLevel=High }" />

<ContentControl Grid.Row="1" Content="{Binding CurrentView, PresentationTraceSources.TraceLevel=High }" />

AfterLifeOP•3y ago

The Changed Is being executed

AfterLifeOP•3y ago

Issue resolved.. I was just dumb trying to update a MainViewModel instance with another one..

canton7•3y ago

Cool! What do you meian, "with another one"?

AfterLifeOP•3y ago

Sorry for the late reply, The issue was in my StartView that has a DataContext set to MainViewModel.. I've ended up creating a "middle-man" class like that

using System;
using System.Collections.Generic;
using _505_GUI_Battleships.MVVM.ViewModel;

namespace _505_GUI_Battleships.Core;

public sealed class ChangeViewModel
{
    public static event EventHandler<object>? ViewChanged;

    public enum ViewType
    {
        Dummy = 0,
        Start = 1
    }

    private static readonly Dictionary<ViewType, Type> Views = new()
    {
        { ViewType.Dummy, typeof(DummyViewModel) },
        { ViewType.Start, typeof(StartViewModel) }
    };

    public static void ChangeView(ViewType viewType)
    {
        var view = Activator.CreateInstance(Views[viewType]);
        if ( view == null ) return;
            OnViewChanged(view);
    }

    private static void OnViewChanged(object e)
    {
        ViewChanged?.Invoke(null, e);
    }
}

using System;
using System.Collections.Generic;
using _505_GUI_Battleships.MVVM.ViewModel;

namespace _505_GUI_Battleships.Core;

public sealed class ChangeViewModel
{
    public static event EventHandler<object>? ViewChanged;

    public enum ViewType
    {
        Dummy = 0,
        Start = 1
    }

    private static readonly Dictionary<ViewType, Type> Views = new()
    {
        { ViewType.Dummy, typeof(DummyViewModel) },
        { ViewType.Start, typeof(StartViewModel) }
    };

    public static void ChangeView(ViewType viewType)
    {
        var view = Activator.CreateInstance(Views[viewType]);
        if ( view == null ) return;
            OnViewChanged(view);
    }

    private static void OnViewChanged(object e)
    {
        ViewChanged?.Invoke(null, e);
    }
}

The MainViewModel Subscribes like that

public MainViewModel()
    {
        ChangeViewModel.ViewChanged += (_, view) =>
        {
            CurrentView = view;
        };
    }

public MainViewModel()
    {
        ChangeViewModel.ViewChanged += (_, view) =>
        {
            CurrentView = view;
        };
    }

And the StartViewModel Executes a Command like that

public static ICommand DummyViewModelCommand => new RelayCommand(_ => ChangeViewModel.ChangeView(ChangeViewModel.ViewType.Dummy));

public static ICommand DummyViewModelCommand => new RelayCommand(_ => ChangeViewModel.ChangeView(ChangeViewModel.ViewType.Dummy));

Beforehand the StartView wrote to a different instance of MainViewModel instead of StartViewModel

canton7•3y ago

This is getting quite painful, haha. I'm not sure I follow why StartView knows anything about MainViewModel at all

AfterLifeOP•3y ago

It doesn't anymore.. That was my mistake.. Because I've had a flaw in thoughts in how to design my MainViewModel. Now that i correctly use MVVM everything works out fine!

canton7•3y ago

A decent mvvm framework will remove this clutter, and handle the mapping of ViewModels -> views for you

AfterLifeOP•3y ago

Yes indeed, the issue with that is that i don't know the restrictions of this project yet. This is going to be a Project for my Collage Module "Graphical User Interfaces" and the Proffesor hasn't disclosed rules about using Frameworks yet. https://github.com/xAfterLife/505-GUI

GitHub

GitHub - xAfterLife/505-GUI: Re-Creating Battle-Ships in C# using t...

Re-Creating Battle-Ships in C# using the WPF Framework and MVVM - GitHub - xAfterLife/505-GUI: Re-Creating Battle-Ships in C# using the WPF Framework and MVVM

canton7•3y ago

Aah, fair enough

AfterLifeOP•3y ago

Never the less thank you for your help! Have a wonderfull day/evening 🙂

Accord•3y ago

✅ This post has been marked as answered!

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WPF MVVM PropertyChangedEventHandler always null - Not updating Binding [Answered]

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