C#•2y ago

❔ WPF multiple data contexts

Hi! In my wpf app i need to have view-specific commands but i am already using my separate ViewModel (MVVM) as a data context? How can I achieve it wanting a button do something view-specific but also something on the model?

<Button>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Click">
            <i:InvokeCommandAction Command="{Binding ViewModelCommand}" />
            <i:InvokeCommandAction Command="{Binding ViewCommand}" /> //
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Button>

<Button>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Click">
            <i:InvokeCommandAction Command="{Binding ViewModelCommand}" />
            <i:InvokeCommandAction Command="{Binding ViewCommand}" /> //
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Button>

21 Replies

Indeed•2y ago

this works for the Click event handler

<Button Click="SayHello" Background="{StaticResource BrushBgHover}">
    <TextBlock Text="{Binding Message}" />
</Button>

<Button Click="SayHello" Background="{StaticResource BrushBgHover}">
    <TextBlock Text="{Binding Message}" />
</Button>

But binding to message does not work

public partial class WeatherHomeView : UserControl {
    public string Message { get; set; } = "Hi";
    public WeatherHomeView() {
        InitializeComponent();
    }

    public void SayHello(object sender, RoutedEventArgs e) {
        MessageBox.Show("Hello from the view!");
    }
}

public partial class WeatherHomeView : UserControl {
    public string Message { get; set; } = "Hi";
    public WeatherHomeView() {
        InitializeComponent();
    }

    public void SayHello(object sender, RoutedEventArgs e) {
        MessageBox.Show("Hello from the view!");
    }
}

I've tried doing it like this

<Button Click="SayHello" Background="{StaticResource BrushBgHover}">
    <Button.DataContext>
        <local:WeatherHomeView/>
    </Button.DataContext>
    <TextBlock Text="{Binding Message}" />
</Button>

<Button Click="SayHello" Background="{StaticResource BrushBgHover}">
    <Button.DataContext>
        <local:WeatherHomeView/>
    </Button.DataContext>
    <TextBlock Text="{Binding Message}" />
</Button>

only to get stackoverflow exception looking into something like this

    <UserControl.DataContext>
        <local:WeatherHomeView x:Name="SelfContext"/>
    </UserControl.DataContext>

    <UserControl.DataContext>
        <local:WeatherHomeView x:Name="SelfContext"/>
    </UserControl.DataContext>

mikernet•2y ago

Not sure I understanding the question The Message prop should be on the VM And Click should fire a handler just on the VM Why do you need behavior in the view The point of a VM is that it encapsulates the behavior And the view just binds whatever it needs on the VM If you need to be showing message boxes, the right way to do that is to do it from the VM by injecting an IAlert implementation and then calling IAlert.ShowMessage() or whatever

Indeed•2y ago

For my specific situation I have a listview and on scroll all the way to left/right i want to fetch more data my current solution

public partial class WeatherHomeView : UserControl, INotifyPropertyChanged {
    public event PropertyChangedEventHandler? PropertyChanged;
    private void OnPropertyChanged(string name) {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }

    private double _maxScrollOffset;
    public double MaxScrollOffset {
        get => _maxScrollOffset;
        set {
            if (value == _maxScrollOffset) {
                return;
            }

            _maxScrollOffset = value;
            OnPropertyChanged(nameof(MaxScrollOffset));
        }
    }

    private double _scrollOffset;
    public double ScrollOffset {
        get => _scrollOffset;
        set {
            if (_scrollOffset == value) {
                return;
            }

            _scrollOffset = value;
            OnPropertyChanged(nameof(ScrollOffset));
        }
    }

    public WeatherHomeView() {
        InitializeComponent();
    }

    private void ScrollChanged(object sender, ScrollChangedEventArgs e) {
        Debug.WriteLine($"ScrollChanged  Offset: {e.HorizontalOffset} VpWidth: {e.ViewportWidth} ExWidth: {e.ExtentWidth}");
        ScrollOffset = e.HorizontalOffset;
        MaxScrollOffset = e.ExtentWidth - e.ViewportWidth;
    }
}

public partial class WeatherHomeView : UserControl, INotifyPropertyChanged {
    public event PropertyChangedEventHandler? PropertyChanged;
    private void OnPropertyChanged(string name) {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }

    private double _maxScrollOffset;
    public double MaxScrollOffset {
        get => _maxScrollOffset;
        set {
            if (value == _maxScrollOffset) {
                return;
            }

            _maxScrollOffset = value;
            OnPropertyChanged(nameof(MaxScrollOffset));
        }
    }

    private double _scrollOffset;
    public double ScrollOffset {
        get => _scrollOffset;
        set {
            if (_scrollOffset == value) {
                return;
            }

            _scrollOffset = value;
            OnPropertyChanged(nameof(ScrollOffset));
        }
    }

    public WeatherHomeView() {
        InitializeComponent();
    }

    private void ScrollChanged(object sender, ScrollChangedEventArgs e) {
        Debug.WriteLine($"ScrollChanged  Offset: {e.HorizontalOffset} VpWidth: {e.ViewportWidth} ExWidth: {e.ExtentWidth}");
        ScrollOffset = e.HorizontalOffset;
        MaxScrollOffset = e.ExtentWidth - e.ViewportWidth;
    }
}

                    <ListView
                        x:Name="ForecastsListView"
                        Grid.RowSpan="3"
                        Grid.Column="1"
                        HorizontalContentAlignment="Stretch"
                        VerticalContentAlignment="Stretch"
                        Background="Transparent"
                        BorderThickness="0"
                        ItemsSource="{Binding Weather}"
                        ScrollViewer.ScrollChanged="ScrollChanged"
                        SelectedItem="{Binding SelectedWeather}">

                        <ListView.ItemsPanel>
                                 ...
                        </ListView.ItemsPanel>
                        <ListView.ItemTemplate>
                                                     ...
                        </ListView.ItemTemplate>

                        <i:DataTrigger
                            Binding="{Binding ScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}"
                            Value="{Binding MaxScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}">
                            <i:InvokeCommandAction Command="" />
                        </i:DataTrigger>
                        <i:DataTrigger
                            Binding="{Binding ScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}"
                            Value="0">
                            <i:InvokeCommandAction Command="" />
                        </i:DataTrigger>
                    </ListView>

                    <ListView
                        x:Name="ForecastsListView"
                        Grid.RowSpan="3"
                        Grid.Column="1"
                        HorizontalContentAlignment="Stretch"
                        VerticalContentAlignment="Stretch"
                        Background="Transparent"
                        BorderThickness="0"
                        ItemsSource="{Binding Weather}"
                        ScrollViewer.ScrollChanged="ScrollChanged"
                        SelectedItem="{Binding SelectedWeather}">

                        <ListView.ItemsPanel>
                                 ...
                        </ListView.ItemsPanel>
                        <ListView.ItemTemplate>
                                                     ...
                        </ListView.ItemTemplate>

                        <i:DataTrigger
                            Binding="{Binding ScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}"
                            Value="{Binding MaxScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}">
                            <i:InvokeCommandAction Command="" />
                        </i:DataTrigger>
                        <i:DataTrigger
                            Binding="{Binding ScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}"
                            Value="0">
                            <i:InvokeCommandAction Command="" />
                        </i:DataTrigger>
                    </ListView>

since I need to use the event to get the value from view specific event and to save it i have to bind to my viewClass for these instances as seen i was able to use ancestorType for it although it is a bit cumbersome to write out Binding="{Binding ScrollOffset, RelativeSource={RelativeSource AncestorType=local:WeatherHomeView}}" other context is assumed to be VM through inheritance I'll gladly take criticism of how it could be done otherwise with respect to mvvm (Gonna think about moving it to VM tmrrw since every collection display afaik has scrollviewer so might be useful to put it there)

mikernet•2y ago

I would still do this in the VM I made this for exactly this kind of scenario: https://github.com/Singulink/Singulink.WPF.Data.MethodBinding Towards the bottom you can see this example:

You can even bind to properties on the event args themselves and pass them as parameters:

You can even bind to properties on the event args themselves and pass them as parameters:

public class ViewModel
{
    public void UpdateSize(Size newSize) { }
}

public class ViewModel
{
    public void UpdateSize(Size newSize) { }
}

<!-- Passes in SizeChangedEventArgs.NewSize as the parameter -->
<Canvas SizeChanged="{s:MethodBinding UpdateSize, {s:EventArgs NewSize}}" />

<!-- Passes in SizeChangedEventArgs.NewSize as the parameter -->
<Canvas SizeChanged="{s:MethodBinding UpdateSize, {s:EventArgs NewSize}}" />

So your VM still stays view agnostic and doesn't need to reference any UI assemblies with the event args types in them Your VM method would probably be something like

public void UpdateScroll(double horizontalOffset, double extentWidth, double viewportWidth)
{
}

public void UpdateScroll(double horizontalOffset, double extentWidth, double viewportWidth)
{
}

And then your binding would be:

<ListView ScrollViewer.ScrollChanged="{s:MethodBinding UpdateScroll, {s:EventArgs HorizontalOffset}, {s:EventArgs ExtentWidth}, {s:EventArgs ViewportWidth}" />

<ListView ScrollViewer.ScrollChanged="{s:MethodBinding UpdateScroll, {s:EventArgs HorizontalOffset}, {s:EventArgs ExtentWidth}, {s:EventArgs ViewportWidth}" />

You can also call VM methods from the view codebehind, FYI.

Indeed•2y ago

Oh, that looks great. Will definitely be useful <3. However I think I would like to settle in the end with VM having a method for fetching data and view specifying when it should be called (maybe with a button click, maybe on scroll to the end, generally something view specific)

and on scroll all the way to left/right i want to fetch more data

However you got me interested into calling VM methods from VC (viewclass), how is it achievable? My context from MainWindow is passed this way

    mc:Ignorable="d">
    <Grid d:DataContext="{d:DesignInstance vms:MainViewModel}">
        <Grid.Resources>
            <DataTemplate DataType="{x:Type vms:WeatherHomeViewModel}">
                <views:WeatherHomeView />
            </DataTemplate>
            <DataTemplate DataType="{x:Type vms:WelcomeViewModel}">
                <views:WelcomeView />
            </DataTemplate>
        </Grid.Resources>

        <ContentControl Content="{Binding CurrentViewModel}" />
    </Grid>

    mc:Ignorable="d">
    <Grid d:DataContext="{d:DesignInstance vms:MainViewModel}">
        <Grid.Resources>
            <DataTemplate DataType="{x:Type vms:WeatherHomeViewModel}">
                <views:WeatherHomeView />
            </DataTemplate>
            <DataTemplate DataType="{x:Type vms:WelcomeViewModel}">
                <views:WelcomeView />
            </DataTemplate>
        </Grid.Resources>

        <ContentControl Content="{Binding CurrentViewModel}" />
    </Grid>

do you mean this way?

        var x = this.DataContext as WeatherHomeViewModel;
        x.method();

        var x = this.DataContext as WeatherHomeViewModel;
        x.method();

mikernet•2y ago

I would write a typed prop for the VM in this case that assigns the DC but that's the idea yeah

Indeed•2y ago

oh okay so it would be something here? <ContentControl SomePropertyToStoreViewModel="{Binding CurrentViewModel}" />

mikernet•2y ago

As for putting behavior in views - I think this boils down to a difference in controls vs views I don't put code behind into views If there's any reason for codebehind then I make a control that I can put into a view Like a real reusable templated control, not a user control I.e. "InfiniteScollListView"

Indeed•2y ago

Oh that's an interesting view, that makes a lot of sense. I do that however it is only at the point of cleaning up my code, when prototyping all goes where it can Hmm, not an user control? templated control haven't heard about it, will definitely have to read on it. Looking at templates in vs only thing sounding similiarily is custom control

mikernet•2y ago

Like, I would subclass ListView, not UserControl, for example

Indeed•2y ago

oh, i haven't seen it be recommended anywhere 👀 , even wpf unleashed didn't mention a word. Seen most people recommend attached properties

mikernet•2y ago

That can work in some cases as well Sometimes a new control is cleaner/better though I mean like

public WeatherHomeViewModel ViewModel
{
    get => DataContext as WeatherHomeViewModel;
    set => DataContext = value;
}

public WeatherHomeViewModel ViewModel
{
    get => DataContext as WeatherHomeViewModel;
    set => DataContext = value;
}

On the view

Indeed•2y ago

oh simply extracting the cast, okay

mikernet•2y ago

Yeah I think it's generally better to register DPs on controls instead of using INPC as well INPC should really only be for VMs

Indeed•2y ago

do DPs two-way bound auto implement INPC?

mikernet•2y ago

If you mean can DPs be two-way bound then yeah, that's how the existing DPs on your controls work They don't implement INPC, it's a different mechanism Mixing the two into one type is a bit of a code smell If its a dependency object with DPs on it then no real reason to use INPC as well DPs are more powerful INPC is a simplified version of DPs for VMs

Indeed•2y ago

Ye, I've had to create my own DP snippet due to using them a lot coz VS still using "propertyName" instead of nameof and 0 instead of default(type) XD

mikernet•2y ago

Yeah

Indeed•2y ago

Thank you a lot for your help! I've definitely learnt a lot from this conversation. Can't wait to try all of the things you've suggested. Some of the things you mentioned I've not seen in the books, will definitely have to write some of them down 💜<a:slickster_swim:1018447771830857728>

mikernet•2y ago

No probskis 🙂

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ WPF multiple data contexts