Multibinding a style class in code

[DarkMortainius]

I'm trying to create a data template in code for a TreeDataGrid, and I want to have my data cells have a style class added if the user has write access to a specific data cell. Essentially, I want to turn this XAML into code, and I'm not sure how (specifically the part concerning the style class). I've already got the converter working for other items. I just need to convert this portion into code

<DataTemplate x:DataType="datapoint:DataPoint">                        
    <Label Content="Sample Text">
        <Label.Classes.Edit>
            <MultiBinding Converter="{StaticResource WriteAccessConverter}">
                <Binding Path=""/>
                <Binding Path="$parent[TreeDataGrid].((vm:ParameterTabViewModel)DataContext).ActiveUser"/>
            </MultiBinding>
        </Label.Classes.Edit>
    </Label>
</DataTemplate>

This is the closest I've been able to get (adding the class without the conditional binding)

var cellTemplate = new FuncDataTemplate<DataRow>((_, _) => {
    var label = new Label { [!Label.ContentProperty] = new Binding(bindingString + ".Text") };
    label.Classes.Add("Edit");
    return new ContentControl() { Content = label };                    
});

[stevemonaco]

Remove the Label. part. eg.

  <Window.Styles>
    <Style Selector="TextBlock.big">
      <Setter Property="FontSize" Value="36" />
    </Style>
  </Window.Styles>

  <TextBlock x:Name="text" Text="Testing MultiBinding">
    <Classes.big>
      <MultiBinding Converter="{x:Static BoolConverters.And}">
        <Binding Path="IsA" />
        <Binding Path="IsB" />
      </MultiBinding>
    </Classes.big>
  </TextBlock>

You also can't use syntax the same way you do in the {Binding} MarkupExtension. You need to fill out the individual properties when using <Binding /> directly, AFAIK. ie. You need to use RelativeSource instead of $parent[TreeDataGrid] and whatnot. If you were trying to do a self-binding in the first <Binding />, then that might still be unsupported. You might check out issues regarding that.

You could also write your own MarkupExtension instead which can support bindings (via {Binding}) as parameters.

[DarkMortainius]

Ok, so my syntax was a little off. But that wasn't what my question was. I'm trying to do this in C# code, not XAML, and I'm not sure how. I can't find any examples of assigning bindings to classes in C# code.

[stevemonaco]

I see, sorry about that. You need to work with StyledElement.BindClass which is an extension method in the Avalonia namespace. Converting my example...

public MainWindow()
{
    InitializeComponent();

    var mb = new MultiBinding()
    {
        Bindings = [new Binding("IsA"), new Binding("IsB")],
        Converter = BoolConverters.And
    };

    text.BindClass("big", mb, null);
}

<Window.Styles>
  <Style Selector="TextBlock.big">
    <Setter Property="FontSize" Value="36" />
  </Style>
</Window.Styles>

<Grid RowDefinitions="auto,*">
  <TextBlock x:Name="text" Text="Testing MultiBinding" />
</Grid>

[DarkMortainius]

That looks exactly like what I need, I'll give it a try. Thank you!

[DarkMortainius]

Do you know what anchor is supposed to be? It's non-nullable, so the compiler is complaining if I try to pass null, and the documentation is no help.

BindClass(IStyledElement, string, IBinding, object)

[DarkMortainius]

After digging into the code, it looks like it doesn't matter what anchor is, it never actually gets used by anything...
Not sure why it even needs to be there.

[stevemonaco]

That's what I got out of it, too. I think anchor was used in the old IBinding system in SourceNode to do something with the binding Source, but now it seems to have faded away with IBinding2 and can't be removed until v12 because it's a breaking change.

I'll post an issue about changing it to object? instead. Maybe it can get into 11.1.

Also, the reference docs site shouldn't be used. It's still on v0.10.x docs as there are some technical issues with re-generating them. You'll need to check source.

[hydrolizer]

My solution:

using Avalonia;
using Avalonia.Xaml.Interactivity;

namespace Attached;

public sealed class StyleClassBinding: Behavior<StyledElement>
{
  public static readonly StyledProperty<bool> IsSuitableProperty
    = AvaloniaProperty.Register<StyleClassBinding, bool>(nameof(IsSuitable));
  
  public bool IsSuitable
  {
    get => GetValue(IsSuitableProperty);
    set => SetValue(IsSuitableProperty, value);
  }

  public string? ClassName { get; set; }

  protected override void OnAttached()
  {
    base.OnAttached();
    if (string.IsNullOrWhiteSpace(ClassName)) return;
    AssociatedObject?.BindClass(ClassName, this.GetObservable(IsSuitableProperty).ToBinding(), null!);
  }
}

Example of use:

<Grid RowDefinitions="*,Auto">
  <StyledElement.Resources>
    <local:BooleanAndConverter x:Key="BooleanAndConverter" />
  </StyledElement.Resources>
  <StyledElement.Styles>
    <Style Selector="Border#TestBorder">
      <Setter Property="Background" Value="Aqua" />
    </Style>
    <Style Selector="Border#TestBorder.allChecked">
      <Setter Property="Background" Value="Green" />
    </Style>
  </StyledElement.Styles>
  <Border x:Name="TestBorder">
    <i:Interaction.Behaviors>
      <local:StyleClassBinding ClassName="allChecked">
        <local:StyleClassBinding.IsSuitable>
          <MultiBinding Converter="{StaticResource BooleanAndConverter}">
            <Binding ElementName="FirstCheckBox" Path="IsChecked" />
            <Binding ElementName="SecondCheckBox" Path="IsChecked" />
          </MultiBinding>
        </local:StyleClassBinding.IsSuitable>
      </local:StyleClassBinding>
    </i:Interaction.Behaviors>
  </Border>
  <StackPanel Grid.Row="1" Margin="0,5,0,0">
    <CheckBox x:Name="FirstCheckBox" Content="First" />
    <CheckBox x:Name="SecondCheckBox" Content="Second" />
  </StackPanel>
</Grid>