Unseal Avalonia.Data.MultiBinding to allow derived classes

[Kryptos-FR]

Is your feature request related to a problem? Please describe.

Before Avalonia 12.0, it was possible to derive from Avalonia.Data.MultiBinding. One of the main benefit was the ability to make it work using a shorter and user-friendlier syntax for multi bindings. It was also based on the same feature being possible in WPF using a similar derivation. Since we are porting a lot of code from WPF (for the cross-platform version of the Stride GameStudio), having the same ability was a huge benefit.

Here is an example:

<Button IsVisible="{sd:MultiBinding {Binding ActiveVersion.DownloadCommand.IsEnabled},
                                    {Binding !ActiveVersion.CanStart},
                                     Converter={sd:AndMulti}}" />

The equivalent using allowed syntax is:

<Button>
  <Button.IsVisible>
    <MultiBinding Converter="{sd:AndMulti}">
      <Binding Path="ActiveVersion.DownloadCommand.IsEnabled" />
      <Binding Path="!ActiveVersion.CanStart" />
    </MultiBinding>
  </Button.IsVisible>
</Button>

It is uglier, more verbose and less practical. Now imagine if you have dozen of properties of a object where you need multi bindings.

Note that it was working fine in Avalonia 11 with compiled bindings.

Describe the solution you'd like

Remove the sealed modifer from Avalonia.Data.MultiBinding to restore the ability to derive from it.

Alternatively, would you consider accepting a PR that adds constructors to Avalonia.Data.MultiBinding as well as an implementation of the ProvideTypedValue() method so that we don't have to use the full-form which is unpractical.

Describe alternatives you've considered

Stay forever on Avalonia 11.

Additional context

Here is how it was implemented in WPF: Stride.Core.Presentation.Wpf/MarkupExtensions/MultiBinding.cs as well as in Avalonia 11: Stride.Core.Presentation.Avalonia/MarkupExtensions/MultiBinding.cs

This is somehow related to #17621, though the syntax is different.

[timunie]

I like the syntax. Let's see if we can agree on an api shape for it.

[MrJul]

For advanced scenarios, prefer authoring your own markup extension over inheriting from the core types.
We want to keep those types under control for any future improvements.

Example:

public class AdvancedMultiBindingEx2Extension
{
    public AdvancedMultiBindingEx2Extension(BindingBase binding1, BindingBase binding2)
    {
        Bindings.Add(binding1);
        Bindings.Add(binding2);
    }

    [Content, AssignBinding]
    public IList<BindingBase> Bindings { get; set; } = new List<BindingBase>();

    public IMultiValueConverter? Converter { get; set; }

    // other properties omitted for brevity

    public MultiBinding ProvideTypedValue()
    {
        var multiBinding = new MultiBinding { Converter = Converter };

        foreach (var binding in Bindings)
            multiBinding.Bindings.Add(binding);

        return multiBinding;
    }
}

Usage:

<TextBlock Text="{local:AdvancedMultiBindingEx2 {Binding Value1}, {Binding Value2}, Converter={StaticResource SumConverter}}" />

[Kryptos-FR]

@MrJul I'll try that. Thanks for the suggestion.

[Kryptos-FR]

I managed to make it work. Thanks for the help.