Setting default/fallback ControlTheme

[vpenades]

I've read many tutorials and examples on how to configure a ControlTheme and almost all I've read follow these steps:

• Create a ResourceDictionary with your ControlTheme inside
• Declare the ResourceDictionary within App.axaml

So whenever you want to change the themes, you switch the files in App.axaml.

The problem is that I want to set a default or fallback ControlTheme for every control, so when no theme is chosen in the App.axaml, the control has a theme to fall back to.

The closest I've came to do this is by doing this in the static constructor of the control:

static CustomControl()
{
    
    var uri = new Uri("avares://AvaloniaControlsLibrary/CustomControl.axaml");
    var include = new ResourceInclude(uri) { Source = uri };
    var resourceDictionary = new ResourceDictionary();
    resourceDictionary.MergedDictionaries.Add(include);

    if (resourceDictionary .TryGetResource(typeof(CustomControl), null, out var res) && res is ControlTheme ctrlTheme)
    {
        ThemeProperty.OverrideDefaultValue<CustomControl>(ctrlTheme);
    }
 }

This code successfully sets the default control theme, not as a fallback but as top level theme, so It cannot be easily overriden downstream by setting another controlTheme at app.axaml

What I would like to do is:

• to assign a default ControlTheme for each control so when the renderer founds nothing else to render with, it uses it as a fallback
• I want to avoid having to require adding a resource dictionary at app.axaml to keep the library easy to use.
• I want to be able to override that default controlthem by defining another controltheme downstream, on another UserControl or at app.axaml

I have been expending several days on this, and I'm out of ideas

[timunie]

I personally have one ResourceDict in the controls lib that bundles all my custom ones via ResourceInclude. And in App.axaml I only have to add one single line 🤗.

[vpenades]

yes it's almost like creating a theme.... yet, I would like to know if it's possible to avoid even that, to just drop the library in a project and begin pulling controls here and there, yet keeping the controls thematizable

[maxkatz6]

If you want to force default ControlTheme, without any resources being referenced from the App.xaml.cs, you can do that.

ThemeProperty is a normal StyledProperty. Meaning you can override its default value.
Control.ThemeProperty.OverrideDefaultValue<CustomControl>(new ControlTheme()) from static constructor.
I haven't tried that code, but it should work without hidden issues.

var uri = new Uri("avares://AvaloniaControlsLibrary/CustomControl.axaml");

I would recommend adding x:Class to your CustomControl.axaml file, and reference it using C# class name. This helps with avoiding reflection and trimming issues.

[maxkatz6]

The closest I've came to do this is by doing this in the static constructor of the control:

Why did you have to add ResourceDictionary in between? You can use ResourceInclude.Loaded to get content with reflection. Or even better x:Class instead.

[vpenades]

@maxkatz6 in either case, although loading the theme to ThemeProperty.OverrideDefaultValue works well in setting the theme to the control, it does not act as a fallback but as a top level value.

What I mean with this is to also be able to define another theme for the control at app.xaml that overrides the default one defined by the control. And that's not happening.

If I define a default "basic" theme to the control using ThemeProperty.OverrideDefaultValue , and then I define an "advanced" theme of that same control at app.xaml, the control stays with the "basic" theme

[vpenades]

Okey... after some tinkering, I've ended up with this solution:

Creating a control and it's default theme:

<!-- CustomControlDefaultTheme.axaml -->
<ControlTheme x:Class="SomeNamespace.CustomControlDefaultTheme"
              xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:controls="using:SomeNamespace.CustomControl"
              TargetType="controls:GroupBox">

  <Setter Property="Margin" Value="4" />

</ControlTheme>

// CustomControl.cs
namespace SomeNamespace;

partial class CustomControlDefaultTheme : ControlTheme {}

class CustomControl : TemplatedControl
{
    static CustomControl()
    {
         // sets CustomControlDefaultTheme as the default theme of CustomControl
         StyledElement.ThemeProperty.OverrideDefaultValue<CustomControl>(new CustomControlDefaultTheme());
    }
}

Overriding default theme

If at a later stage we want to override or replace the default theme, things get tricky:

what does NOT work:

<UserControl>
  <CustomControl>
    <CustomControl.Resources>
         <ControlTheme x:Key={x:Type CustomControl} x:DataType="CustomControl">
             ....
         </ControlTheme>
    </CustomControl.Resources>
  </CustomControl>
</UserControl>

Actually, defining <ControlTheme x:Key={x:Type CustomControl} ... /> anywere, in places where it's supposed to override the default theme, like in app.xaml , does not take any effect.

what DOES work:

<UserControl>
  <CustomControl>
    <CustomControl.Theme>
         <ControlTheme x:DataType="CustomControl">
             ....
         </ControlTheme>
    </CustomControl.Theme>
  </CustomControl>
</UserControl>

But this can only be done within an instance of the control itself.

Essentially, you loose the "cascade" effect of Resource Dictionaries.

My conclusions

Since overriding the default theme apparently does not work if we simply set the new themes in resource dictionaries, the old fashioned
way of creating custom themes is not appropiate anymore.

When working with ControlThemes, I don't know if this approach is appropiate. If it is, which would be the right approach to define additional overriding themes? given that resourcedictionaries can't override the theme's default value?

I think there's a need for an updated, end to end tutorial of good practices to deal with ControlThemes.