IValueConverter not called on RelativeSource Ancestor - wpf

Question

I am trying to set the color of an SVG in a toggle button whenever the forecolor of the button is changed (e.g. on hover or button pressed). The interface of the SVGImage class requires to wrap the brush into a dictionary and assign it to it's CustomBrushes property. I have written a value converter for this. However, the Convert method of this converter is not called on events like mouse hovering or if the button is pressed although the text below the image changes it's color.
As I understand I have to set the relative source for the binding like this:
<ToggleButton Style="{StaticResource MyToggleButtonStyle}">
<StackPanel>
<svg:SVGImage Source="/CommonResources;component/Svg/MyIcon.svg"
CustomBrushes="{Binding
Path=Foreground,
RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ToggleButton}},
Converter={StaticResource BrushToDictionaryConverter}}" />
<TextBlock>Button Caption</TextBlock>
</StackPanel>
</ToggleButton>
For completeness although the problem should be in the first snippet. The style looks like this:
<Style x:Key="MyToggleButtonStyle" TargetType="ToggleButton">
<Setter Property="Foreground" Value="White" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Background="{TemplateBinding Background}">
<ContentPresenter/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Foreground" Value="Yellow" />
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter Property="Foreground" Value="Red" />
</Trigger>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Foreground" Value="Green" />
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsChecked" Value="True"/>
<Condition Property="IsPressed" Value="True"/>
</MultiTrigger.Conditions>
<MultiTrigger.Setters>
<Setter Property="Foreground" Value="Red" />
</MultiTrigger.Setters>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
and the converter like this:
/// <summary>
/// Converter to wrap a color into a dictionary.
/// </summary>
public class BrushToDictionaryConverter : IValueConverter
{
/// <summary>
/// Returns a dictionary containing a brush as value and
/// a string representation of the brush as key.
/// </summary>
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is SolidColorBrush scb)
return new Dictionary<string, Brush> { { scb.Color.ToString(), scb } };
else if (value is Brush b)
return new Dictionary<string, Brush> { { "custom", b } };
return new Dictionary<string, Brush> { { "empty", new SolidColorBrush(Colors.Transparent) } };
}
/// <summary>
/// Returns a brush from the dictionary assuming the provided value is a dictionary containing
/// brushes as values.
/// </summary>
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is Dictionary<string, Brush> dict && dict.Any())
return dict.First().Value;
return new SolidColorBrush(Colors.Transparent);
}
}
Does anyone see why the Convert method is not even called although the style is working and the button's caption changes it's color on user interaction?

Answer 1

I found the solution after I added diag:PresentationTraceSources.TraceLevel=High to the binding which gave me this output:
System.Windows.Data Warning: 61 : BindingExpression (hash=14000148): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=14000148): Attach to SVGImage.SVG.SVGImage.CustomBrushes (hash=57434139)
System.Windows.Data Warning: 66 : BindingExpression (hash=14000148): RelativeSource (FindAncestor) requires tree context
System.Windows.Data Warning: 65 : BindingExpression (hash=14000148): Resolve source deferred
System.Windows.Data Warning: 79 : BindingExpression (hash=14000148): Deactivate
System.Windows.Data Warning: 63 : BindingExpression (hash=14000148): Detach
Finally, I also added the source code of the DotnetProjects.SVGImage nuget package it turned out to be a glitch in the SVGImage's OnInitialized method. At least it is working if I comment the last line of that method (which probably means that I have broken something else but for me it works.)