How to bind a child `UserControl`'s Dependency Property, using `Style`, to a property of the host element's view-model?

Question

How to bind a child UserControl's Dependency Property, using Style, to a property of the host element's view-model?

I tried the code below, and MyBlock.BlockIsOnlyOne should be bound to the view-model's property - MyContainerViewModel.ViewModelIsOnlyOne, via the Style's Setter. But from some reason it just doesn't work - the MyBlock.BlockIsOnlyOne's value never changes, although the MyContainerViewModel.ViewModelIsOnlyOne changes...

The Container XAML:

<UserControl x:Class="MyNs.MyContainer"
             ...
             >
    <UserControl.DataContext>
        <vc:MyContainerViewModel x:Name="TheDataContext"/>
    </UserControl.DataContext>

    <Grid>
        <Grid.Resources>
            <Style TargetType="{x:Type vc:MyBlock}">
                <Setter Property="BlockIsOnlyOne" Value="{Binding ViewModelIsOnlyOne}"/>
                <!-- Tried this too, with no success: -->
                <!-- <Setter Property="BlockIsOnlyOne" Value="{Binding ViewModelIsOnlyOne, ElementName=TheDataContext}"/> -->
            </Style>
        </Grid.Resources>
        ...
        <vc:MyBlock DataContext="{Binding PortA[0]}" />
    </Grid>
</UserControl>

The Container's ViewModel (only the important part...):

[NotifyPropertyChangedAspect] // handles the INotifyPropertyChanged implementation...
class MyContainerViewModel{
    ...
    public bool ViewModelIsOnlyOne { get; private set; }
    ...
}

The MyBlock UserControl:

class MyBlock : UserControl{
    ...
    public static readonly DependencyProperty BlockIsOnlyOneProperty = DependencyProperty.Register(
        "BlockIsOnlyOne", typeof (bool), typeof (MyBlock), 
        new PropertyMetadata(default(bool), BlockIsOnlyOne_PropertyChangedCallback));

    private static void BlockIsOnlyOne_PropertyChangedCallback(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs a)
    {
        var @this = dependencyObject as MyBlock;
        if (@this == null) return;
        Trace.WriteLine(string.Format("Old: {0}, New: {1}", a.OldValue, a.NewValue)); // never seems to fire...
    }

    public bool BlockIsOnlyOne
    {
        get { return (bool) GetValue(BlockIsOnlyOneProperty); }
        set { SetValue(BlockIsOnlyOneProperty, value); }
    }
    ...
}

Answer 1

You should be able to reach your view model property from the UserControl using a RelativeSource Binding. The idea is to search for the parent view that the view model is set as the DataContext using the AncestorType property... try this:

<Style TargetType="{x:Type vc:MyBlock}">
    <Setter Property="DataContext.BlockIsOnlyOne" Value="{Binding ViewModelIsOnlyOne, 
    RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}" />
</Style>

If that picks up the child UserControl.DataContext instead, then you could either set the RelativeSource.AncestorLevel Property to the appropriate level, or use the name/type of your parent UserControl instead:

<Style TargetType="{x:Type vc:MyBlock}">
    <Setter Property="BlockIsOnlyOne" Value="{Binding DataContext.ViewModelIsOnlyOne, 
    RelativeSource={RelativeSource AncestorType={x:Type YourPrefix:MyContainer}}}" />
</Style>