ChangePropertyAction does not affect FontWeight

Question

In my Universal Windows Platform app, I have a TextBlock whose DataContext is a custom class:

<TextBlock Text="{Binding OpeningHourDescription}" FontWeight="Light">
    ...
</TextBlock>

My custom class has a property IsOpen. If it is true, the TextBlock should change its color and font weight. I tried to define this by replacing the following XAML code within the TextBlock element:

<Interactivity:Interaction.Behaviors>
    <Core:DataTriggerBehavior Binding="{Binding IsOpen}" Value="true">
        <Core:ChangePropertyAction PropertyName="Foreground" >
            <Core:ChangePropertyAction.Value>
                <Color>Lime</Color>
            </Core:ChangePropertyAction.Value>
        </Core:ChangePropertyAction>
        <Core:ChangePropertyAction PropertyName="FontWeight">
            <Core:ChangePropertyAction.Value>
                <FontWeight>ExtraBold</FontWeight>
            </Core:ChangePropertyAction.Value>
        </Core:ChangePropertyAction>
    </Core:DataTriggerBehavior>
</Interactivity:Interaction.Behaviors>

The behavior itself is working: If IsOpen is true, the text is green. However, the FontWeight is not being set, the text never looks bold.

Curiously, swapping both ChangePropertyAction results in both actions not being applied.

Changing the FontWeight statically on the TextBlock works as expected. The issue also occurs when FontWeight="Light" is not set on the TextBlock.

What am I doing wrong? How can I modify the FontWeight with a ChangePropertyAction?

Answer 1

If you look into the Windows API, you'll see that a TextBlock.FontWeight property is of type FontWeight, which is a struct containing an Int16.

public struct FontWeight
{
    public System.UInt16 Weight;
}

If you set the XAML property on the control, it manages to convert the string to a FontWeights static property.

public sealed class FontWeights : IFontWeights
{
    public static FontWeight Black { get; }
    public static FontWeight Bold { get; }
    public static FontWeight ExtraBlack { get; }
    public static FontWeight ExtraBold { get; }
    public static FontWeight ExtraLight { get; }
    public static FontWeight Light { get; }
    public static FontWeight Medium { get; }
    public static FontWeight Normal { get; }
    public static FontWeight SemiBold { get; }
    public static FontWeight SemiLight { get; }
    public static FontWeight Thin { get; }
}

However in the trigger you're stating that the string you're entering is actually a FontWeight struct (and not a FontWeights property), thus the system has an error trying to convert the string.

<Core:ChangePropertyAction.Value>
     <FontWeight>ExtraBold</FontWeight>
</Core:ChangePropertyAction.Value>

---

Solution: You could use a GoToStateAction and VisualStates to fix your issue.

<TextBlock x:Name="MyTextBlock"  Text="{Binding OpeningHourDescription}" FontWeight="Light">
    <interactivity:Interaction.Behaviors>
        <core:DataTriggerBehavior Binding="{Binding IsOpen}" Value="true">
            <core:GoToStateAction StateName="Open" >
            </core:GoToStateAction>
        </core:DataTriggerBehavior>
    </interactivity:Interaction.Behaviors>
</TextBlock>

And add the correct states to your container control.

<VisualStateManager.VisualStateGroups>
    <VisualStateGroup x:Name="VisualStateGroup">
        <VisualState x:Name="Open">
            <VisualState.Setters>
                <Setter Target="MyTextBlock.Foreground" Value="Lime" />
                <Setter Target="MyTextBlock.FontWeight" Value="Bold" />
            </VisualState.Setters>
        </VisualState>
        ...
    </VisualStateGroup>
</VisualStateManager.VisualStateGroups>