UWP User Control ConverterParameter Binding is null

Question

I have a UserControl which has a property Theme. The Theme is just an Enum of ElementTheme.

public sealed partial class PlaylistControl : UserControl, MediaControlListener
{
    public ElementTheme Theme { get; set; }
    public static readonly DependencyProperty ThemeProperty = DependencyProperty.Register("Theme", typeof(ElementTheme), typeof(PlaylistControl), new PropertyMetadata(null));

    ...
}

And inside the xaml of that control, I have a TextBlock whose Foreground is Foreground="{x:Bind IsPlaying, Converter={StaticResource RowColorConverter}, ConverterParameter={Binding Theme}, Mode=OneWay}"

However, the converter doesn't seem to get the theme. It is always null.

class RowColorConverter : Windows.UI.Xaml.Data.IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        return value.Equals(true) ? Helper.GetHighlightBrush() :
                                    parameter is ElementTheme && (ElementTheme)parameter == ElementTheme.Dark ? Helper.WhiteSmokeBrush : Helper.BlackBrush;
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        return null;
    }
}

What is wrong?

Answer 1

You used new PropertyMetadata(null) when creating Theme.

Its meaning is to set the default value of Theme to null. Please try to modify it to new PropertyMetadata(ElementTheme.Light).

Update

I don't know which color to convert back when the music is not being played so I am trying this way

I try to simplify the content, assuming you don't need theme resources. This is the solution to the idea:

First define two brush resources in App.xaml, which are unplayed state and play state.

<SolidColorBrush x:Key="UnplayForeground" Color="White"/>
<SolidColorBrush x:Key="PlayingForeground" Color="Red"/>

Write a method to retrieve the resource with the specified name from the current Resource resource:

public static Brush GetResourceBrush(string key)
{
    return (Brush)Application.Current.Resources[key];
}

Then directly convert it with the IsPlaying property.

public class RowColorConverter:IValueConverter
{
    if(value is bool isPlaying)
    {
        return isPlaying ? GetResourceBrush("PlayingForeground") : 
                           GetResourceBrush("UnplayForeground");
    }
    return GetResourceBrush("UnplayForeground");
}

Update2

But how is your key different from my Theme?

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.ThemeDictionaries>
            <ResourceDictionary x:Key="Light">
                <SolidColorBrush x:Key="PlayingForeground" Color="Green"/>
                <SolidColorBrush x:Key="UnplayForeground" Color="Black"/>
            </ResourceDictionary>
            <ResourceDictionary x:Key="Dark">
                <SolidColorBrush x:Key="PlayingForeground" Color="Red"/>
                <SolidColorBrush x:Key="UnplayForeground" Color="White"/>
            </ResourceDictionary>
        </ResourceDictionary.ThemeDictionaries>
    </ResourceDictionary>
</Application.Resources>

The theme switch needs to create two resource dictionaries named Light and Dark respectively. Within these two resource dictionaries, there are resources with the same name.

When you need to change the theme, you should modify the RequestedTheme property of the Control instead of creating a new Theme property.

When the theme is switched, the software also switches the corresponding ResourceDictionary, and the name of the resource is unchanged. This is why GetResourceBrush(string key) always gets the current theme resource based on key.

Best regards.

Answer 2

Thank Richard for answering my question. As is mentioned by him, I can't pass the bound theme to the ConverterParameter. So there is no way to solve my problem using ConverterParameter.

The reason why I am posting an answer is I just want to show my workaround to get the problem done.

Since I have only one PlaylistControl displayed at the same time, I can declare another static variable UITheme CurrentTheme. When the control is Loaded, assign its member variable Theme to the CurrentTheme and I can easily switch color in the converter:

public object Convert(object value, Type targetType, object parameter, string language)
{
    return value.Equals(true) ? Helper.GetHighlightBrush() :
                                CurrentTheme == ElementTheme.Dark ? Helper.WhiteSmokeBrush : Helper.BlackBrush;
}