Behavior bindable property with DynamicResource not supported?

[Cybrosys]

Hi,

I raised this Q&A and also a bug in the MCT repo:
CommunityToolkit/Maui#1054

It is a question related to doing a DynamicResource binding against a bindable property on the StatusBarBehaior, if it is supported or a bug. If it is a bug I think it belongs here in the main MAUI repo.

Here is a link to the bug in the MCT Repo:
CommunityToolkit/Maui#1056

So, is this a bug in the MAUI repo or a limitation? For me as a developer and consumer, since the XAML does not show any error I would expect and assume that I would be able to use a DynamicResource binding for the StatusBarBehavior's StatusBarColor bindable property.

I also expressed in one of my comments that if I wanted to start investigating this, where would I start? I do not know who would own the DynamicResource binding. When doing a normal Bind I would expect it to be owned by the UI element itself, targeting a defined BindingContext. However, DynamicResource bindings seem a bit more "magical" in that they work without a BindingContext being defined/assigned.

There are several issues currently related to DynamicResource bindings not working in .NET 7 but in .NET 6. So I do not know if this problem is related to Behavors specifically or a core issue with DynamicResource.

Update

I got information here CommunityToolkit/Maui#1056 that it can't use DynamicResource because it does not inherit from VisualElement or implement IResourcesProvider.

I would argue that you should be able to use DynamicResource even when not inheriting from VisualElement. The reason being that you can put any type of value into a ResourceDictionary, so in essence it is just a normal binding where the source is the parent or app's ResourceDictionary.

[RsZoli]

well, after 2,5 years, i too would like to know about this!

[stephenquan]

On Mar 7, 2023 @TheCodeTraveler writes CommunityToolkit/Maui#1056 (comment):

Behavior in .NET MAUI does not have support for ResourceDictionary and thus do not support StaticResource or DynamicResource

In .NET MAUI, Behavior inherits from BindableObject.

For a class to support ResourceDictionary, it needs to either inherit from VisualElement where public ResourceDictionary Resources { get; set; } exists, or implement IResourceProvider.

This explains why DynamicResource cannot be assigned to Behaviors, but it also suggests an interesting workaround. It works by assigning the DynamicResource to a property in a hidden VisualElement (e.g., Button.CommandParameter) and then extracting the StatusBarColor from it. For it to work, the VisualElement must be connected to see the merge dictionaries, hence the Parent line.

<ContentPage x:Name="thisPage" ...>
    <ContentPage.Behaviors>
        <toolkit:StatusBarBehavior
           StatusBarColor="{Binding CommandParameter,
                                    x:DataType=Button,
                                    Source={Button CommandParameter={DynamicResource Tertiary},
                                                   Parent={Reference thisPage}}}" />
    </ContentPage.Behaviors>
</ContentPage>

Another solution is to write an alternate DynamicResource that carries its own VisualElement with a settable Parent, e.g.

<ContentPage.Behaviors>
    <toolkit:StatusBarBehavior StatusBarColor="{local:DynamicResourceAlt Tertiary}" />
</ContentPage.Behaviors>

[ContentProperty(nameof(Key))]
[RequireService([typeof(IReferenceProvider), typeof(IProvideValueTarget)])]
public partial class DynamicResourceAlt : Button, IMarkupExtension<BindingBase>
{
    [BindableProperty] public partial string Key { get; set; } = string.Empty;
    public DynamicResourceAlt()
    {
        this.Parent = Application.Current;
        this.PropertyChanged += (s, e) =>
        {
            if (e.PropertyName == nameof(Key) && !string.IsNullOrEmpty(Key))
            {
                SetDynamicResource(Button.CommandParameterProperty, Key);
            }
        };
    }
    public object ProvideValue(IServiceProvider serviceProvider) => (this as IMarkupExtension<BindingBase>).ProvideValue(serviceProvider);
    BindingBase IMarkupExtension<BindingBase>.ProvideValue(IServiceProvider serviceProvider)
    {
        if (serviceProvider.GetService(typeof(IProvideValueTarget)) is IProvideValueTarget provideValueTarget && provideValueTarget.TargetObject is BindableObject targetObject)
        {
            this.SetBinding(BindingContextProperty, static (BindableObject t) => t.BindingContext, BindingMode.OneWay, source: targetObject);
        }
        return BindingBase.Create(static (DynamicResourceAlt t) => t.CommandParameter, BindingMode.OneWay, source: this);
    }
}

[stephenquan]

Suggestion, if we create a new IBehavior interface, we can change VisualElement.Behaviors as follows

public partial class VisualElement : NavigableElement, IAnimatable, IVisualElementController, IResourcesProvider, IStyleElement, IFlowDirectionController, IPropertyPropagationController, IVisualController, IWindowController, IView, IControlsVisualElement, IConstrainedView
{
    // public IList<Behavior> Behaviors
   public IList<IBehavior> Behaviors
   /* ... */

Then someone can subclass VisualElement and an IBehavior interface as a new Behavior, and this may "help" support DynamicResource. How we may utilize this could be

//public partial class IconTintColorBehavior : Behavior<View>
public partial class IconTintColorBehavior : VisualElement, IBehavior<View>
{
    /* ... */
}

<Image x:Name="globe" Source="globe_32x32.png">
    <Image.Behaviors>
        <toolkit:IconTintColorBehavior
           TintColor="{DynamicResource TargetColor}",
           Parent="{Reference globe}" />
    </Image.Behaviors>
</Image>

References:

• https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/VisualElement/VisualElement.cs
• Experimental PR (for discussion): Experimental interface IBehavior #32350