ControlTemplate not propagating the BindingContext correctly if set through Binding

strajk- commented 5 months ago

Description

It's a repeat of the Issue #12470 that was supposedly fixed with the Pull #12536 except in my case the propagation doesn't seem to work correctly if the ControlTemplate is set through Binding.

Steps to Reproduce

Create a Template without a BindingContext, add it as a Resource in your ContentPage

<ContentPage.Resources>
<ResourceDictionary>
     <ControlTemplate  x:Key="String_Template" >
         <Entry Text="{Binding Value, Mode=TwoWay}"/>
     </ControlTemplate>
</ResourceDictionary>
</ContentPage.Resources>

A Property to Bind to that handles the ControlTemplate swapping


public static readonly BindableProperty TypeTemplateProperty = BindableProperty.Create(
nameof(TypeTemplate),
typeof(ControlTemplate),
typeof(ContentPage),
defaultValue: null,
defaultBindingMode: BindingMode.OneWay);

public ControlTemplate TypeTemplate { get => (ControlTemplate)GetValue(TypeTemplateProperty); set => SetValue(TypeTemplateProperty, value); }

3. Add a ContentView that will show the Template set in TypeTemplate
```xaml
<Grid BindingContext="{Binding Source={RelativeSource Mode=FindAncestor,AncestorType={x:Type ContentPage}}}">
    <ContentView ControlTemplate="{Binding TypeTemplate}"/>
</Grid>

Some logic to set our TypeTemplate

Resources.TryGetValue("String_Template", out object template);
if (template != null && template is ControlTemplate cTemplate) {
TypeTemplate = cTemplate;
}

Value Property that we want to show in our ControlTemplate
```
public object Value { get; set; } = "Test";
```

Link to public reproduction project repository

No response

Version with bug

8.0.10 SR3

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

Unknown/Other

Affected platforms

Android, I was not able test on other platforms

Affected platform versions

API 34 (Android 14.0) - Didn't test others

Did you find any workaround?

Adding the BindingContext on the Control that's defined inside the ControlTemplate to "renew" the BindingContext fixes it.

<ControlTemplate  x:Key="String_Template" >
    <Entry Text="{Binding Value, Mode=TwoWay}" BindingContext="{Binding BindingContext, Source={RelativeSource Mode=TemplatedParent}}"/>
</ControlTemplate>

It also works if I were to set the ControlTemplate as a StaticResource like this:

<Grid BindingContext="{Binding Source={RelativeSource Mode=FindAncestor,AncestorType={x:Type ContentView}}}">
    <ContentView ControlTemplate="{StaticResource String_Template}"/>
</Grid>

But that defeats the purpose of being able to dynamically switch between ControlTemplates without extra C# code.

Relevant log output

No response

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