In the context of an Items Collection, how do I bind to a command on the view model?

[bsampica]

So, I think this should be easy, but I just can't work out the syntax. Here's the setup

<UserControl...>
    <Design.DataContext>
          <viewModels:HomePageViewModel />
    </Design.DataContext>


    <ItemsRepeater ItemsSource={Binding ACollectionOnTheViewModel }>
       <ItemsRepeater.ItemTemplate>
          <DataTemplate>
                <Button Command="{Binding ??????}" /> <!-- This needs to be an exposed command on the ViewModel, not in the collection of items -->
          </DataTemplate>
       </ItemsRepeater.ItemTemplate>
</UserControl>

I've been round and round on using the TemplatedParent which throws an error about being in a data template, and trying to use the RelativeSource and ancestors to back up to the ViewModel which is on the UserControl DataContext.

Any help would be appreciated, even if you tell me I'm architecting the solution wrong and I need to rework it to make it work without code stinking' hacks.

[stevemonaco]

You need to use some form of binding indirection through a control that contains the VM as its DataContext. I'll use the ElementName shorthand syntax here, but you could use RelativeSource parent instead. You can check out the docs here.

<ItemsRepeater x:Name="items" ItemsSource={Binding ACollectionOnTheViewModel }>
	<ItemsRepeater.ItemTemplate>
		<DataTemplate>
			<Button Command="{Binding #items.DataContext.SomeCommand}" /> <!-- With ReflectionBinding -->
			<Button Command="{Binding #items.((vm:SomeViewModel)DataContext).SomeCommand}" /> <!-- With CompiledBinding -->
		</DataTemplate>
	</ItemsRepeater.ItemTemplate>
</ItemsRepeater>

[bsampica]

Thank you....one of the two works in this scenario.

<Button Command="{Binding #items.((vm:SomeViewModel)DataContext).SomeCommand}" /> <!-- With CompiledBinding -->

WORKS GREAT!

<Button Command="{Binding #items.DataContext.SomeCommand}" /> <!-- With ReflectionBinding -->

The first button in your example produces an error that says

Unable to resolve field or property 'NavigateTankDetails' in data context of type 'object'

NavigateTankDetails is the ReactiveCommand on the ViewModel that I'm trying to reference. The second however does work. It looks like it's doing a cast?

[stevemonaco]

So that means you probably have compiled bindings enabled as a default. In that case, Binding falls back to CompiledBinding. Remember the DataContext here is an object unless cast (or via reflection). So if you were to use reflection binding, you need to use it explicitly. eg.

<Button Command="{ReflectionBinding #items.DataContext.SomeCommand}" />

[timunie]

fyi: TemplateParent is only for ControlTemplates, where you replace the entire look of a control in your own Style. Using named element would be what I suggest also, or use the $parent[ItemsPresenter] syntax

[bsampica]

In the past in situations like this, I've broken layer barriers slightly, and developed a ViewDataAdapter that turned a Data Model into a View Data Model, that had all the properties, commands, timers, subscriptions...whatever, on it the view needed. Has anyone done something like this in Avalonia?

[timunie]

In WPF it was way harder to achive than in Avalonia, so I had such an adapter for WPF but never needed it for Avalonia until today.

[NickBarton]

Thanks chaps, I had two nested ItemsControl and the trick to solve the problem is to define an x:Name on the outer one.
I was trying to navigate using $parent, nightmare.