ItemsRepeater crashes when using custom ItemTemplateSelector that does not implement SelectTemplateCore(item) override

[jtorjo]

Describe the bug
I want to create a combination of treeview/listview that would behave like this: have a few roots (such as: Effects/Transitions/Text Effects). Each will expand to a few effect packs. An effect pack will show a (wrap panel) list of effects.

I wanted to implemement this as an ItemsRepeater, but no matter what I do, it crashes at startup (the app can't even start).

Steps to reproduce the bug
It could be that I'm doing something wrong, but I don't know what that would be. I've trimmed off pretty much everything, just to make it easy to debug.

<UserControl
	x:Class="gridsplittertest.browse_effects_ctrl"
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:local="using:gridsplittertest"
	xmlns:muxc="using:Microsoft.UI.Xaml.Controls"
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	mc:Ignorable="d"
	d:DesignHeight="300"
	d:DesignWidth="400">
	<UserControl.DataContext>
		<local:browse_effects_view_model/>
	</UserControl.DataContext>
	<UserControl.Resources>

		<DataTemplate x:Key="et" x:DataType="local:effect_type">
			<Grid Height="10" Width="200" MaxHeight="10" MaxWidth="10">
			</Grid>
		</DataTemplate>

		<DataTemplate x:Key="ep" x:DataType="local:effect_pack">
			<Grid Height="10" Width="200" MaxHeight="10" MaxWidth="10">
			</Grid>

		</DataTemplate>

		<DataTemplate x:Key="ei" x:DataType="local:effect_info">
			<Grid></Grid>
		</DataTemplate>

		<local:effect_data_template_selector x:Key="edts" effect_type_template="{StaticResource et}" effect_pack_template="{StaticResource ep}" effect_item_template="{StaticResource ei}"
		/>

	</UserControl.Resources>
	<Grid>
		<muxc:ItemsRepeater ItemsSource="{Binding ui_effect_types}" ItemTemplate="{StaticResource edts}">
			<muxc:ItemsRepeater.Layout>
				<muxc:StackLayout Orientation="Vertical"/>
			</muxc:ItemsRepeater.Layout>
		</muxc:ItemsRepeater>
	</Grid>
</UserControl>

To my knowledge, the above should work. Am I doing something wrong?

I've attached the whole sample project I'm using.
gridsplittertest.zip

Windows 10 version	Saw the problem?
November 2019 Update (18363)	Yes

Device form factor	Saw the problem?
Desktop	Yes

[ranjeshj]

@jtorjo I was able to make it work by using the other override of DataTemplateSelector. ItemsRepeater does not have a container, so it uses this override. We should be providing a better error in this case though or handle this scenario (we can use this bug to track that)

After I made the update below and changed the data templates to show something, i was able to see the items.

 protected override DataTemplate SelectTemplateCore(object item)
        {
            if (item is effect_type)
                return effect_type_template;
            if (item is effect_pack)
                return effect_type_template;
            if (item is effect_info)
                return effect_item_template;

            Debug.Assert(false);
            return null;
        }

[marcelwgn]

Couldn't we just support the SelectTemplateCore(object item, DependencyObject container) method by passing nullptr as the container? Not supporting this method and crashing/returning an error seems a bit overkill here.

[ranjeshj]

Couldn't we just support the SelectTemplateCore(object item, DependencyObject container) method by passing nullptr as the container? Not supporting this method and crashing/returning an error seems a bit overkill here.

We could look at one and fallback to the other. Yes.

[jtorjo]

@ranjeshj

I was able to make it work by using the other override of DataTemplateSelector. ItemsRepeater does not have a container, so it uses this override. We should be providing a better error in this case though or handle this scenario (we can use this bug to track that)

After I made the update below and changed the data templates to show something, i was able to see the items.

Awesome, thanks! Yeah, the error should be reported better - in my case, it basically stops the app from starting. There's no way to understand anything.

[marcelwgn]

Would the following be a suitable fix:

1. Call SelectTemplate(IInspectable item)
2. If we received a non nullptr, use that, otherwise try SelectTemplate(IInspectable item, DependencyObject container)
3. Check if that got us a valid pointer, if so use that
4. If we still got a nullptr, throw some exception like "No valid template returned"

@jtorjo @ranjeshj

If that would be a fix you are fine with, I would be more than happy to implement that.

[ranjeshj]

That sounds reasonable. We will have to pass null as the container and there could be app code that fails because of that, but that should be easier to debug from the app side.

[jtorjo]

@chingucoding Sounds perfect!