OnSelectTemplate does not get called for each item in ItemsSource?

[Valentin]

Hello, I'm running into a weird issue:

I'm using a slide view with an item template selector to select the data template for each item in the items source.

For some reason, the OnSelectTemplate method in the template selector will never be called more than three times ever. The first call is made with the last item in the items source, the second call with the first item and the third call with the second item.

My items source collection has at four items, so the third template is never selected...

Here's the XAML:

<telerikPrimitives:RadSlideView HorizontalContentOptions="CenterAndExpand" VerticalContentOptions="CenterAndExpand" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
    <telerikPrimitives:RadSlideView.ItemsSource>
        <x:Array Type="{x:Type x:Int32}">
            <x:Int32>1</x:Int32>
            <x:Int32>2</x:Int32>
            <x:Int32>3</x:Int32>
            <x:Int32>4</x:Int32>
            <x:Int32>5</x:Int32>
            <x:Int32>6</x:Int32>
            <x:Int32>7</x:Int32>
            <x:Int32>8</x:Int32>
        </x:Array>
    </telerikPrimitives:RadSlideView.ItemsSource>
    <telerikPrimitives:RadSlideView.ItemTemplateSelector>
        <local:TemplateSelector>
            <local:TemplateSelector.Template1>
                <DataTemplate>
                    <ContentView>
                        <Label Text="Bug" HorizontalTextAlignment="Center" VerticalTextAlignment="Center"/>
                    </ContentView>
                </DataTemplate>
            </local:TemplateSelector.Template1>
        </local:TemplateSelector>
    </telerikPrimitives:RadSlideView.ItemTemplateSelector>
</telerikPrimitives:RadSlideView>

 

And here's the template selector class:

public class TemplateSelector : DataTemplateSelector
{
    public DataTemplate Template1 { get; set; }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        return Template1;
    }
}

[Valentin]

Rereading my original post I realize the example code could be misinterpreted.I understand that I would need eight different item templates in XAML and to return them appropriately from the template selector. I chose to keep the code shorter though.

My point is really about the number of calls made to OnSelectTemplate. I expect eight calls and I see only three.

[Lance | Senior Manager Technical Support]

Hi Valentin,

You're on the right path, but you've only got one DataTemplate defined. You return the different templates inside OnSelectTemplate

public class TemplateSelector : DataTemplateSelector
{
    public DataTemplate FallBackTemplate { get; set; }
    public DataTemplate Template1 { get; set; }
    public DataTemplate Template2 { get; set; }
    public DataTemplate Template3 { get; set; }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        if (item is int value)
        {
            switch (value)
            {
                case 1:
                    return Template1;
                case 2:
                    return Template2;
                case 3:
                    return Template3;
            }
        }
        return FallBackTemplate;
    }
}

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

[Valentin]

Thank you Lance, as I mentioned in my follow up post, I understand I need to return different templates - I should've make that clear in my original post. The problem is that OnSelectTemplate does not appear to be called more than 3 times ever, regardless of how many items are in the ItemsSource collection.

[Lance | Senior Manager Technical Support]

Hello Valentin,

My apologies! I'll test this now. So that I can make sure I'm using the right environment, can you reply with the following info:

What version of Xamarin.Forms are You using?
What platform are you targeting?
   -- Is this this Android, iOS or UWP?
   -- What version of the OS/SDK?

In the meantime, I'll spin up the project using your initial code example using the defaults.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

[Valentin]

Thanks. Here are the details you requested:

Telerik_UI_for_Xamarin_2018_2_516_2_Dev
Shared code project targets .NET Standard 1.4
Xamarin Forms 2.5.1.527436
Android and iOS
Android 8.1 (API Level 27 - Oreo) - on emulator
iOS 11 - on simulator
Did not yet try on real devices.

[Lance | Senior Manager Technical Support]

Hi Valentin,

 I have been able to reproduce the issue using your initial code example and occurs on all platforms. It appears to reuse the template for several items in a row and then makes another call again for subsequent items.  

I have logged this with the development team, you can follow the bug report here: SlideView: ItemTemplateSelector Not Working Correctly. I have also updated your Telerik points as a small token of appreciation for bringing this to our attention.


Workaround

Unfortunately, I am not able to come up with a workaround for the selector.   An alternative would be to use a single ItemTemplate and ValueConverters to change the content's properties.

For example:

public class Item
{
    public string DisplayValue { get; set; }
    public int Value { get; set; }
}

public class MyContentConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (int)value % 2 == 0 ? Color.Blue : Color.Red;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

<ResourceDictionary>
    <portable:MyContentConverter x:Key="MyContentConverter"/>
</ResourceDictionary>

...

<telerikPrimitives:RadSlideView.ItemTemplate>
    <DataTemplate>
        <ContentView>
            <StackLayout VerticalOptions="Center" HorizontalOptions="Center">
                <Label Text="{Binding DisplayValue}"
                    TextColor="{Binding Value, Converter={StaticResource MyContentConverter}}"/>
            </StackLayout>
        </ContentView>
    </DataTemplate>
</telerikPrimitives:RadSlideView.ItemTemplate>

This would result in a different color being used for every other item.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

[Valentin]

Thank you for confirming it's a bug and for trying to find a workaround. Unfortunately my templates are very different from slide  to slide. I'll have to re-arrange my controls to fit on three templates for now.

[Valentin]

Hi Lance, I can't seem to see the Telerik points earned for reporting this issue in my account profile...

[Yana]

Hello Valentin,

I have updated your points for the report.  In addition, I'd like to inform a fix will be available in the upcoming minor release expected this week.

Regards,
Yana
Progress Telerik