Unable to set RadTreeViewItem's expander style to a StaticResource?

[Yanming]

I learned how to set a RadTreeViewItem's expander style dynamically by WPF data binding, see the link below.
http://blogs.telerik.com/blogs/11-01-17/controlling-treeview-s-expanderstyle-dynamically.aspx

However, if I change ExpanderStyle to StaticResource, it doesn't work. (But it works for RadTreeView)

<Style x:Key="ContainerStyle" TargetType="telerik:RadTreeViewItem">
    <Style.Setters>
        <!--<Setter Property="ExpanderStyle" Value="{Binding MyCustomExpanderStyle, Mode=TwoWay}"/>-->
        <Setter Property="ExpanderStyle" Value="{StaticResource ExpanderStyle_3}" />
    </Style.Setters>
</Style>

​What's interesting is when I click "Clear Local Item Style", the style is applied.

How can I set an item's expander style to static resource just like the tree view?

[Boris]

Hello Yanming,

In the WPF project in the article that Hristo Milyakov wrote he used an MVVM approach with a ViewModel(class SampleViewModel) which inherits an ObservableCollection with business objects (class BusinessItem) defined in it. In the class BusinessItem there is a MyCustomExpanderStyle property which is of type Style and it notifies the UI when it is changed. The style changes thanks to the binding of that property in MainWindow.xaml file and the INotifyPropertyChanged interface. In order to change the styles at run time, you also could use binding to the same property like it's shown in the WPF project in the blog post.

On the other hand, I am not sure that I fully understand what you mean by saying: "But it works for RadTreeView". Could you please elaborate more on that?

I'm looking forward to your response.

Regards,
Boris Penev
Telerik

[Yanming]

Hi Boris,

Thank you for your reply. Let me explain my problem more precisely.

"It works for RadTreeView" means the ExpanderStyle property take effect for a RadTreeView. Take Hristo's code as an example, if I write the code below, myTreeView's expander will have the style "ExpanderStyle_1".

<telerik:RadTreeView x:Name="myTreeView" Width="250" BorderThickness="1" BorderBrush="#FF989898"
                     ItemTemplate="{StaticResource ItemsTemplate}"
                     ExpanderStyle="{StaticResource ExpanderStyle_1}"
                     IsVirtualizing="True" />

But when I tried to set ExpanderStyle for RadTreeViewItem like below, no expander style is applied.

<telerik:RadTreeView x:Name="myTreeView" Width="250" BorderThickness="1" BorderBrush="#FF989898"
                        IsVirtualizing="True" >
    <telerik:RadTreeViewItem Header="Sport Categories" ExpanderStyle="{StaticResource ExpanderStyle_1}">
        <telerik:RadTreeViewItem Header="Football">
            <telerik:RadTreeViewItem Header="Futsal"/>
            <telerik:RadTreeViewItem Header="Soccer"/>
        </telerik:RadTreeViewItem>
    </telerik:RadTreeViewItem>
</telerik:RadTreeView>

Hristo's code work well through data binding. But I just want to apply the style at compile time.
My initial objective is to hide the expand icon for root item (root item should be expanded by default), but preserve the icon for child items.

[Boris]

Hello Yanming,

The ExpanderStyle_1 style isn't applying because you are setting the IsVirtualizing property of the RadTreeView to "True" while you aren't in a data bound scenario. If you remove this property the code snippet you posted should work and apply the style.

More info about UI Virtualization  in RadTreeView you can find in these support resources:

• IsVirtualizing property API Reference
• Virtualization feature 

Please let us know if this helps you.

Regards,
Boris Penev
Telerik

[Yanming]

Hi Boris,

Removing the IsVirtualizing property exactly solved my problem!

However, I found even I was in data bound scenario, expander style can't be applied to RadTreeViewItem if IsVirtualizing is set to True.
So it seems to me that if I want to apply expander style for RadTreeViewItem by StaticResource, I can't utilize the Virtualization feature of RadTreeView?

[Boris]

Hello Yanming,

You can use the Virtualization feature of the RadTreeView in a data-bound scenario and apply the ExpanderStyle as StaticResource. If we use Hristo Milyakov's WPF example, which is using virtualization and applying the ExpanderStyles runtime we can change it to apply one of the ExpanderStyles by commenting the unnecessary code and doing something like this:

<telerik:RadTreeView x:Name="myTreeView" Width="250" BorderThickness="1" BorderBrush="#FF989898"
                             ItemTemplate="{StaticResource ItemsTemplate}"
                             IsVirtualizing="True"
                             ExpanderStyle="{StaticResource ExpanderStyle_3}"
                             />

And by doing so we are using a StaticResource which will be applied to all RadTreeViewItems on load. 

I hope this helps.

Regards,
Boris Penev
Telerik

[Yanming]

Hi Boris,

I know ExpanderStyle can be applied to RadTreeView, but I want to apply it to RadTreeViewItem.
Again take the WPF example. I define the ItemsTemplate as below.

<Style x:Key="ContainerStyle" TargetType="telerik:RadTreeViewItem">
    <Style.Setters>
        <!--Successfully apply ExpanderStyle through data binding-->
        <!--<Setter Property="ExpanderStyle" Value="{Binding MyCustomExpanderStyle, Mode=TwoWay}"/>-->

        <!--Failed to apply this style by StaticResource-->
        <Setter Property="ExpanderStyle" Value="{StaticResource ExpanderStyle_3}" />
    </Style.Setters>
</Style>

<HierarchicalDataTemplate x:Key="ItemsTemplate" ItemsSource="{Binding Items}" ItemContainerStyle="{StaticResource ContainerStyle}">
    <TextBlock Text="{Binding Name}"/>
</HierarchicalDataTemplate>

As I commented, data binding works well, but ExpanderStyle_3 can't be applied as StaticResource, unless I set IsVitrualizing property as false.
Can you tell me why ExpanderStyle's behavior is not consistent on RadTreeView and RadTreeViewItem?

[Boris]

Hello Yanming,

It seams that we have misunderstood at first what you had in mind. We reviewed the RadTreeView control and we can confirm that the reported behavior is actually a bug. We logged it in our feedback portal where you can track its status. We also updated your telerik account points as a thanks for your cooperation in reporting this issue. 

However, there is a possible workaround which you can examine in the attached project. In it we bind the ExpanderStyle property of every RadTreeViewItem and use a Converter - the ModelToExpanderStyleConverter which selects the correct ExpanderStyle based on the type of the ViewModel for the particular RadTreeViewItem.

<Style TargetType="telerik:RadTreeViewItem" >
           <Setter Property="ExpanderStyle" Value="{Binding Converter={StaticResource converter}}" />
           <Setter Property="IsExpanded" Value="True" />
       </Style>

public class ModelToExpanderStyleConverter : IValueConverter

   {

       public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)

       {

           if (value is League)

           {

               return this.leagueStyle;

           }

           else if (value is Division)

           {

               return this.divisionStyle;
....

Could you please check out the workaround and let us if it is viable for your scenario?

Regards,
Boris Penev
Telerik

[Yanming]

Hi Boris,

We can apply this workaround in our project.
The example is so detailed, I really appreciate your work!