Default ItemTemplate
When using the ItemTemplate property of the TreeViewDescriptor it overrides the default template. For example, if you are using the default template with CheckBoxes, when overriding it, the CheckBoxes will be no longer shown. In order to properly visualize them you will need the original ItemTemplate.
This topic gives an overview of the default ItemTemplate of the TreeViewDescriptor, so you can copy it to your app and make changes.
The original ItemTemplate
The XAML defined below relies on the following namespace to be set:
xmlns:telerikPrimitives="clr-namespace:Telerik.XamarinForms.Primitives;assembly=Telerik.XamarinForms.Primitives"
xmlns:telerikDataControls="clr-namespace:Telerik.XamarinForms.DataControls;assembly=Telerik.XamarinForms.DataControls"
xmlns:telerikTreeView="clr-namespace:Telerik.XamarinForms.DataControls.TreeView;assembly=Telerik.XamarinForms.DataControls"
<ResourceDictionary>
<telerikTreeView:LevelToMarginConverter x:Key="levelToMarginConverter" />
<DataTemplate x:Key="CustomControlTemplate">
<StackLayout Orientation="Horizontal"
Margin="{Binding Path=Level, Converter={StaticResource levelToMarginConverter}}">
<StackLayout.HeightRequest>
<OnPlatform x:TypeArguments="x:Double">
<On Platform="iOS" Value="44"></On>
<On Platform="Android" Value="40"></On>
<On Platform="UWP" Value="40"></On>
</OnPlatform>
</StackLayout.HeightRequest>
<telerikTreeView:ExpandCollapseIndicator FontSize="Medium"
WidthRequest="10"
Margin="15,0"
VerticalTextAlignment="Center"
IsLoading="{Binding Path=IsLoading}"
IsLoadOnDemandEnabled="{Binding Path=IsLoadOnDemandEnabled}"
IsExpanded="{Binding Path=IsExpanded}"
IsLeaf="{Binding Path=IsLeaf}">
<AutomationProperties.HelpText>
<OnPlatform x:TypeArguments="x:String">
<On Platform="iOS" Value="RadTreeView"></On>
</OnPlatform>
</AutomationProperties.HelpText>
</telerikTreeView:ExpandCollapseIndicator>
<telerikPrimitives:RadCheckBox IsChecked="{Binding Path=IsChecked, Mode=TwoWay}"
IsVisible="{Binding Path=IsCheckBoxVisible}"
VerticalOptions="Center">
<AutomationProperties.HelpText>
<OnPlatform x:TypeArguments="x:String">
<On Platform="iOS" Value="RadTreeView"></On>
</OnPlatform>
</AutomationProperties.HelpText>
</telerikPrimitives:RadCheckBox>
<telerikTreeView:ItemText Text="{Binding Path=Header}"
VerticalTextAlignment="Center">
<AutomationProperties.HelpText>
<OnPlatform x:TypeArguments="x:String">
<On Platform="iOS" Value="RadTreeView"></On>
</OnPlatform>
</AutomationProperties.HelpText>
</telerikTreeView:ItemText>
</StackLayout>
</DataTemplate>
</ResourceDictionary>
Finally, use the custom Item ControlTemplate as a StaticResource on any instance of TreeViewDescriptor ItemTemplate:
<telerikDataControls:RadTreeView x:Name="treeView">
<telerikDataControls:TreeViewDescriptor ItemTemplate="{StaticResource CustomControlTemplate}"/>
</telerikDataControls:RadTreeView>