Bind IsExpaned to my own custom TreeViewItem in Wpf? - wpf

Question

I want to bind my custom TreeViewItem to IsExpanded.
The normal way without a custom TreeView Item would look like this.
<TreeView.ItemContainerStyle>
<Style TargetType="TreeViewItem">
<Setter Property="IsExpanded" Value="{Binding IsExpanded}"></Setter>
</Style>
</TreeView.ItemContainerStyle>
But I want to bind it to my own TreeViewItem for example named CoolTreeItemModel.
CoolTreeItemModel could look like this:
public class CoolTreeItemModel : XY
{
public LocalTreeItemModel()
{
TreeViewItems = new List<CoolTreeItemModel>();
}
public List<CoolTreeItemModel> TreeViewItems { get; set; }
public SomeType IsValid { get; set; }
public bool IsExpanded { get; set; }
}
How CoolTreeItemModel is bound:
<TreeView HorizontalAlignment="Stretch" VerticalAlignment="Stretch" ItemsSource="{Binding TreeRoots}">
<TreeView.ItemTemplate>
<HierarchicalDataTemplate DataType="models:CoolTreeItemModel"
ItemsSource="{Binding TreeViewItems}"></HierarchicalDataTemplate>
</TreeView.ItemTemplate>
</TreeView>
How can I bind to the IsExpanded property of CoolTreeItemModel ?
Thanks for your help.

Answer 1

The "normal way" is applicable in this case. Each CoolTreeItemModel will be implicitly wrapped in a TreeViewItem container so you should be able to bind to your IsExpanded property. You may want to set the Mode of the binding to TwoWay though:
<TreeView.ItemContainerStyle>
<Style TargetType="TreeViewItem">
<Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}"></Setter>
</Style>
</TreeView.ItemContainerStyle>

Answer 2

Binding requires the use of dependency properties. Dependency Properties On MSDN You would need to define a dependency property like so:
public ClassName
{
public static readonly DependencyProperty IsExpandedProperty = DependencyProperty.Register("IsExpanded", typeof(bool), typeof(ClassName));
public bool IsExpanded
{
get { return (bool)GetValue(IsExpandedProperty); }
set { SetValue(IsExpandedProperty, value); }
}
}
From there you can then include the namespace of your class in the top of your xaml and then bind to IsExpanded like normal
<MyNamespace:ClassName IsExpanded="{Binding Value}" />
I Would like to raise one concern of my though. When developers new to XAML/WPF start developing their own controls, the distinction between view data and business data gets muddled. If this is your own control to be consumed by others, there is an entire style template that may need to be created and used. All of this binding should be taking place in this style template, following the example set forth by Microsoft. Also a custom control should have no view model or defined data context as it is expected to be provided by the consumer, meaning a lot of your logic is going to be in the code behind, which is okay as a custom control is only view related and shouldn't have any business logic in it.