How to bind to IsExpanded property - to persist the TreeView's expanded state? - xaml

Question

In the ViewModel I have defined a bool ShouldExpand property, bound to it in from the TreeView's itemcontainerstyle - it does not work.
This answer (WinRT XAML Toolkit TreeView Save state) would've answered my question, but it does not work...
Is there a WORKING code snippet?
public class AgendaItemTreeView : ViewModelBase
{
private string _title;
private Visibility _documentGridVisibility = Visibility.Collapsed;
private bool _shouldExpand;
// Binding property - to display in a TreeView
public string Title
{
get { return _title; }
set { Set(ref _title, value); }
}
// To expand/collapse documents GridView
public Visibility DocumentGridVisibility
{
get { return _documentGridVisibility; }
set { Set(ref _documentGridVisibility, value); }
}
// Property to expand/collapse a node in a TreeView - does not work!
public bool ShouldExpand
{
get { return _shouldExpand; }
set { Set(ref _shouldExpand, value); }
}
// Nested agenda items
public ObservableCollection<AgendaItemTreeView> AgendaItems { get; set; } = new ObservableCollection<AgendaItemTreeView>();
// Documents of current agenda item
public ObservableCollection<DocumentViewModel> Documents { get; set; } = new ObservableCollection<DocumentViewModel>();
public DocumentViewModel SelectedItem { get; set; }
public AgendaItemTreeView(AgendaItem agendaItem, string selectedTitle = "")
{
// Constructor populating the nested AgendaItem & Documents
}
}
XAML:
<UserControl
<UserControl.Resources>
<!--<Style x:Key="TreeViewItemContainerStyle" TargetType="controls:TreeViewItem">
<Setter Property="IsExpanded" Value="{Binding ShouldExpand, Mode=TwoWay}"/>
</Style>-->
<!-- Folder type Node, that can contain other 'folders' and 'files' -->
<DataTemplate x:Key="TreeViewItemTemplate" x:DataType="vm:AgendaItemTreeView">
<data:DataTemplateExtensions.Hierarchy>
<data:HierarchicalDataTemplate ItemsSource="{Binding AgendaItems}" />
<!-- When the next line used together with the Style TreeViewItemContainerStyle - the TreeView still does not expand the top level tree. The app C
<!--<data:HierarchicalDataTemplate ItemsSource="{Binding AgendaItems}" ItemContainerStyle="{StaticResource TreeViewItemContainerStyle}"/>-->
</data:DataTemplateExtensions.Hierarchy>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<ToggleButton x:Name="titleToggleBtn" Tapped="{x:Bind ExpandDocumentsCommand}">
<StackPanel Orientation="Horizontal">
<SymbolIcon Symbol="{x:Bind DocumentsIcon}" Margin="5,0"/>
<TextBlock Text="{Binding Title}"/>
</StackPanel>
</ToggleButton>
<GridView Grid.Row="1" x:Name="DocumentsGrid" ItemsSource="{Binding Documents}" ItemTemplate="{StaticResource ZoomedInTemplate}" Visibility="{Binding DocumentGridVisibility}" SelectedItem="{Binding SelectedItem}"/>
</Grid>
</DataTemplate>
<!-- Nested 'file' type Node data template -->
<DataTemplate x:Key="ZoomedInTemplate" x:DataType="vm:DocumentViewModel">
<Border BorderBrush="Black" Padding="5" BorderThickness="1" Width="100">
<Grid Tapped="{x:Bind TappedCommand}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition />
</Grid.RowDefinitions>
<Image Source="{Binding Thumbnail}" Width="60" Height="80" />
<TextBlock Grid.Row="1" Text="{x:Bind Title, Mode=OneWay}" VerticalAlignment="Center" Tapped="{x:Bind TappedCommand}"/>
</Grid>
</Border>
</DataTemplate>
</UserControl.Resources>
<Grid d:DataContext="{d:DesignData /SampleData/AgendaItemTreeViewSampleData.xaml}">
<controls:TreeView x:Name="myTreeView"
ItemsSource="{x:Bind TreeItems}"
ItemTemplate="{StaticResource TreeViewItemTemplate}" />
</Grid>
</UserControl>