WPF: Select TreeViewItem broken past the root level

Question

I'm trying to select a TreeViewItem by ID, but having problems getting it to work past the first (root) level. I've done so much reading round on this and am using the method below.

private static bool SetSelected(ItemsControl parent, INestable itemToSelect) {
    if(parent == null || itemToSelect == null) {
        return false;
    }
    foreach(INestable item in parent.Items) {
        if(item.ID == itemToSelect.ID) { // just comparing instances failed
            TreeViewItem container = parent.ItemContainerGenerator.ContainerFromItem(item) as TreeViewItem;
            if(container != null) {
                container.IsSelected = true;
                container.Focus();
                return true;
            }
        }
        ItemsControl childControl = parent.ItemContainerGenerator.ContainerFromItem(item) as ItemsControl;
        if(SetSelected(childControl, itemToSelect))
            return true;
    }
    return false;
}

INestable is the base level interface, implemented by IGroup and IAccount:

public interface INestable {
        string ID { get; set; }
    ...
}
public interface IAccount : INestable { 
    ...
}
public interface IGroup : INestable { 
    public IList<INestable> Children
    ...
}

I think it must have something to do with the datatemplates (perhaps):

<HierarchicalDataTemplate DataType="{x:Type loc:IGroup}" ItemsSource="{Binding Children}" x:Key="TreeViewGroupTemplate">
<HierarchicalDataTemplate DataType="{x:Type loc:IAccount}" x:Key="TreeViewAccountTemplate">

The Template selector for the treeview returns thr group template for IGroups and the account template for IAccounts:
<conv:TreeTemplateSelector x:Key="TreeTemplateSelector" AccountTemplate="{StaticResource TreeViewAccountTemplate}" GroupTemplate="{StaticResource TreeViewGroupTemplate}"/>
<TreeView ItemTemplateSelector="{StaticResource TreeTemplateSelector}">

It works for all top level items, just nothing below that, and debugging confirms parent.ItemContainerGenerator does contain the items for all levels.

I know there's a lot of code but I'm burning through hours trying to get this to work. Thanks for any help. :)

Answer 1

Hi Echilon.

I think it doesn't work since item is collapsed and its container is not instantiated. Thus trying to select TreeViewItem directly is definitely not the best way to go.

Instead we use MVVM approach. Every viewmodel object should have IsSelected property. Then you bind TreeViewItem.IsSelected propery to it.

In your case it would go like this

CS:

public interface INestable : INotifyPropertyChanged
{
  string ID { get; set; }

  // Make sure you invoke PropertyChanged in setter
  bool IsSelected { get; set; } 

  event PropertyChangedEventHandler PropertyChanged;
  ...
}

XAML:

<TreeView ...>
  <TreeView.ItemContainerStyle>
   <Style TargetType="{x:Type TreeViewItem}">
     <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}" />
   </Style>
  </TreeView.ItemContainerStyle>
</TreeView>

Now you can go through your model and set IsSelected property there.

You may also want to track IsExpanded property in a same way...

To get more information about TreeView read this wonderful article by Josh Smith: Simplifying the WPF TreeView by Using the ViewModel Pattern

Hope this helps.

Answer 2

The problem is that the nested ItemContainerGenerators aren't generated all at the beginning, they are generated on-demand. And even more so, they are generated in a separate thread, so you have to listen to a StatusChanged on the generator to be sure it is ready =(

Some people suggest to play with the Dispatcher (like in this Bea's post). I tried to implement the Dispatcher solution, but it didn't work for some reason... the generators are still empty =(

So I ended up with another one, where you specifically ask the tree to update its layout, which causes the generation for the expanded nodes. Here's the final method... you might want to test it a little to verify that it suites your needs. It might collapse some nodes that were expanded before its run.

    private static bool SetSelected(TreeView treeView, ItemsControl parentControl, INestable itemToSelect)
    {
        if (parentControl == null || itemToSelect == null)
        {
            return false;
        }
        foreach (INestable item in parentControl.Items)
        {
            TreeViewItem container = parentControl.ItemContainerGenerator.ContainerFromItem(item) as TreeViewItem;

            if (item.ID == itemToSelect.ID)
            { // just comparing instances failed
                    container.IsSelected = true;
                    container.Focus();
                    return true;
            }
            container.IsExpanded = true;
            treeView.UpdateLayout();
            WaitForPriority(DispatcherPriority.Background);
            if (SetSelected(treeView, container, itemToSelect))
                return true;
            else
                container.IsExpanded = false;
        }
        return false;
    }