Prevent Automatic Horizontal Scroll in TreeView is not working. MVVM, XAML approach

Question

Whenever a node is selected in my treeview, it automatically does a horizontal scroll to that item. I've found the way to disable this. If I use this code in code behind, it works perfectly:

<TreeView>
  <TreeView.ItemContainerStyle>
      <Style TargetType="TreeViewItem">
         <EventSetter Event="RequestBringIntoView" Handler="TreeViewItem_RequestBringIntoView"/>
      </Style>
   </TreeView.ItemContainerStyle>
</TreeView>

private void TreeViewItem_RequestBringIntoView(object sender, RequestBringIntoViewEventArgs e)
{
    e.Handled = true;
}

However, if I use MVVM, I cannot disable horizontal scroll to the item:

My window:

<Window x:Class="TreeViewWpfApplication.MainWindow"
    . . . . .  
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:ei=http://schemas.microsoft.com/expression/2010/interactions>
  <TreeView Grid.Column="1" Margin="5" Background="Green">
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="RequestBringIntoView">
                <ei:CallMethodAction MethodName="RequestBringIntoView_Handler" TargetObject="{Binding}"/>
            </i:EventTrigger>
        </i:Interaction.Triggers> 

<TreeView>
     <TreeViewItem Header="---Level 1" >
       <TreeViewItem Header="--- Level 2.1" >
          <TreeViewItem Header="--- Level 3.1" >
          </TreeViewItem>
        </TreeViewItem>
     </TreeViewItem>  
     <TreeViewItem Header="Level 2.3" />        
   </TreeView>
 </Window>

ViewModel:

public void RequestBringIntoView_Handler(object sender, RequestBringIntoViewEventArgs e)
{
    e.Handled = true;         
}

Why cannot I stop automatic horizontal scroll to the item by MVVM approach?

Answer 1

I believe you can always loop through all TreeViewItem of the TreeView and clone each TriggerBase from Interaction.Triggers attached on the TreeView before attaching each cloned one to the Interaction.Triggers of each TreeViewItem.

I'm so disappointed about Microsoft, that name has made me proud of and been my endless inspiration for many years since I started beginning to learn how to program. But frankly speaking there are many things Microsoft made us disappointed. Your code should have actually worked fine. Why? I've tried it and the event RequestBringIntoView actually bubbles up from TreeViewItem to TreeView. And in fact when you add the event handler on TreeView directly, you'll see the event handler is fired OK. But the very equivalent form of setting handler using Interaction does not work that way. That's so terrible. It's obvious that it's designed to setup event handler in MVVM way but it's so limited.

I had to make a work-around in which we use a custom attached property to allow to set the Interaction.Triggers in Style. However I have to say that it's not very pretty. You need to explicitly declare an Array of TriggerBase (I've done something like this before but never found a better solution for this). Next you need to use a proxy to bind TargetObject for EventTrigger (because we put the trigger in an Array and it's detached from visual tree).

Here is the code for the custom attached property:

//add some using alias like this first
//using i = System.Windows.Interactivity;
public static class InteractionX 
{
    public static readonly DependencyProperty TriggersProperty
        = DependencyProperty.RegisterAttached("Triggers", typeof(i.TriggerBase[]), 
          typeof(InteractionX), new PropertyMetadata(triggersChanged));
    public static i.TriggerBase[] GetTriggers(DependencyObject o){
        return o.GetValue(TriggersProperty) as i.TriggerBase[];
    }
    public static void SetTriggers(DependencyObject o, i.TriggerBase[] value)
    {
        o.SetValue(TriggersProperty, value);
    }
    static void triggersChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        var triggers = e.NewValue as i.TriggerBase[];
        var currentTriggers = i.Interaction.GetTriggers(o);
        currentTriggers.Clear();
        foreach (var t in triggers)
        {
            t.Detach();
            currentTriggers.Add(t);
        }
    }
}

Here is the XAML:

<TreeView>
  <TreeView.Resources>
     <DiscreteObjectKeyFrame x:Key="proxy" Value="{Binding}"/> 
  </TreeView.Resources>
  <TreeView.ItemContainerStyle>
      <Style TargetType="TreeViewItem">
         <Setter Property="local:InteractionX.Triggers">
                <Setter.Value>            
                    <x:Array Type="{x:Type i:TriggerBase}">
                        <i:EventTrigger EventName="RequestBringIntoView">
                            <ei:CallMethodAction MethodName="bringIntoViewHandler" 
                             TargetObject="{Binding Value, Source={StaticResource proxy}}"/>
                        </i:EventTrigger>
                    </x:Array>
                </Setter.Value>
            </Setter>   
      </Style>
  </TreeView.ItemContainerStyle>
</TreeView>

It appears that the Interaction.Triggers set on TreeViewItem can handle bubbling-up RequestBringIntoView from the descendant TreeViewItems but as I said it's a pity that setting that on TreeView does not work.