WPF MVVM design question

Question

In my View I have a TreeView with a event "TreeView_MouseLeftButtonDown". If it fires it proofs if the mouse clicked on a TreeViewItem. If not it deselects the last TreeViewItem. My question is, should i do this in the code-behind or call a static methode in the viewmodel-class? How would you solve this?

The Methode:

private void treeView_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    if (sender != null)
    {
        var treeView = sender as TreeView;
        if (treeView != null && treeView.SelectedItem != null)
            TreeViewHelper.ReturnTreeViewItem(ref treeView, (XmlNode)treeView.SelectedItem).IsSelected = false;
    }
} 

XAML:

<TreeView ... KeyDown="TreeView_KeyDown" 
              MouseLeftButtonDown="TreeView_MouseLeftButtonDown" 
              SelectedItemChanged="TreeView_SelectedItemChanged" />

Answer 1

You are trying to add a behaviour to the TreeView.

The way I would implement this would be using Attached Properties. I would create an attached property called VerifiesLeftClick or similar and implement the logic in there. This way you do not need an event in the code behind.

See here for samples.

Answer 2

I made for you solution using attached behaviors which were pretty well described here Introduction to Attached Behaviors in WPF by Josh Smith

My solution:

public static class TreeViewBehavior
{
    public static bool GetIsResetMouseLeftButtonDown(TreeView treeView)
    {
        return (bool)treeView.GetValue(IsResetMouseLeftButtonDownProperty);
    }
    public static void SetIsResetMouseLeftButtonDown(TreeView treeViewItem, bool value)
    {
        treeViewItem.SetValue(IsResetMouseLeftButtonDownProperty, value);
    }
    public static readonly DependencyProperty IsResetMouseLeftButtonDownProperty =
        DependencyProperty.RegisterAttached("PreviewMouseLeftButtonDown", typeof(bool), typeof(TreeViewBehavior),
        new UIPropertyMetadata(false, OnIsMouseLeftButtonDownChanged));
    static void OnIsMouseLeftButtonDownChanged(DependencyObject depObj, DependencyPropertyChangedEventArgs e)
    {
        TreeView item = depObj as TreeView;
        if (item == null)
            return;
        if (e.NewValue is bool == false)
            return;

        if ((bool)e.NewValue)
        {
            item.PreviewMouseLeftButtonDown += OnMouseLeftButtonDown;
        }
        else
        {
            item.MouseLeftButtonDown -= OnMouseLeftButtonDown;
        }
    }
    static void OnMouseLeftButtonDown(object sender, RoutedEventArgs e)
    {
        var tempItem = e.Source as TreeViewItem;
        if (tempItem != null && tempItem.IsSelected == false)
        {
            tempItem.IsSelected = true;
        }
        else
        {
            var tree = e.Source as TreeView;
            if (tree != null && tree.SelectedItem != null)
            {
                var selItem = (tree.SelectedItem as TreeViewItem);
                if (selItem != null)
                {
                    selItem.IsSelected = false;
                }
            }
        }
    }
}

and then in View you should add this:

<TreeView local:TreeViewBehavior.IsResetMouseLeftButtonDown="True">

I hope my solution do what you are trying to achieve.