Moving nodes around a TreeView

Question

I'm trying to enable users to move TreeViewItems around a TreeView (specifically, by holding control and pressing arrow keys). I can move nodes into and out of other nodes, and I can move nodes up and down on the top level, but when I try to move nodes up within a subnode, it does nothing, and if I try to move nodes down within a subnode, I get the following exception:

Element already has a logical parent. It must be detached from the old parent before it is attached to a new one.

which occurs when I attempt to add the node back to its original collection (after removing it, of course). Here's the original implementation:

private TreeViewItem getParent(TreeViewItem item)
{
    for (int i=0; i<fragment_tree.Items.Count; ++i)
    {
        TreeViewItem r = getParent((TreeViewItem)(fragment_tree.Items[i]), item);
        if (r != null)
        {
            return r;
        }
    }
    return null;
}
private TreeViewItem getParent(TreeViewItem test, TreeViewItem item)
{
    for (int i=0; i<test.Items.Count; ++i)
    {
        if (test.Items[i] == item)
        {
            return test;
        }
    }
    for (int i=0; i<test.Items.Count; ++i)
    {
        TreeViewItem r = getParent((TreeViewItem)(test.Items[i]), item);
        if (r != null)
        {
            return r;
        }
    }
    return null;
}
private ItemCollection getContainingList(TreeViewItem item, out int id)
{
    return getContainingList(fragment_tree.Items, item, out id);
}
private ItemCollection getContainingList(ItemCollection test, TreeViewItem item, out int id)
{
    for (int i=0; i<test.Count; ++i)
    {
        if (test[i] == item)
        {
            id = i;
            return test;
        }
    }
    for (int i=0; i<test.Count; ++i)
    {
        ItemCollection r = getContainingList((TreeViewItem)(test[i]), out id);
        if (r != null)
        {
            return r;
        }
    }
    id = -1;
    return null;
}
private void fragment_tree_PreviewKeyDown(object sender, KeyEventArgs e)
{
    TreeViewItem selected_item = (TreeViewItem)(fragment_tree.SelectedItem);
    if (selected_item.Header is String)
    {
        if (e.KeyboardDevice.IsKeyDown(Key.LeftCtrl) || e.KeyboardDevice.IsKeyDown(Key.RightCtrl))
        {
            if (e.Key == Key.Up)
            {
                int id;
                ItemCollection collection = getContainingList(selected_item, out id);
                if (collection != null) // it'll never be null, but w/e
                {
                    if (id > 0)
                    {
                        collection.RemoveAt(id);
                        collection.Insert(id-1, selected_item);
                        selected_item.IsSelected = true;
                    }
                }
                e.Handled = true;
            }
            else if (e.Key == Key.Down)
            {
                int id;
                ItemCollection collection = getContainingList(selected_item, out id);
                if (collection != null) // it'll never be null, but w/e
                {
                    if (id < collection.Count)
                    {
                        collection.RemoveAt(id);
                        collection.Insert(id+1, selected_item); // here is the exception
                        selected_item.IsSelected = true;
                    }
                }
                e.Handled = true;
            }
            else if (e.Key == Key.Left)
            {
                TreeViewItem parent = getParent(selected_item);
                if (parent != null)
                {
                    int id;
                    ItemCollection collection = getContainingList(parent, out id);
                    parent.Items.RemoveAt(id);
                    collection.Insert(id, selected_item);
                    selected_item.IsSelected = true;
                }
                e.Handled = true;
            }
            else if (e.Key == Key.Right)
            {
                int id;
                ItemCollection collection = getContainingList(selected_item, out id);
                if (id+1 < collection.Count)
                {
                    TreeViewItem next_item = (TreeViewItem)(collection[id+1]);
                    collection.RemoveAt(id);
                    next_item.Items.Insert(0, selected_item);
                    next_item.IsExpanded = true;
                    selected_item.IsSelected = true;
                }
                e.Handled = true;
            }
        }
    }

I tried making a deep clone of the selected TreeViewItem (though I'd rather not incur the overhead), and got some strange behavior. When I try to move an item up or down within it's subtree, it jumps out to the parent. When I try to move a node on the top level up or down, it deletes its neighbors. I feel as though there's something fundamental I'm missing

private TreeViewItem cloneTreeViewItem(TreeViewItem item)
{
    TreeViewItem r = new TreeViewItem();
    r.Header = item.Header;
    r.Tag = item.Tag;
    for (int i=0; i<item.Items.Count; ++i)
    {
        r.Items.Add(cloneTreeViewItem((TreeViewItem)(item.Items[i])));
    }
    return r;
}

....
if (e.Key == Key.Up)
{
    int id;
    ItemCollection collection = getContainingList(selected_item, out id);
    if (collection != null) // it'll never be null, but w/e
    {
        if (id > 0)
        {
            collection.RemoveAt(id);
            TreeViewItem clone = cloneTreeViewItem(selected_item);
            collection.Insert(id-1, clone);
            clone.IsSelected = true;
        }
    }
    e.Handled = true;
}
else if (e.Key == Key.Down)
{
    int id;
    ItemCollection collection = getContainingList(selected_item, out id);
    if (collection != null) // it'll never be null, but w/e
    {
        if (id < collection.Count)
        {
            collection.RemoveAt(id);
            TreeViewItem clone = cloneTreeViewItem(selected_item);
            collection.Insert(id+1, clone);
            clone.IsSelected = true;
        }
    }
    e.Handled = true;
}
.....

I did spend at least an hour researching this topic, I understand that the TreeViewItem must be detached from its logical parent, and I know from testing that the Parent of a TreeViewItem is always a TreeView, I just don't know how to detach it. I know you try to keep the site free of frivolous questions, and I hope mine isn't one. Any insight would be appreciated.

Answer 1

Well thanks for the input, however after further debugging, I realized my issue was the recursive call within getContainingList,

ItemCollection r = getContainingList((TreeViewItem)(test[i]), out id);

should be

ItemCollection r = getContainingList(((TreeViewItem)(test[i])).Items, item, out id);

that's what I get for coding at 2 am

Answer 2

The QUICK answer is

[whatevertheparentofyourUIElement].Children.Remove(childrenitem);

The CORRECT answer is: do NOT manipulate UIElements in code. You have a conceptual (design) flaw: your UI is not data, your data is data, and your UI is just a nice way to show your data in the screen. Wouldn't it be a lot easier if you just had an ObservableCollection<T> somewhere in your ViewModel level and had to manipulate that? Instead, you're manipulating complicated WPF objects, doing a lot of unnecessary spaghetti code, and you're gonna end up with a lot of problems due to UI Virtualization and stuff like that.

If you had implemented your TreeView the right way, this task would be reduced to 2 lines of code: removing the selected item from an ObservableCollection and inserting it in another ObservableCollection located somewhere else in the hierarchical ViewModel structure.