Materialize TreeViewItems on selection.

If we're selecting a particular tree view item, then materialize and expand the item's ancestors as `SelectionModel` requests children. Only do this if a particular item is being selected, not when an item is selected as part of a range select.

To do this, needed to add a `FinalIndex`  property to `SelectionModelChildrenRequestedEventArgs` in order to know if we're selecting a descendent of the item whose children are being requested.

This is a massive hack, but I can't think of a better way to do it with the current `TreeView` implementation.

src/Avalonia.Controls/IndexPath.cs

@@ -123,6 +123,26 @@ namespace Avalonia.Controls
             }
         }
 
+        public bool IsAncestorOf(in IndexPath other)
+        {
+            if (other.GetSize() <= GetSize())
+            {
+                return false;
+            }
+
+            var size = GetSize();
+
+            for (int i = 0; i < size; i++)
+            {
+                if (GetAt(i) != other.GetAt(i))
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         public override string ToString()
         {
             if (_path != null)

src/Avalonia.Controls/SelectionModel.cs

@@ -141,7 +141,7 @@ namespace Avalonia.Controls
                     while (current?.AnchorIndex >= 0)
                     {
                         path.Add(current.AnchorIndex);
-                        current = current.GetAt(current.AnchorIndex, false);
+                        current = current.GetAt(current.AnchorIndex, false, default);
                     }
 
                     anchor = new IndexPath(path);
@@ -420,7 +420,7 @@ namespace Avalonia.Controls
             for (int i = 0; i < path.GetSize() - 1; i++)
             {
                 var childIndex = path.GetAt(i);
-                node = node.GetAt(childIndex, realizeChild: false);
+                node = node.GetAt(childIndex, false, default);
 
                 if (node == null)
                 {
@@ -455,7 +455,7 @@ namespace Avalonia.Controls
             }
 
             var isSelected = (bool?)false;
-            var childNode = _rootNode.GetAt(groupIndex, realizeChild: false);
+            var childNode = _rootNode.GetAt(groupIndex, false, default);
 
             if (childNode != null)
             {
@@ -474,7 +474,7 @@ namespace Avalonia.Controls
             for (int i = 0; i < path.GetSize() - 1; i++)
             {
                 var childIndex = path.GetAt(i);
-                node = node.GetAt(childIndex, realizeChild: false);
+                node = node.GetAt(childIndex, false, default);
 
                 if (node == null)
                 {
@@ -598,7 +598,10 @@ namespace Avalonia.Controls
             ApplyAutoSelect(true);
         }
 
-        internal IObservable<object?>? ResolvePath(object data, IndexPath dataIndexPath)
+        internal IObservable<object?>? ResolvePath(
+            object data,
+            IndexPath dataIndexPath,
+            IndexPath finalIndexPath)
         {
             IObservable<object?>? resolved = null;
 
@@ -607,18 +610,22 @@ namespace Avalonia.Controls
             {
                 if (_childrenRequestedEventArgs == null)
                 {
-                    _childrenRequestedEventArgs = new SelectionModelChildrenRequestedEventArgs(data, dataIndexPath, false);
+                    _childrenRequestedEventArgs = new SelectionModelChildrenRequestedEventArgs(
+                        data,
+                        dataIndexPath,
+                        finalIndexPath,
+                        false);
                 }
                 else
                 {
-                    _childrenRequestedEventArgs.Initialize(data, dataIndexPath, false);
+                    _childrenRequestedEventArgs.Initialize(data, dataIndexPath, finalIndexPath, false);
                 }
 
                 ChildrenRequested(this, _childrenRequestedEventArgs);
                 resolved = _childrenRequestedEventArgs.Children;
 
                 // Clear out the values in the args so that it cannot be used after the event handler call.
-                _childrenRequestedEventArgs.Initialize(null, default, true);
+                _childrenRequestedEventArgs.Initialize(null, default, default, true);
             }
 
             return resolved;
@@ -683,7 +690,7 @@ namespace Avalonia.Controls
                 ClearSelection(resetAnchor: true);
             }
 
-            var childNode = _rootNode.GetAt(groupIndex, realizeChild: true);
+            var childNode = _rootNode.GetAt(groupIndex, true, new IndexPath(groupIndex, itemIndex));
             var selected = childNode!.Select(itemIndex, select);
 
             if (selected)
@@ -764,7 +771,7 @@ namespace Avalonia.Controls
 
             for (int groupIdx = startGroupIndex; groupIdx <= endGroupIndex; groupIdx++)
             {
-                var groupNode = _rootNode.GetAt(groupIdx, realizeChild: true)!;
+                var groupNode = _rootNode.GetAt(groupIdx, true, new IndexPath(endGroupIndex, endItemIndex))!;
                 int startIndex = groupIdx == startGroupIndex ? startItemIndex : 0;
                 int endIndex = groupIdx == endGroupIndex ? endItemIndex : groupNode.DataCount - 1;
                 groupNode.SelectRange(new IndexRange(startIndex, endIndex), select);

src/Avalonia.Controls/SelectionModelChildrenRequestedEventArgs.cs

@@ -16,15 +16,17 @@ namespace Avalonia.Controls
     {
         private object? _source;
         private IndexPath _sourceIndexPath;
+        private IndexPath _finalIndexPath;
         private bool _throwOnAccess;
         
         internal SelectionModelChildrenRequestedEventArgs(
             object source,
             IndexPath sourceIndexPath,
+            IndexPath finalIndexPath,
             bool throwOnAccess)
         {
             source = source ?? throw new ArgumentNullException(nameof(source));
-            Initialize(source, sourceIndexPath, throwOnAccess);
+            Initialize(source, sourceIndexPath, finalIndexPath, throwOnAccess);
         }
 
         /// <summary>
@@ -65,9 +67,26 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Gets the index of the final object which is being attempted to be retrieved.
+        /// </summary>
+        public IndexPath FinalIndex
+        {
+            get
+            {
+                if (_throwOnAccess)
+                {
+                    throw new ObjectDisposedException(nameof(SelectionModelChildrenRequestedEventArgs));
+                }
+
+                return _finalIndexPath;
+            }
+        }
+
         internal void Initialize(
             object? source,
             IndexPath sourceIndexPath,
+            IndexPath finalIndexPath,
             bool throwOnAccess)
         {
             if (!throwOnAccess && source == null)
@@ -77,6 +96,7 @@ namespace Avalonia.Controls
 
             _source = source;
             _sourceIndexPath = sourceIndexPath;
+            _finalIndexPath = finalIndexPath;
             _throwOnAccess = throwOnAccess;
         }
     }

src/Avalonia.Controls/SelectionNode.cs

@@ -162,7 +162,7 @@ namespace Avalonia.Controls
         // create a bunch of leaf node instances - instead i use the same instance m_leafNode to avoid 
         // an explosion of node objects. However, I'm still creating the m_childrenNodes 
         // collection unfortunately.
-        public SelectionNode? GetAt(int index, bool realizeChild)
+        public SelectionNode? GetAt(int index, bool realizeChild, IndexPath finalIndexPath)
         {
             SelectionNode? child = null;
             
@@ -192,7 +192,7 @@ namespace Avalonia.Controls
                     if (childData != null)
                     {
                         var childDataIndexPath = IndexPath.CloneWithChildIndex(index);
-                        resolver = _manager.ResolvePath(childData, childDataIndexPath);
+                        resolver = _manager.ResolvePath(childData, childDataIndexPath, finalIndexPath);
                     }
 
                     if (resolver != null)
@@ -864,7 +864,7 @@ namespace Avalonia.Controls
                     int notSelectedCount = 0;
                     for (int i = 0; i < ChildrenNodeCount; i++)
                     {
-                        var child = GetAt(i, realizeChild: false);
+                        var child = GetAt(i, false, default);
 
                         if (child != null)
                         {

src/Avalonia.Controls/TreeView.cs

@@ -395,10 +395,17 @@ namespace Avalonia.Controls
 
         private void OnSelectionModelChildrenRequested(object sender, SelectionModelChildrenRequestedEventArgs e)
         {
-            var container = ItemContainerGenerator.Index.ContainerFromItem(e.Source) as ItemsControl;
+            var container = ItemContainerGenerator.Index.ContainerFromItem(e.Source) as TreeViewItem;
 
             if (container is object)
             {
+                if (e.SourceIndex.IsAncestorOf(e.FinalIndex))
+                {
+                    container.IsExpanded = true;
+                    container.ApplyTemplate();
+                    container.Presenter?.ApplyTemplate();
+                }
+
                 e.Children = Observable.CombineLatest(
                     container.GetObservable(TreeViewItem.IsExpandedProperty),
                     container.GetObservable(ItemsProperty),

src/Avalonia.Controls/Utils/SelectionTreeHelper.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Controls.Utils
 
                 if (depth < path.GetSize() - 1)
                 {
-                    node = node.GetAt(childIndex, realizeChildren)!;
+                    node = node.GetAt(childIndex, realizeChildren, path)!;
                 }
             }
         }
@@ -50,7 +50,7 @@ namespace Avalonia.Controls.Utils
                 int count = realizeChildren ? nextNode.Node.DataCount : nextNode.Node.ChildrenNodeCount;
                 for (int i = count - 1; i >= 0; i--)
                 {
-                    var child = nextNode.Node.GetAt(i, realizeChildren);
+                    var child = nextNode.Node.GetAt(i, realizeChildren, nextNode.Path);
                     var childPath = nextNode.Path.CloneWithChildIndex(i);
                     if (child != null)
                     {
@@ -90,7 +90,7 @@ namespace Avalonia.Controls.Utils
 
                     for (int i = endIndex; i >= startIndex; i--)
                     {
-                        var child = node.GetAt(i, realizeChild: true);
+                        var child = node.GetAt(i, true, end);
                         if (child != null)
                         {
                             var childPath = currentPath.CloneWithChildIndex(i);
@@ -112,7 +112,7 @@ namespace Avalonia.Controls.Utils
                 int endIndex = depth < end.GetSize() && isEndPath ? end.GetAt(depth) : info.Node.DataCount - 1;
                 for (int i = endIndex; i >= startIndex; i--)
                 {
-                    var child = info.Node.GetAt(i, realizeChild: true);
+                    var child = info.Node.GetAt(i, true, end);
                     if (child != null)
                     {
                         var childPath = info.Path.CloneWithChildIndex(i);