Refactor DevTools logical/visual tree nodes.

- Previously we created a node for every logical/visual control immediately, now only create them when the `TreeView` requests nodes
- Dispose the child node collections when the DevTools window closes

src/Avalonia.Base/Collections/AvaloniaListExtensions.cs

@@ -140,6 +140,7 @@ namespace Avalonia.Collections
             }
         }
 
+        [Obsolete("Causes memory leaks. Use DynamicData or similar instead.")]
         public static IAvaloniaReadOnlyList<TDerived> CreateDerivedList<TSource, TDerived>(
             this IAvaloniaReadOnlyList<TSource> collection,
             Func<TSource, TDerived> select)

src/Avalonia.Controls/Utils/IEnumerableUtils.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections;
+using System.Collections.Generic;
 using System.Linq;
 
 namespace Avalonia.Controls.Utils
@@ -15,12 +16,14 @@ namespace Avalonia.Controls.Utils
         {
             if (items != null)
             {
-                var collection = items as ICollection;
-
-                if (collection != null)
+                if (items is ICollection collection)
                 {
                     return collection.Count;
                 }
+                else if (items is IReadOnlyCollection<object> readOnly)
+                {
+                    return readOnly.Count;
+                }
                 else
                 {
                     return Enumerable.Count(items.Cast<object>());

src/Avalonia.Diagnostics/Diagnostics/ViewModels/LogicalTreeNode.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia.Collections;
 using Avalonia.Controls;
 using Avalonia.LogicalTree;
@@ -9,7 +10,7 @@ namespace Avalonia.Diagnostics.ViewModels
         public LogicalTreeNode(ILogical logical, TreeNode parent)
             : base((Control)logical, parent)
         {
-            Children = logical.LogicalChildren.CreateDerivedList(x => new LogicalTreeNode(x, this));
+            Children = new LogicalTreeNodeCollection(this, logical);
         }
 
         public static LogicalTreeNode[] Create(object control)
@@ -17,5 +18,31 @@ namespace Avalonia.Diagnostics.ViewModels
             var logical = control as ILogical;
             return logical != null ? new[] { new LogicalTreeNode(logical, null) } : null;
         }
+
+        internal class LogicalTreeNodeCollection : TreeNodeCollection
+        {
+            private readonly ILogical _control;
+            private IDisposable _subscription;
+
+            public LogicalTreeNodeCollection(TreeNode owner, ILogical control)
+                : base(owner)
+            {
+                _control = control;
+            }
+
+            public override void Dispose()
+            {
+                base.Dispose();
+                _subscription?.Dispose();
+            }
+
+            protected override void Initialize(AvaloniaList<TreeNode> nodes)
+            {
+                _subscription = _control.LogicalChildren.ForEachItem(
+                    (i, item) => nodes.Insert(i, new LogicalTreeNode(item, Owner)),
+                    (i, item) => nodes.RemoveAt(i),
+                    () => nodes.Clear());
+            }
+        }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNode.cs

@@ -3,14 +3,13 @@ using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Reactive;
 using System.Reactive.Linq;
-using Avalonia.Collections;
 using Avalonia.Controls;
 using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
-    internal class TreeNode : ViewModelBase
+    internal class TreeNode : ViewModelBase, IDisposable
     {
         private string _classes;
         private bool _isExpanded;
@@ -49,7 +48,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public IAvaloniaReadOnlyList<TreeNode> Children
+        public TreeNodeCollection Children
         {
             get;
             protected set;
@@ -104,6 +103,11 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
+        public void Dispose()
+        {
+            Children.Dispose();
+        }
+
         private static int IndexOf(IReadOnlyList<TreeNode> collection, TreeNode item)
         {
             var count = collection.Count;

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.ComponentModel;
+using Avalonia.Collections;
+
+namespace Avalonia.Diagnostics.ViewModels
+{
+    internal abstract class TreeNodeCollection : IAvaloniaReadOnlyList<TreeNode>, IDisposable
+    {
+        private AvaloniaList<TreeNode> _inner;
+
+        public TreeNodeCollection(TreeNode owner) => Owner = owner;
+
+        public TreeNode this[int index]
+        {
+            get
+            {
+                EnsureInitialized();
+                return _inner[index];
+            }
+        }
+
+        public int Count
+        {
+            get
+            {
+                EnsureInitialized();
+                return _inner.Count;
+            }
+        }
+
+        protected TreeNode Owner { get; }
+
+        public event NotifyCollectionChangedEventHandler CollectionChanged
+        {
+            add => _inner.CollectionChanged += value;
+            remove => _inner.CollectionChanged -= value;
+        }
+
+        public event PropertyChangedEventHandler PropertyChanged
+        {
+            add => _inner.PropertyChanged += value;
+            remove => _inner.PropertyChanged -= value;
+        }
+
+        public virtual void Dispose()
+        {
+            if (_inner is object)
+            {
+                foreach (var node in _inner)
+                {
+                    node.Dispose();
+                }
+            }
+        }
+
+        public IEnumerator<TreeNode> GetEnumerator()
+        {
+            EnsureInitialized();
+            return _inner.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+        protected abstract void Initialize(AvaloniaList<TreeNode> nodes);
+
+        private void EnsureInitialized()
+        {
+            if (_inner is null)
+            {
+                _inner = new AvaloniaList<TreeNode>();
+                Initialize(_inner);
+            }
+        }
+    }
+}

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreePageViewModel.cs

@@ -62,7 +62,15 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public void Dispose() => _details?.Dispose();
+        public void Dispose()
+        {
+            foreach (var node in Nodes)
+            {
+                node.Dispose();
+            }
+
+            _details?.Dispose();
+        }
 
         public TreeNode FindNode(IControl control)
         {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/VisualTreeNode.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia.Collections;
 using Avalonia.Styling;
 using Avalonia.VisualTree;
@@ -9,16 +10,7 @@ namespace Avalonia.Diagnostics.ViewModels
         public VisualTreeNode(IVisual visual, TreeNode parent)
             : base(visual, parent)
         {
-            var host = visual as IVisualTreeHost;
-
-            if (host?.Root == null)
-            {
-                Children = visual.VisualChildren.CreateDerivedList(x => new VisualTreeNode(x, this));
-            }
-            else
-            {
-                Children = new AvaloniaList<VisualTreeNode>(new[] { new VisualTreeNode(host.Root, this) });
-            }
+            Children = new VisualTreeNodeCollection(this, visual);
 
             if ((Visual is IStyleable styleable))
             {
@@ -33,5 +25,30 @@ namespace Avalonia.Diagnostics.ViewModels
             var visual = control as IVisual;
             return visual != null ? new[] { new VisualTreeNode(visual, null) } : null;
         }
+
+        internal class VisualTreeNodeCollection : TreeNodeCollection
+        {
+            private readonly IVisual _control;
+            private IDisposable _subscription;
+
+            public VisualTreeNodeCollection(TreeNode owner, IVisual control)
+                : base(owner)
+            {
+                _control = control;
+            }
+
+            public override void Dispose()
+            {
+                _subscription?.Dispose();
+            }
+
+            protected override void Initialize(AvaloniaList<TreeNode> nodes)
+            {
+                _subscription = _control.VisualChildren.ForEachItem(
+                    (i, item) => nodes.Insert(i, new VisualTreeNode(item, Owner)),
+                    (i, item) => nodes.RemoveAt(i),
+                    () => nodes.Clear());
+            }
+        }
     }
 }