Use dictionary to track item containers.

Because even when virtualized we were still creating a list the size of
the Items collection to store the containers. Using Dictionary here
still isn't ideal - we'd ideally use some sort of sparse array but that
can be optimized later.

src/Avalonia.Controls/Generators/ItemContainerGenerator.cs

@@ -16,7 +16,7 @@ namespace Avalonia.Controls.Generators
     /// </summary>
     public class ItemContainerGenerator : IItemContainerGenerator
     {
-        private List<ItemContainerInfo> _containers = new List<ItemContainerInfo>();
+        private Dictionary<int, ItemContainerInfo> _containers = new Dictionary<int, ItemContainerInfo>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemContainerGenerator"/> class.
@@ -30,7 +30,7 @@ namespace Avalonia.Controls.Generators
         }
 
         /// <inheritdoc/>
-        public IEnumerable<ItemContainerInfo> Containers => _containers.Where(x => x != null);
+        public IEnumerable<ItemContainerInfo> Containers => _containers.Values;
 
         /// <inheritdoc/>
         public event EventHandler<ItemContainerEventArgs> Materialized;
@@ -60,7 +60,7 @@ namespace Avalonia.Controls.Generators
             var i = selector != null ? selector.Select(item) : item;
             var container = new ItemContainerInfo(CreateContainer(i), item, index);
 
-            AddContainer(container);
+            _containers.Add(container.Index, container);
             Materialized?.Invoke(this, new ItemContainerEventArgs(container));
 
             return container;
@@ -73,11 +73,8 @@ namespace Avalonia.Controls.Generators
 
             for (int i = startingIndex; i < startingIndex + count; ++i)
             {
-                if (i < _containers.Count &&_containers[i] != null)
-                {
-                    result.Add(_containers[i]);
-                    _containers[i] = null;
-                }
+                result.Add(_containers[i]);
+                _containers.Remove(i);
             }
 
             Dematerialized?.Invoke(this, new ItemContainerEventArgs(startingIndex, result));
@@ -88,18 +85,47 @@ namespace Avalonia.Controls.Generators
         /// <inheritdoc/>
         public virtual void InsertSpace(int index, int count)
         {
-            _containers.InsertRange(index, Enumerable.Repeat<ItemContainerInfo>(null, count));
+            if (count > 0)
+            {
+                var toMove = _containers.Where(x => x.Key >= index).ToList();
+
+                foreach (var i in toMove)
+                {
+                    _containers.Remove(i.Key);
+                    i.Value.Index += count;
+                    _containers[i.Value.Index] = i.Value;
+                }
+            }
         }
 
         /// <inheritdoc/>
         public virtual IEnumerable<ItemContainerInfo> RemoveRange(int startingIndex, int count)
         {
-            List<ItemContainerInfo> result = new List<ItemContainerInfo>();
+            var result = new List<ItemContainerInfo>();
 
-            if (startingIndex < _containers.Count)
+            if (count > 0)
             {
-                result.AddRange(_containers.GetRange(startingIndex, count));
-                _containers.RemoveRange(startingIndex, count);
+                for (var i = startingIndex; i < startingIndex + count; ++i)
+                {
+                    ItemContainerInfo found;
+
+                    if (_containers.TryGetValue(i, out found))
+                    {
+                        result.Add(found);
+                    }
+
+                    _containers.Remove(i);
+                }
+
+                var toMove = _containers.Where(x => x.Key >= startingIndex).ToList();
+
+                foreach (var i in toMove)
+                {
+                    _containers.Remove(i.Key);
+                    i.Value.Index -= count;
+                    _containers.Add(i.Value.Index, i.Value);
+                }
+
                 Dematerialized?.Invoke(this, new ItemContainerEventArgs(startingIndex, result));
             }
 
@@ -119,8 +145,8 @@ namespace Avalonia.Controls.Generators
         /// <inheritdoc/>
         public virtual IEnumerable<ItemContainerInfo> Clear()
         {
-            var result = _containers.Where(x => x != null).ToList();
-            _containers = new List<ItemContainerInfo>();
+            var result = Containers.ToList();
+            _containers.Clear();
 
             if (result.Count > 0)
             {
@@ -133,27 +159,20 @@ namespace Avalonia.Controls.Generators
         /// <inheritdoc/>
         public IControl ContainerFromIndex(int index)
         {
-            if (index < _containers.Count)
-            {
-                return _containers[index]?.ContainerControl;
-            }
-
-            return null;
+            ItemContainerInfo result;
+            _containers.TryGetValue(index, out result);
+            return result?.ContainerControl;
         }
 
         /// <inheritdoc/>
         public int IndexFromContainer(IControl container)
         {
-            var index = 0;
-
             foreach (var i in _containers)
             {
-                if (i?.ContainerControl == container)
+                if (i.Value.ContainerControl == container)
                 {
-                    return index;
+                    return i.Key;
                 }
-
-                ++index;
             }
 
             return -1;
@@ -176,33 +195,6 @@ namespace Avalonia.Controls.Generators
             return result;
         }
 
-        /// <summary>
-        /// Adds a container to the index.
-        /// </summary>
-        /// <param name="container">The container.</param>
-        protected void AddContainer(ItemContainerInfo container)
-        {
-            Contract.Requires<ArgumentNullException>(container != null);
-
-            while (_containers.Count < container.Index)
-            {
-                _containers.Add(null);
-            }
-
-            if (_containers.Count == container.Index)
-            {
-                _containers.Add(container);
-            }
-            else if (_containers[container.Index] == null)
-            {
-                _containers[container.Index] = container;
-            }
-            else
-            {
-                throw new InvalidOperationException("Container already created.");
-            }
-        }
-
         /// <summary>
         /// Moves a container.
         /// </summary>
@@ -213,10 +205,11 @@ namespace Avalonia.Controls.Generators
         protected ItemContainerInfo MoveContainer(int oldIndex, int newIndex, object item)
         {
             var container = _containers[oldIndex];
-            var newContainer = new ItemContainerInfo(container.ContainerControl, item, newIndex);
-            _containers[oldIndex] = null;
-            AddContainer(newContainer);
-            return newContainer;
+            container.Index = newIndex;
+            container.Item = item;
+            _containers.Remove(oldIndex);
+            _containers.Add(newIndex, container);
+            return container;
         }
 
         /// <summary>
@@ -227,7 +220,7 @@ namespace Avalonia.Controls.Generators
         /// <returns>The containers.</returns>
         protected IEnumerable<ItemContainerInfo> GetContainerRange(int index, int count)
         {
-            return _containers.GetRange(index, count);
+            return _containers.Where(x => x.Key >= index && x.Key <= index + count).Select(x => x.Value);
         }
 
         /// <summary>

src/Avalonia.Controls/Generators/ItemContainerInfo.cs

@@ -35,11 +35,11 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Gets the item that the container represents.
         /// </summary>
-        public object Item { get; }
+        public object Item { get; internal set; }
 
         /// <summary>
         /// Gets the index of the item in the <see cref="ItemsControl.Items"/> collection.
         /// </summary>
-        public int Index { get; }
+        public int Index { get; internal set; }
     }
 }

tests/Avalonia.Controls.UnitTests/Generators/ItemContainerGeneratorTests.cs

@@ -1,11 +1,9 @@
 // Copyright (c) The Avalonia Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using Avalonia.Controls.Generators;
-using Avalonia.Controls.Templates;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests.Generators
@@ -41,22 +39,6 @@ namespace Avalonia.Controls.UnitTests.Generators
             Assert.Equal(containers[2].ContainerControl, target.ContainerFromIndex(2));
         }
 
-        private IList<ItemContainerInfo> Materialize(
-            IItemContainerGenerator generator,
-            int index,
-            string[] items)
-        {
-            var result = new List<ItemContainerInfo>();
-
-            foreach (var item in items)
-            {
-                var container = generator.Materialize(index++, item, null);
-                result.Add(container);
-            }
-
-            return result;
-        }
-
         [Fact]
         public void IndexFromContainer_Should_Return_Index()
         {
@@ -98,6 +80,20 @@ namespace Avalonia.Controls.UnitTests.Generators
             Assert.Equal(expected, result);
         }
 
+        [Fact]
+        public void InsertSpace_Should_Alter_Successive_Container_Indexes()
+        {
+            var items = new[] { "foo", "bar", "baz" };
+            var owner = new Decorator();
+            var target = new ItemContainerGenerator(owner);
+            var containers = Materialize(target, 0, items);
+
+            target.InsertSpace(1, 3);
+
+            Assert.Equal(3, target.Containers.Count());
+            Assert.Equal(new[] { 0, 4, 5 }, target.Containers.Select(x => x.Index));
+        }
+
         [Fact]
         public void RemoveRange_Should_Alter_Successive_Container_Indexes()
         {
@@ -111,6 +107,23 @@ namespace Avalonia.Controls.UnitTests.Generators
             Assert.Equal(containers[0].ContainerControl, target.ContainerFromIndex(0));
             Assert.Equal(containers[2].ContainerControl, target.ContainerFromIndex(1));
             Assert.Equal(containers[1], removed);
+            Assert.Equal(new[] { 0, 1 }, target.Containers.Select(x => x.Index));
+        }
+
+        private IList<ItemContainerInfo> Materialize(
+            IItemContainerGenerator generator,
+            int index,
+            string[] items)
+        {
+            var result = new List<ItemContainerInfo>();
+
+            foreach (var item in items)
+            {
+                var container = generator.Materialize(index++, item, null);
+                result.Add(container);
+            }
+
+            return result;
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Presenters/CarouselPresenterTests.cs

@@ -155,7 +155,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
         }
 
         [Fact]
-        public void Should_have_correct_index_itemscontainer()
+        public void Should_Have_Correct_ItemsContainer_Index()
         {
             ObservableCollection<string> items = new ObservableCollection<string>();
 
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             items.Remove(items[0]);
             Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
             Assert.Equal(1, target.Panel.Children.Count);
-            Assert.Equal(1, target.ItemContainerGenerator.Containers.First().Index);
+            Assert.Equal(0, target.ItemContainerGenerator.Containers.First().Index);
 
             items.Remove(items[0]);
             Assert.Equal(0, target.ItemContainerGenerator.Containers.Count());