WIP

samples/XamlTestApplicationPcl/Views/MainWindow.cs

@@ -25,6 +25,17 @@ namespace XamlTestApplication.Views
             AvaloniaXamlLoader.Load(this);
             _exitMenu = this.FindControl<MenuItem>("exitMenu");
             _exitMenu.Click += (s, e) => Application.Current.Exit();
+
+            var vadd = this.FindControl<Button>("vadd");
+            var vsp = this.FindControl<VirtualizingStackPanel>("vsp");
+            var ivp = (IVirtualizingPanel)vsp;
+            var index = 0;
+
+            vadd.Click += (s, e) =>
+            {
+                vsp.Children.Add(new TextBlock { Text = "Hello " + ++index });
+                vadd.IsEnabled = !ivp.IsFull;
+            };
         }
     }
 }

samples/XamlTestApplicationPcl/Views/MainWindow.xaml

@@ -24,6 +24,17 @@
       <TabControl.Transition>
         <PageSlide Duration="0.25" />
       </TabControl.Transition>
+
+
+      <TabItem Header="Virtualization">
+        <DockPanel LastChildFill="True">
+          <StackPanel DockPanel.Dock="Right" Orientation="Vertical">
+            <Button Name="vadd">Add Item</Button>
+          </StackPanel>
+          <Thingamybob></Thingamybob>
+        </DockPanel>
+      </TabItem>
+      
       <TabItem Header="Buttons">
         <ScrollViewer CanScrollHorizontally="False">
           <StackPanel Margin="10" Gap="4">

src/Avalonia.Controls/Avalonia.Controls.csproj

@@ -57,6 +57,7 @@
     <Compile Include="INameScope.cs" />
     <Compile Include="IPseudoClasses.cs" />
     <Compile Include="DropDownItem.cs" />
+    <Compile Include="IVirtualizingPanel.cs" />
     <Compile Include="LayoutTransformControl.cs" />
     <Compile Include="Mixins\ContentControlMixin.cs" />
     <Compile Include="NameScope.cs" />
@@ -161,6 +162,7 @@
     <Compile Include="Templates\ITemplate`1.cs" />
     <Compile Include="Templates\ITreeDataTemplate.cs" />
     <Compile Include="Templates\FuncTreeDataTemplate.cs" />
+    <Compile Include="Thingamybob.cs" />
     <Compile Include="ToolTip.cs" />
     <Compile Include="UserControl.cs" />
     <Compile Include="Templates\TemplateExtensions.cs" />
@@ -171,6 +173,7 @@
     <Compile Include="TopLevel.cs" />
     <Compile Include="Primitives\PopupRoot.cs" />
     <Compile Include="Utils\UndoRedoHelper.cs" />
+    <Compile Include="VirtualizingStackPanel.cs" />
     <Compile Include="WindowState.cs" />
     <Compile Include="Window.cs" />
     <Compile Include="RowDefinition.cs" />

src/Avalonia.Controls/IVirtualizingPanel.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace Avalonia.Controls
+{
+    public interface IVirtualizingPanel : IPanel
+    {
+        bool IsFull { get; }
+
+        int OverflowCount { get; }
+
+        Action ArrangeCompleted { get; set; }
+    }
+}

src/Avalonia.Controls/Panel.cs

@@ -79,12 +79,28 @@ namespace Avalonia.Controls
             set { SetValue(BackgroundProperty, value); }
         }
 
+        /// <summary>
+        /// Renders the visual to a <see cref="DrawingContext"/>.
+        /// </summary>
+        /// <param name="context">The drawing context.</param>
+        public override void Render(DrawingContext context)
+        {
+            var background = Background;
+            if (background != null)
+            {
+                var renderSize = Bounds.Size;
+                context.FillRectangle(background, new Rect(renderSize));
+            }
+
+            base.Render(context);
+        }
+
         /// <summary>
         /// Called when the <see cref="Children"/> collection changes.
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected virtual void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
         {
             List<Control> controls;
 
@@ -122,21 +138,5 @@ namespace Avalonia.Controls
 
             InvalidateMeasure();
         }
-
-        /// <summary>
-        /// Renders the visual to a <see cref="DrawingContext"/>.
-        /// </summary>
-        /// <param name="context">The drawing context.</param>
-        public override void Render(DrawingContext context)
-        {
-            var background = Background;
-            if (background != null)
-            {
-                var renderSize = Bounds.Size;
-                context.FillRectangle(background, new Rect(renderSize));
-            }
-
-            base.Render(context);
-        }
     }
 }

src/Avalonia.Controls/StackPanel.cs

@@ -181,6 +181,7 @@ namespace Avalonia.Controls
         /// <returns>The space taken.</returns>
         protected override Size ArrangeOverride(Size finalSize)
         {
+            var orientation = Orientation;
             double arrangedWidth = finalSize.Width;
             double arrangedHeight = finalSize.Height;
             double gap = Gap;
@@ -199,11 +200,11 @@ namespace Avalonia.Controls
                 double childWidth = child.DesiredSize.Width;
                 double childHeight = child.DesiredSize.Height;
 
-                if (Orientation == Orientation.Vertical)
+                if (orientation == Orientation.Vertical)
                 {
                     double width = Math.Max(childWidth, arrangedWidth);
                     Rect childFinal = new Rect(0, arrangedHeight, width, childHeight);
-                    child.Arrange(childFinal);
+                    ArrangeChild(child, childFinal, finalSize, orientation);
                     arrangedWidth = Math.Max(arrangedWidth, childWidth);
                     arrangedHeight += childHeight + gap;
                 }
@@ -211,13 +212,13 @@ namespace Avalonia.Controls
                 {
                     double height = Math.Max(childHeight, arrangedHeight);
                     Rect childFinal = new Rect(arrangedWidth, 0, childWidth, height);
-                    child.Arrange(childFinal);
+                    ArrangeChild(child, childFinal, finalSize, orientation);
                     arrangedWidth += childWidth + gap;
                     arrangedHeight = Math.Max(arrangedHeight, childHeight);
                 }
             }
 
-            if (Orientation == Orientation.Vertical)
+            if (orientation == Orientation.Vertical)
             {
                 arrangedHeight = Math.Max(arrangedHeight - gap, finalSize.Height);
             }
@@ -228,5 +229,14 @@ namespace Avalonia.Controls
 
             return new Size(arrangedWidth, arrangedHeight);
         }
+
+        internal virtual void ArrangeChild(
+            IControl child,
+            Rect rect,
+            Size panelSize,
+            Orientation orientation)
+        {
+            child.Arrange(rect);
+        }
     }
 }

src/Avalonia.Controls/Thingamybob.cs

@@ -0,0 +1,67 @@
+using Avalonia.Media;
+using System;
+
+namespace Avalonia.Controls
+{
+    public class Thingamybob : Decorator
+    {
+        private int _lastIndex;
+
+        public override void ApplyTemplate()
+        {
+            if (Child == null)
+            {
+                Child = new VirtualizingStackPanel();
+                ((IVirtualizingPanel)Child).ArrangeCompleted = CheckPanel;
+            }
+        }
+
+        protected override Size ArrangeOverride(Size finalSize)
+        {
+            var result = base.ArrangeOverride(finalSize);
+            CreateItems();
+            return result;
+        }
+
+        private void CreateItems()
+        {
+            var panel = Child as IVirtualizingPanel;
+            var randomColor = Color.FromUInt32(
+                (uint)(0xff000000 + new Random().Next(0xffffff)));
+
+            while (!panel.IsFull)
+            {
+                panel.Children.Add(new TextBlock
+                {
+                    Text = "Item " + ++_lastIndex,
+                    Background = new SolidColorBrush(randomColor),
+                });
+            }
+        }
+
+        private void RemoveItems()
+        {
+            var panel = Child as IVirtualizingPanel;
+            var remove = panel.OverflowCount;
+
+            panel.Children.RemoveRange(
+                panel.Children.Count - remove,
+                panel.OverflowCount);
+            _lastIndex -= remove;
+        }
+
+        private void CheckPanel()
+        {
+            var panel = Child as IVirtualizingPanel;
+
+            if (!panel.IsFull)
+            {
+                CreateItems();
+            }
+            else if (panel.OverflowCount > 0)
+            {
+                RemoveItems();
+            }
+        }
+    }
+}

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -0,0 +1,148 @@
+// 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 Avalonia.Controls.Primitives;
+using System;
+using System.Collections.Specialized;
+
+namespace Avalonia.Controls
+{
+    public class VirtualizingStackPanel : StackPanel, IScrollable, IVirtualizingPanel
+    {
+        private double _takenSpace;
+        private int _canBeRemoved;
+
+        bool IVirtualizingPanel.IsFull
+        {
+            get
+            {
+                return Orientation == Orientation.Horizontal ?
+                    _takenSpace >= Bounds.Width :
+                    _takenSpace >= Bounds.Height;
+            }
+        }
+
+        int IVirtualizingPanel.OverflowCount => _canBeRemoved;
+
+        Action IVirtualizingPanel.ArrangeCompleted { get; set; }
+
+        Action IScrollable.InvalidateScroll
+        {
+            get;
+            set;
+        }
+
+        Size IScrollable.Extent => new Size(_takenSpace, _takenSpace);
+
+        Vector IScrollable.Offset
+        {
+            get { return default(Vector); }
+            set { }
+        }
+
+        Size IScrollable.Viewport => Bounds.Size;
+
+        Size IScrollable.ScrollSize => new Size(1, 1);
+
+        Size IScrollable.PageScrollSize => new Size(1, 1);
+
+        protected override Size ArrangeOverride(Size finalSize)
+        {
+            _canBeRemoved = 0;
+            _takenSpace = 0;
+            var result = base.ArrangeOverride(finalSize);
+            ((IVirtualizingPanel)this).ArrangeCompleted?.Invoke();
+            return result;
+        }
+
+        protected override void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        {
+            base.ChildrenChanged(sender, e);
+
+            switch (e.Action)
+            {
+                case NotifyCollectionChangedAction.Add:
+                    foreach (IControl control in e.NewItems)
+                    {
+                        UpdatePhysicalSizeForAdd(control);
+                    }
+
+                    break;
+
+                case NotifyCollectionChangedAction.Remove:
+                    foreach (IControl control in e.OldItems)
+                    {
+                        UpdatePhysicalSizeForRemove(control);
+                    }
+
+                    break;
+            }
+        }
+
+        internal override void ArrangeChild(
+            IControl child, 
+            Rect rect,
+            Size panelSize,
+            Orientation orientation)
+        {
+            base.ArrangeChild(child, rect, panelSize, orientation);
+
+            if (orientation == Orientation.Horizontal)
+            {
+                if (rect.X >= panelSize.Width)
+                {
+                    ++_canBeRemoved;
+                }
+
+                if (rect.Right >= _takenSpace)
+                {
+                    _takenSpace = rect.Right;
+                }
+            }
+            else
+            {
+                if (rect.Y >= panelSize.Height)
+                {
+                    ++_canBeRemoved;
+                }
+
+                if (rect.Bottom >= _takenSpace)
+                {
+                    _takenSpace = rect.Bottom;
+                }
+            }
+        }
+
+        private void UpdatePhysicalSizeForAdd(IControl child)
+        {
+            var bounds = Bounds;
+            var gap = Gap;
+
+            child.Measure(bounds.Size);
+
+            if (Orientation == Orientation.Vertical)
+            {
+                _takenSpace += child.DesiredSize.Height + gap;
+            }
+            else
+            {
+                _takenSpace += child.DesiredSize.Width + gap;
+            }
+        }
+
+        private void UpdatePhysicalSizeForRemove(IControl child)
+        {
+            var bounds = Bounds;
+            var gap = Gap;
+
+            if (Orientation == Orientation.Vertical)
+            {
+                _takenSpace -= child.DesiredSize.Height + gap;
+            }
+            else
+            {
+                _takenSpace -= child.DesiredSize.Width + gap;
+            }
+        }
+    }
+}