Added Carousel.IsVirtualized.

src/Perspex.Controls/Carousel.cs

@@ -14,6 +14,12 @@ namespace Perspex.Controls
     /// </summary>
     public class Carousel : SelectingItemsControl
     {
+        /// <summary>
+        /// Defines the <see cref="IsVirtualized"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsVirtualizedProperty =
+            PerspexProperty.Register<Carousel, bool>(nameof(IsVirtualized), true);
+
         /// <summary>
         /// Defines the <see cref="Transition"/> property.
         /// </summary>
@@ -35,6 +41,18 @@ namespace Perspex.Controls
             SelectionModeProperty.OverrideDefaultValue<Carousel>(SelectionMode.AlwaysSelected);
             ItemsPanelProperty.OverrideDefaultValue<Carousel>(PanelTemplate);
         }
+        
+        /// <summary>
+        /// Gets or sets a value indicating whether the items in the carousel are virtualized.
+        /// </summary>
+        /// <remarks>
+        /// When the carousel is virtualized, only the active page is held in memory.
+        /// </remarks>
+        public bool IsVirtualized
+        {
+            get { return GetValue(IsVirtualizedProperty); }
+            set { SetValue(IsVirtualizedProperty, value); }
+        }
 
         /// <summary>
         /// Gets or sets the transition to use when moving between pages.

src/Perspex.Controls/Generators/ItemContainerGenerator.cs

@@ -29,7 +29,7 @@ namespace Perspex.Controls.Generators
         }
 
         /// <inheritdoc/>
-        public IEnumerable<ItemContainer> Containers => _containers;
+        public IEnumerable<ItemContainer> Containers => _containers.Where(x => x != null);
 
         /// <inheritdoc/>
         public event EventHandler<ItemContainerEventArgs> Materialized;

src/Perspex.Controls/Presenters/CarouselPresenter.cs

@@ -20,6 +20,12 @@ namespace Perspex.Controls.Presenters
     /// </summary>
     public class CarouselPresenter : Control, IItemsPresenter
     {
+        /// <summary>
+        /// Defines the <see cref="IsVirtualized"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsVirtualizedProperty =
+            Carousel.IsVirtualizedProperty.AddOwner<CarouselPresenter>();
+
         /// <summary>
         /// Defines the <see cref="Items"/> property.
         /// </summary>
@@ -96,6 +102,18 @@ namespace Perspex.Controls.Presenters
             }
         }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether the items in the carousel are virtualized.
+        /// </summary>
+        /// <remarks>
+        /// When the carousel is virtualized, only the active page is held in memory.
+        /// </remarks>
+        public bool IsVirtualized
+        {
+            get { return GetValue(IsVirtualizedProperty); }
+            set { SetValue(IsVirtualizedProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the items to display.
         /// </summary>
@@ -212,13 +230,17 @@ namespace Perspex.Controls.Presenters
                 if (toIndex != -1)
                 {
                     var item = Items.Cast<object>().ElementAt(toIndex);
-                    to = generator.ContainerFromIndex(toIndex) ??
-                         generator.Materialize(toIndex, new[] { item }, MemberSelector)
-                            .FirstOrDefault()?.ContainerControl;
+                    to = generator.ContainerFromIndex(toIndex);
 
-                    if (to != null)
+                    if (to == null)
                     {
-                        Panel.Children.Add(to);
+                        to = generator.Materialize(toIndex, new[] { item }, MemberSelector)
+                           .FirstOrDefault()?.ContainerControl;
+
+                        if (to != null)
+                        {
+                            Panel.Children.Add(to);
+                        }
                     }
                 }
 
@@ -233,10 +255,16 @@ namespace Perspex.Controls.Presenters
 
                 if (from != null)
                 {
-                    Panel.Children.Remove(from);
-                    generator.Dematerialize(fromIndex, 1);
+                    if (IsVirtualized)
+                    {
+                        Panel.Children.Remove(from);
+                        generator.Dematerialize(fromIndex, 1);
+                    }
+                    else
+                    {
+                        from.IsVisible = false;
+                    }
                 }
-
             }
         }
 

src/Perspex.Themes.Default/Carousel.xaml

@@ -4,7 +4,8 @@
       <Border Background="{TemplateBinding Background}"
               BorderBrush="{TemplateBinding BorderBrush}"
               BorderThickness="{TemplateBinding BorderThickness}">
-        <CarouselPresenter Items="{TemplateBinding Items}"
+        <CarouselPresenter IsVirtualized="{TemplateBinding IsVirtualized}"
+                           Items="{TemplateBinding Items}"
                            ItemsPanel="{TemplateBinding ItemsPanel}"
                            Margin="{TemplateBinding Padding}"
                            MemberSelector="{TemplateBinding MemberSelector}"

tests/Perspex.Controls.UnitTests/CarouselTests.cs

@@ -54,14 +54,53 @@ namespace Perspex.Controls.UnitTests
             Assert.Equal("Foo", ((TextBlock)child).Text);
         }
 
+        [Fact]
+        public void Should_Remove_NonCurrent_Page_When_IsVirtualized_True()
+        {
+            var target = new Carousel
+            {
+                Template = new FuncControlTemplate<Carousel>(CreateTemplate),
+                Items = new[] { "foo", "bar" },
+                IsVirtualized = true,
+                SelectedIndex = 0,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+            target.SelectedIndex = 1;
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+        }
+
+        [Fact]
+        public void Should_Not_Remove_NonCurrent_Page_When_IsVirtualized_False()
+        {
+            var target = new Carousel
+            {
+                Template = new FuncControlTemplate<Carousel>(CreateTemplate),
+                Items = new[] { "foo", "bar" },
+                IsVirtualized = false,
+                SelectedIndex = 0,
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+            target.SelectedIndex = 1;
+            Assert.Equal(2, target.ItemContainerGenerator.Containers.Count());
+        }
+
         private Control CreateTemplate(Carousel control)
         {
             return new CarouselPresenter
             {
                 Name = "PART_ItemsPresenter",
-                [~ItemsPresenter.ItemsProperty] = control[~ItemsControl.ItemsProperty],
-                [~ItemsPresenter.ItemsPanelProperty] = control[~ItemsControl.ItemsPanelProperty],
-                [~CarouselPresenter.SelectedIndexProperty] = control[~SelectingItemsControl.SelectedIndexProperty],
+                [~CarouselPresenter.IsVirtualizedProperty] = control[~Carousel.IsVirtualizedProperty],
+                [~CarouselPresenter.ItemsProperty] = control[~Carousel.ItemsProperty],
+                [~CarouselPresenter.ItemsPanelProperty] = control[~Carousel.ItemsPanelProperty],
+                [~CarouselPresenter.SelectedIndexProperty] = control[~Carousel.SelectedIndexProperty],
                 [~CarouselPresenter.TransitionProperty] = control[~Carousel.TransitionProperty],
             };
         }

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

@@ -1,6 +1,7 @@
 // Copyright (c) The Perspex Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System.Linq;
 using Moq;
 using Perspex.Controls.Generators;
 using Perspex.Controls.Presenters;
@@ -78,6 +79,43 @@ namespace Perspex.Controls.UnitTests.Presenters
             Assert.Equal("bar", ((TextBlock)target.Panel.Children[0]).Text);
         }
 
+        [Fact]
+        public void Should_Remove_NonCurrent_Page_When_IsVirtualized_True()
+        {
+            var target = new CarouselPresenter
+            {
+                Items = new[] { "foo", "bar" },
+                IsVirtualized = true,
+                SelectedIndex = 0,
+            };
+
+            target.ApplyTemplate();
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+            target.SelectedIndex = 1;
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+        }
+
+        [Fact]
+        public void Should_Not_Remove_NonCurrent_Page_When_IsVirtualized_False()
+        {
+            var target = new CarouselPresenter
+            {
+                Items = new[] { "foo", "bar" },
+                IsVirtualized = false,
+                SelectedIndex = 0,
+            };
+
+            target.ApplyTemplate();
+            Assert.Equal(1, target.ItemContainerGenerator.Containers.Count());
+            Assert.Equal(1, target.Panel.Children.Count);
+            target.SelectedIndex = 1;
+            Assert.Equal(2, target.ItemContainerGenerator.Containers.Count());
+            Assert.Equal(2, target.Panel.Children.Count);
+            target.SelectedIndex = 0;
+            Assert.Equal(2, target.ItemContainerGenerator.Containers.Count());
+            Assert.Equal(2, target.Panel.Children.Count);
+        }
+
         private class TestItem : ContentControl
         {
         }