Implemented LazyView

samples/XCT.Sample/Pages/Views/TabView/LazyTabPage.xaml

@@ -0,0 +1,57 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<pages:BasePage
+    x:Class="Xamarin.CommunityToolkit.Sample.Pages.Views.TabView.LazyTabPage"
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    xmlns:local="clr-namespace:Xamarin.CommunityToolkit.Sample.Pages.Views.TabView"
+    xmlns:pages="clr-namespace:Xamarin.CommunityToolkit.Sample.Pages"
+    xmlns:vm="clr-namespace:Xamarin.CommunityToolkit.Sample.ViewModels.Views.Tabs"
+    xmlns:xct="http://xamarin.com/schemas/2020/toolkit">
+    <pages:BasePage.Content>
+        <Grid>
+            <xct:TabView
+                TabContentBackgroundColor="Yellow"
+                TabIndicatorColor="Yellow"
+                TabStripBackgroundColor="Blue"
+                TabStripHeight="60"
+                TabStripPlacement="Bottom">
+                <xct:TabViewItem
+                    FontSize="12"
+                    Icon="triangle.png"
+                    Text="Tab Normal 1"
+                    TextColor="White"
+                    TextColorSelected="Yellow">
+                    <local:NormalTestView />
+                </xct:TabViewItem>
+                <xct:TabViewItem
+                    FontSize="12"
+                    Icon="circle.png"
+                    Text="Tab Lazy 2"
+                    TextColor="White"
+                    TextColorSelected="Yellow">
+                    <xct:LazyView
+                        x:TypeArguments="local:LazyTestView"
+                        BindingContext="{x:Static vm:LazyTestViewModel.Current}"
+                        IsLoaded="{Binding Loaded}" />
+                </xct:TabViewItem>
+                <xct:TabViewItem
+                    FontSize="12"
+                    Icon="triangle.png"
+                    Text="Tab Normal 3"
+                    TextColor="White"
+                    TextColorSelected="Yellow">
+                    <local:NormalTestView />
+                </xct:TabViewItem>
+
+                <xct:TabViewItem
+                    FontSize="12"
+                    Icon="triangle.png"
+                    Text="Tab Lazy 4"
+                    TextColor="White"
+                    TextColorSelected="Yellow">
+                    <xct:LazyView x:TypeArguments="local:LazyTestView" BindingContext="{x:Static vm:LazyTestViewModel.Current}" />
+                </xct:TabViewItem>
+            </xct:TabView>
+        </Grid>
+    </pages:BasePage.Content>
+</pages:BasePage>

samples/XCT.Sample/Pages/Views/TabView/LazyTabPage.xaml.cs

@@ -0,0 +1,7 @@
+namespace Xamarin.CommunityToolkit.Sample.Pages.Views.TabView
+{
+	public partial class LazyTabPage
+	{
+		public LazyTabPage() => InitializeComponent();
+	}
+}

samples/XCT.Sample/Pages/Views/TabView/LazyTestView.xaml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<ContentView
+    x:Class="Xamarin.CommunityToolkit.Sample.Pages.Views.TabView.LazyTestView"
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    xmlns:xct="http://xamarin.com/schemas/2020/toolkit">
+    <ContentView.Content>
+        <Grid RowDefinitions="auto, *">
+            <Label
+                FontSize="Title"
+                HorizontalTextAlignment="Center"
+                Text="{Binding Title}"
+                TextColor="Black" />
+            <xct:UniformGrid x:Name="uniformGrid" Grid.Row="1" />
+        </Grid>
+    </ContentView.Content>
+</ContentView>

samples/XCT.Sample/Pages/Views/TabView/LazyTestView.xaml.cs

@@ -0,0 +1,29 @@
+using Xamarin.CommunityToolkit.Sample.ViewModels.Views.Tabs;
+using Xamarin.Forms;
+
+namespace Xamarin.CommunityToolkit.Sample.Pages.Views.TabView
+{
+	public partial class LazyTestView : ContentView
+	{
+		public LazyTestView()
+		{
+			InitializeComponent();
+
+			Build();
+			NormalTestViewModel.Current.LoadedViews += "LazyView Loaded \n";
+		}
+
+		void Build()
+		{
+			for (var i = 0; i < 117; i++)
+			{
+				var box = new BoxView
+				{
+					BackgroundColor = i % 2 == 0 ? Color.Blue : Color.Fuchsia
+				};
+
+				uniformGrid.Children.Add(box);
+			}
+		}
+	}
+}

samples/XCT.Sample/Pages/Views/TabView/NormalTestView.xaml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<ContentView
+    x:Class="Xamarin.CommunityToolkit.Sample.Pages.Views.TabView.NormalTestView"
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
+    <ContentView.Content>
+        <StackLayout>
+            <Label Text="{Binding LoadedViews}" />
+        </StackLayout>
+    </ContentView.Content>
+</ContentView>

samples/XCT.Sample/Pages/Views/TabView/NormalTestView.xaml.cs

@@ -0,0 +1,16 @@
+using Xamarin.CommunityToolkit.Sample.ViewModels.Views.Tabs;
+using Xamarin.Forms;
+
+namespace Xamarin.CommunityToolkit.Sample.Pages.Views.TabView
+{
+	public partial class NormalTestView : ContentView
+	{
+		public NormalTestView()
+		{
+			InitializeComponent();
+			BindingContext = NormalTestViewModel.Current;
+
+			NormalTestViewModel.Current.LoadedViews += "NormalTestLoaded \n";
+		}
+	}
+}

samples/XCT.Sample/ViewModels/Views/TabViewViewModel.cs

@@ -39,7 +39,10 @@ public class TabViewViewModel : BaseGalleryViewModel
 				"Customize the tabs width"),
 
 			new SectionModel(typeof(NoContentPage), "Tab without Content",
-				"Only the TabStrip is visible")
+				"Only the TabStrip is visible"),
+
+			new SectionModel(typeof(LazyTabPage), "LazyLoadingTab",
+				"See how you can implement LazyViews that are loaded just when you navigate to them"),
 		};
 	}
 }

samples/XCT.Sample/ViewModels/Views/Tabs/LazyTestViewModel.cs

@@ -0,0 +1,27 @@
+using Xamarin.CommunityToolkit.ObjectModel;
+
+namespace Xamarin.CommunityToolkit.Sample.ViewModels.Views.Tabs
+{
+	sealed class LazyTestViewModel : ObservableObject
+	{
+		public static LazyTestViewModel Current { get; } = new LazyTestViewModel();
+
+		string title;
+
+		public string Title
+		{
+			get => title;
+			set => SetProperty(ref title, value);
+		}
+
+		bool loaded;
+
+		public bool Loaded
+		{
+			get => loaded;
+			set => SetProperty(ref loaded, value);
+		}
+
+		public LazyTestViewModel() => Title = "Lazy Tab Sample";
+	}
+}

samples/XCT.Sample/ViewModels/Views/Tabs/NormalTestViewModel.cs

@@ -0,0 +1,17 @@
+using Xamarin.CommunityToolkit.ObjectModel;
+
+namespace Xamarin.CommunityToolkit.Sample.ViewModels.Views.Tabs
+{
+	sealed class NormalTestViewModel : ObservableObject
+	{
+		public static NormalTestViewModel Current { get; } = new NormalTestViewModel();
+
+		string loadedViews;
+
+		public string LoadedViews
+		{
+			get => loadedViews;
+			set => SetProperty(ref loadedViews, value);
+		}
+	}
+}

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/LazyView/BaseLazyView.shared.cs

@@ -0,0 +1,32 @@
+using System;
+using Xamarin.Forms;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.CommunityToolkit.UI.Views
+{
+	[Preserve(Conditional =true)]
+	public abstract class BaseLazyView : ContentView, IDisposable
+	{
+		internal static readonly BindablePropertyKey IsLoadedPropertyKey = BindableProperty.CreateReadOnly(nameof(IsLoaded), typeof(bool), typeof(BaseLazyView), default);
+
+		public static readonly BindableProperty IsLoadedProperty = IsLoadedPropertyKey.BindableProperty;
+
+		public bool IsLoaded => (bool)GetValue(IsLoadedProperty);
+
+		internal void SetIsLoaded(bool isLoaded) => SetValue(IsLoadedPropertyKey, isLoaded);
+
+		public abstract void LoadView();
+
+		public void Dispose()
+		{
+			if (Content is IDisposable disposable)
+				disposable.Dispose();
+		}
+
+		protected override void OnBindingContextChanged()
+		{
+			if (Content != null && !(Content is ActivityIndicator))
+				Content.BindingContext = BindingContext;
+		}
+	}
+}

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/LazyView/LazyView.shared.cs

@@ -0,0 +1,16 @@
+using Xamarin.Forms;
+
+namespace Xamarin.CommunityToolkit.UI.Views
+{
+	public class LazyView<TView> : BaseLazyView where TView : View, new()
+	{
+		public override void LoadView()
+		{
+			View view = new TView { BindingContext = BindingContext };
+
+			Content = view;
+
+			SetIsLoaded(true);
+		}
+	}
+}

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/TabView/TabView.shared.cs

@@ -172,6 +172,9 @@ public void Dispose()
 
 			if (TabItems != null)
 				TabItems.CollectionChanged -= OnTabItemsCollectionChanged;
+
+			var lazyView = (((TabViewItem)contentContainer.CurrentItem).Content as BaseLazyView) ?? (TabItems[SelectedIndex].Content as BaseLazyView);
+			lazyView?.Dispose();
 		}
 
 		public ObservableCollection<TabViewItem> TabItems { get; set; }
@@ -775,6 +778,8 @@ void UpdateSelectedIndex(int position, bool hasCurrentItem = false)
 
 					var tabViewItem = TabItems[SelectedIndex];
 
+					var lazyView = (currentItem?.Content as BaseLazyView) ?? (tabViewItem?.Content as BaseLazyView);
+
 					contentIndex = contentTabItems.IndexOf(currentItem ?? tabViewItem);
 					tabStripIndex = TabItems.IndexOf(currentItem ?? tabViewItem);
 
@@ -788,6 +793,8 @@ void UpdateSelectedIndex(int position, bool hasCurrentItem = false)
 							TabItems[index].IsSelected = false;
 					}
 
+					if (!lazyView?.IsLoaded ?? false)
+						lazyView?.LoadView();
 					var currentTabItem = TabItems[position];
 					currentTabItem.SizeChanged += OnCurrentTabItemSizeChanged;
 					UpdateTabIndicatorPosition(currentTabItem);