VirtualListView for .NET MAUI

This is an experiment in creating a virtualized ListView control for .NET MAUI to support simple, fast, multi-templated, uneven item sized lists by not adding too many bells and whistles and using an adapter pattern data source.

Vroooom!

In the sample, each item (and header/footer) is measured as it is recycled. Performance is pretty great considering! In the future there will be an option to tell the ListView if your template(s) are a consistent size so that the measure can be skipped for even better performance.

Native controls

The implementation uses fast native controls in its renderers and optimizes for the native platform's recycling strategies. Items are cached through the platform's recycling mechanisms so that they can be reused efficiently. This also means the MAUI representation of items are cached as well. Each type of template (Item, Section Header, Section Footer) is cached individually so that they are reused efficiently.

Controls used on each platform:

iOS: UICollectionView
Android: RecyclerView
WinAppSDK: ItemsRepeaterScrollHost with IElementFactory

Setup

To add the Virtual List View control to your project, you need to add .UseVirtualListView() to your app builder:

		public static MauiApp Create()
		{
			var builder = MauiApp.CreateBuilder();
			builder
				.UseMauiApp<App>()
				.UseVirtualListView(); // <--- THIS
			return builder.Build();
		}

Windows App SDK 1.3.*

There is a bug that is fixed in Windows App SDK 1.3.x and newer with IElementFactory. You will need to explicitly specify a newer version of Microsoft.WindowsAppSdk NuGet package to use than is implicitly referenced by .NET MAUI at this time:

<ItemGroup Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">
  <PackageReference Include="Microsoft.WindowsAppSdk" Version="1.3.230602002" />
</ItemGroup>

Adapter / Data Source

Instead of starting with a typical C# collection such as ObservableCollection, the VirtualListView takes the adapter approach that is common to iOS and Android has the concept of grouping built in (called Sections).

This pattern is optimal since it allows for easily creating adapters backed by direct access data stores such as databases. Instead of trying to load data from the actual datastore, and trying to deal with cache invalidation for an in memory collection you can write your adapter directly against any type of storage.

To create an adapter for the VirtualListView, you need to implement the following interface:

public interface IVirtualListViewAdapter
{
	int GetNumberOfSections();

	object GetSection(int sectionIndex);

	int GetNumberOfItemsInSection(int sectionIndex);

	object GetItem(int sectionIndex, int itemIndex);

	event EventHandler OnDataInvalidated;

	void InvalidateData();
}

There are a few implementations included in the box to use:

`VirtualListViewAdapter`

This is a basic adapter backed by an IList<TItem>:

var adapter = new VirtualListViewAdapter<string>(
	new [] {
		"Item 1",
		"Item 2",
		"Item 3",
		//...
	});

`ObservableCollectionAdapter`

Many developers are accustomed to using ObservableCollection. While I recommend against using this if possible, there is a built in adapter that takes in an ObservableCollection<TItem> instance to help map it to the adapter pattern:

var items = new ObservableCollection<string>();
items.Add("Item 1");
items.Add("Item 2");

var adapter = new ObservableCollectionAdapter<string>(items);

items.Add("Item 3");

When using this adapter, the adapter will automatically invalidate itself (by calling this.InvalidateData() when the collection changes via the CollectionView.CollectionChanged event).

Custom Adapter

For many scenarios, it is ideal to create your own adapter implementation. You can implement the IVirtualListViewAdapter directly, or subclass VirtualListViewAdapterBase<TSection, TItem>.

Here's an example of a custom adapter for a flat list (no sections/grouping). Notice that we cache commonly used data such as ItemsForSection and we will reset the cache anytime the data is invalidated:

public class SQLiteAdapter : VirtualListViewAdapterBase<object, ItemInfo>
{
	public SQLiteAdapter() : base()
	{
		Db = new Database(...);
	}

	public Database Db { get; }

	int? cachedItemCount = null;

	// No sections/grouping, so disregard the sectionIndex
	public override int GetNumberOfItemsInSection(int sectionIndex)
		=> cachedItemCount ??= Db.ExecuteScalar<int>("SELECT COUNT(Id) FROM Items");

	public override string GetItem(int sectionIndex, int itemIndex)
		=> Db.FindWithQuery<ItemInfo>("SELECT * FROM Items ORDER BY Id LIMIT 1 OFFSET ?", itemIndex);

	public override void InvalidateData()
	{
		// Clear our item count cache
		// Also do this any time we may insert or delete data 
		cachedItemCount = null;
		base.InvalidateData();
	}
}

Here's an example of a more sophisticated adapter with grouping/sections. Again, notice we cache Section count and Item count per section:

public class SQLiteSectionedAdapter : VirtualListViewAdapterBase<GroupInfo, ItemInfo>
{
	public SQLiteSectionedAdapter() : base()
	{
		Db = new Database(...);
	}

	public Database Db { get; }

	Dictionary<int, GroupInfo> cachedSectionSummaries = new ();

	int? cachedNumberOfSections = null;

	public int GetNumberOfSections()
		=> cachedNumberOfSections ??= Db.ExecuteScalar<int>("SELECT DISTINCT COUNT(GroupId) FROM Items");

	// No sections/grouping, so disregard the sectionIndex
	public override int GetNumberOfItemsInSection(int sectionIndex)
		=> cachedItemCount ??= Db.ExecuteScalar<int>("SELECT COUNT(Id) FROM Items");

	public GroupInfo GetSection(int sectionIndex)
	{
		if (cachedSectionSummaries.ContainsKey(sectionIndex))
			return cachedSectionSummaries[sectionIndex];

		var sql = @"
				SELECT DISTINCT g.GroupId, g.GroupName, Count(i.Id) as ItemCount
				FROM Items g
					INNER JOIN Items i ON i.GroupId = g.GroupId
				GROUP BY g.GroupId
				ORDER BY g.GroupName
				LIMIT 1 OFFSET ?
		";

		var groupInfo = Db.FindWithQuery<GroupInfo>(sql, sectionIndex);

		if (groupInfo != null)
			cachedSectionSummaries.Add(sectionIndex, groupInfo);

		return groupInfo;
	}

	public override string GetItem(int sectionIndex, int itemIndex)
		=> Db.FindWithQuery<ItemInfo>("SELECT * FROM Items WHERE GroupId=? ORDER BY Id LIMIT 1 OFFSET ?", sectionIndex, itemIndex);

	public override void InvalidateData()
	{
		// Clear our caches
		// Also do this any time we may insert or delete data 
		cachedItemCount = null;
		cachedNumberOfSections.Clear();

		base.InvalidateData();
	}
}

Templates

DataTemplates are available for Items along with an Item Template Selector (which is a custom class based on the adapter pattern to select templates).

Different templates can be specified for:

Global Header
Global Footer
Section Header
Section Footer
Item

In addition, it's possible to use Template Selectors which allow you to use different DataTemplates depending on the section / item index being displayed.

Template selectors are available for:

Section Headers and Footers
Items

For Item template selectors, sublcass the AdapterItemDataTemplateSelector:

public class MyItemTemplateSelector
{
	PersonTemplate personTemplate = new PersonTemplate();
	GenericTemplate genericTemplate = new GenericTemplate();

	public override DataTemplate SelectItemTemplate(IVirtualListViewAdapter adapter, int sectionIndex, int itemIndex)
	{
		var item = adapter.GetItem(sectionIndex, itemIndex);

		if (item is Person)
			return personTemplate;
		
		return genericTemplate;
	}
}

For section template selectors, subclass AdapterSectionDataTemplateSelector.

Virtual ViewCells

All templates can contain a single IView, or alternatively you can use VirtualViewCell to wrap your view.

The VirtualViewCell's ResourceDictionary will contain a set of values which are are useful for adapting your views for things like separators and selection state:

int SectionIndex
int ItemIndex
bool IsGlobalHeader
bool IsGlobalFooter
bool IsSectionHeader
bool IsSectionFooter
bool IsItem
bool IsLastItemInSection
bool IsNotLastItemInSection
bool IsFirstItemInSection
bool IsNotFirstItemInSection
bool IsSelected

NOTE: These are also available as properties on VirtualViewCell itself, since it implements IPositionInfo

You can access these properties from your templates. Here's an example of displaying an item separator using these properties, as well as changing the background color based on the selection state and a converter:

<?xml version="1.0" encoding="UTF-8"?>
<xct:VirtualViewCell
	xmlns:xct="clr-namespace:Microsoft.Maui.Controls;assembly=VirtualListView"
	xmlns="http://xamarin.com/schemas/2014/forms" 
	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
	x:Class="VirtualListViewSample.GenericViewCell">
  <xct:VirtualViewCell>
		<VerticalStackLayout
			Spacing="0"
			BackgroundColor="{Binding Source={x:Reference self}, Path=IsSelected, Converter={StaticResource selectedColorConverter}}">

			<BoxView
				HorizontalOptions="FillAndExpand"
				HeightRequest="1"
				BackgroundColor="#f8f8f8"
				IsVisible="{DynamicResource IsNotFirstItemInSection}" <!-- Use the automatic property -->
 				/>

			<Border Background="#f0f0f0" StrokeShape="{RoundedRectangle CornerRadius=14}" Margin="10,5,10,5" Padding="10">
				<Label Text="{Binding TrackName}" />
			</Border>

		</VerticalStackLayout>
	</xct:VirtualViewCell>
</xct:VirtualViewCell>

Notice the IsVisible="{DynamicResource IsNotFirstItemInSection}" references a resource which has been automatically populated by the VirtualViewCell.

Selection

There are 3 selection modes: None, Single, and Multiple. Only Item types are selectable.

There are SelectedItem and SelectedItems bindable properties. There's an OnSelectedItemsChanged event fired whenever these change.

Refreshing

Pull to refresh is enabled for iOS/MacCatalyst and Android. WindowsAppSDK does not have the equivalent feature so there is no support for it. You can use the RefreshCommand or subscribe to the OnRefresh event to perform your logic while the refresh indicator displays. You must set IsRefreshEnabled to true to enable the gesture.
You can also set the RefreshAccentColor to change the color of the refresh indicator.

Empty View

If your adapter has <= 1 section and no items, an empty view can be displayed automatically:

<vlv:VirtualListView.EmptyView>
  <Grid>
    <Label HorizontalOptions="Center" VerticalOptions="Center" Text="EMPTY" />
  </Grid>
</vlv:VirtualListView.EmptyView>

Scrolled

Scrolled notifications can be observed with ScrolledCommand which will pass a ScrolledEventArgs parameter, or the OnScrolled event with a parameter of the same type. The event args contain the X/Y position scrolled.

Future

Looking ahead, there are a few goals:

Even Rows - by default every cell is assumed uneven and measured every time the context changes or the cell is recycled. Adding an option to assume each template type is the same size will make performance even better, but will be an explicit opt-in
Supporting "size of content" constraints

Some current non-goals but considerations for even later:

Grid / Column support
Sticky section headers

Redth/Maui.VirtualListView

Redth

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