Add a caching adapter base class

VirtualListView/Adapters/CachingVirtualListViewAdapter.cs

@@ -0,0 +1,61 @@
+namespace Microsoft.Maui.Adapters;
+
+public abstract class CachingVirtualListViewAdapter<TSection, TItem> : VirtualListViewAdapterBase<TSection, TItem>
+	where TSection : class
+	where TItem : class
+{
+	int? cachedNumberOfSections = null;
+	LRUCache<int, int> cachedNumberOfItemsInSection;
+	LRUCache<int, TSection> cachedSections;
+
+	public CachingVirtualListViewAdapter(VirtualListViewAdapterBase<TSection, TItem> sourceAdapter, int sectionCacheSize = 1000)
+		: this((IVirtualListViewAdapter)sourceAdapter, sectionCacheSize)
+	{
+	}
+
+	public CachingVirtualListViewAdapter(IVirtualListViewAdapter sourceAdapter, int sectionCacheSize = 1000)
+	{
+		SourceAdapter = sourceAdapter;
+
+		cachedSections = new LRUCache<int, TSection>(sectionCacheSize);
+		cachedNumberOfItemsInSection = new LRUCache<int, int>(sectionCacheSize);
+	}
+
+	public readonly IVirtualListViewAdapter SourceAdapter;
+
+	public sealed override int GetNumberOfSections()
+		=> cachedNumberOfSections ??= SourceAdapter.GetNumberOfSections();
+
+
+	public sealed override void InvalidateData()
+	{
+		SourceAdapter.InvalidateData();
+		base.InvalidateData();
+	}
+
+	public sealed override TItem GetItem(int sectionIndex, int itemIndex)
+	{
+		return SourceAdapter.GetItem(sectionIndex, itemIndex) as TItem;
+	}
+
+	public sealed override int GetNumberOfItemsInSection(int sectionIndex)
+	{
+		if (cachedNumberOfItemsInSection.TryGet(sectionIndex, out var itemCount))
+			return itemCount;
+
+		itemCount = SourceAdapter.GetNumberOfItemsInSection(sectionIndex);
+		cachedNumberOfItemsInSection.AddReplace(sectionIndex, itemCount);
+		return itemCount;
+	}
+
+	public sealed override TSection GetSection(int sectionIndex)
+	{
+		if (cachedSections.TryGet(sectionIndex, out var section))
+			return section;
+
+		section = SourceAdapter.GetSection(sectionIndex) as TSection;
+
+		cachedSections.AddReplace(sectionIndex, section);
+		return section;
+	}
+}