Fixed CollectionView does not update layout correctly when ItemsSource changes

src/Controls/src/Core/Handlers/Items2/ItemsViewHandler2.iOS.cs

@@ -184,6 +184,14 @@ public override Size GetDesiredSize(double widthConstraint, double heightConstra
 		{
 			var contentSize = Controller.GetSize();
 
+			// If contentSize comes back null, it means none of the content has been realized yet;
+			// we need to return the expansive size the collection view wants by default to get
+			// it to start measuring its content
+			if (contentSize.Height == 0 || contentSize.Width == 0)
+			{
+				return base.GetDesiredSize(widthConstraint, heightConstraint);
+			}
+
 			// Our target size is the smaller of it and the constraints
 			var width = contentSize.Width <= widthConstraint ? contentSize.Width : widthConstraint;
 			var height = contentSize.Height <= heightConstraint ? contentSize.Height : heightConstraint;

src/Controls/tests/TestCases.HostApp/Issues/Issue30953.cs

@@ -0,0 +1,70 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 30953, "CollectionView does not update layout correctly when ItemsSource changes", PlatformAffected.iOS | PlatformAffected.macOS)]
+public class Issue30953 : ContentPage
+{
+	private ObservableCollection<string> Countries = new();
+
+	public Issue30953()
+	{
+		LoadCountries();
+		var collectionView = new CollectionView2();
+		var button = new Button
+		{
+			Text = "Set ItemsSource",
+			AutomationId = "Issue30953Button",
+			HorizontalOptions = LayoutOptions.Center
+		};
+
+		button.Clicked += (sender, args) =>
+		{
+			collectionView.ItemsSource = Countries;
+		};
+
+		var label = new Label
+		{
+			Text = "The test passed if the CollectionView ItemsSource is set and the items are displayed correctly in runtime.",
+			HorizontalOptions = LayoutOptions.Center
+		};
+
+		var stack = new VerticalStackLayout
+		{
+			Children = {
+				label,
+				button,
+				collectionView
+			}
+		};
+
+		Content = stack;
+	}
+
+	void LoadCountries()
+	{
+		Countries = new ObservableCollection<string>
+		{
+			"United States",
+			"Canada",
+			"United Kingdom",
+			"Germany",
+			"France",
+			"Italy",
+			"Spain",
+			"Japan",
+			"Australia",
+			"Brazil",
+			"India",
+			"China",
+			"Russia",
+			"Mexico",
+			"Argentina",
+			"South Africa",
+			"Egypt",
+			"Turkey",
+			"Netherlands",
+			"Sweden"
+		};
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue30953.cs

@@ -0,0 +1,22 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue30953 : _IssuesUITest
+{
+	public Issue30953(TestDevice testDevice) : base(testDevice)
+	{
+	}
+	public override string Issue => "CollectionView does not update layout correctly when ItemsSource changes";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void EnsureCollectionViewLayoutOnItemsSourceChange()
+	{
+		App.WaitForElement("Issue30953Button");
+		App.Tap("Issue30953Button");
+		App.WaitForElement("United States");
+	}
+}