Add Visual Tree Helper (Based on #1820) (#1845)

Co-authored-by: Stephane Delcroix <stephane@delcroix.org>

src/Controls/src/Core/Application.cs

@@ -10,7 +10,7 @@
 
 namespace Microsoft.Maui.Controls
 {
-	public partial class Application : Element, IResourcesProvider, IApplicationController, IElementConfiguration<Application>
+	public partial class Application : Element, IResourcesProvider, IApplicationController, IElementConfiguration<Application>, IVisualTreeElement
 	{
 		readonly WeakEventManager _weakEventManager = new WeakEventManager();
 		Task<IDictionary<string, object>> _propertiesTask;
@@ -437,5 +437,7 @@ protected internal virtual void CleanUp()
 
 			NavigationProxy = null;
 		}
+
+		IReadOnlyList<Maui.IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => this.Windows;
 	}
 }

src/Controls/src/Core/Element.cs

@@ -9,7 +9,7 @@
 
 namespace Microsoft.Maui.Controls
 {
-	public abstract partial class Element : BindableObject, IElement, INameScope, IElementController
+	public abstract partial class Element : BindableObject, IElement, INameScope, IElementController, IVisualTreeElement
 	{
 		public static readonly BindableProperty MenuProperty = BindableProperty.CreateAttached(nameof(Menu), typeof(Menu), typeof(Element), null);
 
@@ -319,6 +319,10 @@ public new void SetDynamicResource(BindableProperty property, string key)
 			base.SetDynamicResource(property, key);
 		}
 
+		IReadOnlyList<Maui.IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => LogicalChildren;
+
+		IVisualTreeElement IVisualTreeElement.GetVisualParent() => this.Parent;
+
 		protected override void OnBindingContextChanged()
 		{
 			this.PropagateBindingContext(LogicalChildrenInternal, (child, bc) =>

src/Controls/src/Core/ItemsView.cs

@@ -1,4 +1,6 @@
+using System;
 using System.Collections;
+using System.Collections.Generic;
 using System.ComponentModel;
 using Microsoft.Maui.Controls.Internals;
 

src/Controls/src/Core/Layout.cs

@@ -61,7 +61,7 @@ Size ILayoutManager.ArrangeChildren(Rectangle childBounds)
 		}
 	}
 
-	public abstract class Layout : View, ILayout, ILayoutController, IPaddingElement, IFrameworkElement
+	public abstract class Layout : View, ILayout, ILayoutController, IPaddingElement, IFrameworkElement, IVisualTreeElement
 	{
 		public static readonly BindableProperty IsClippedToBoundsProperty =
 			BindableProperty.Create(nameof(IsClippedToBounds), typeof(bool), typeof(Layout), false);
@@ -122,6 +122,8 @@ public bool CascadeInputTransparent
 
 		public void ForceLayout() => SizeAllocated(Width, Height);
 
+		IReadOnlyList<Maui.IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => Children.ToList().AsReadOnly();
+
 		[Obsolete("OnSizeRequest is obsolete as of version 2.2.0. Please use OnMeasure instead.")]
 		[EditorBrowsable(EditorBrowsableState.Never)]
 		public sealed override SizeRequest GetSizeRequest(double widthConstraint, double heightConstraint)

src/Controls/src/Core/Layout/Layout.cs

@@ -1,13 +1,14 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Maui.Graphics;
 using Microsoft.Maui.Layouts;
 
 namespace Microsoft.Maui.Controls
 {
 	[ContentProperty(nameof(Children))]
-	public abstract class Layout : View, Microsoft.Maui.ILayout, IList<IView>, IBindableLayout, IPaddingElement
+	public abstract class Layout : View, Microsoft.Maui.ILayout, IList<IView>, IBindableLayout, IPaddingElement, IVisualTreeElement
 	{
 		protected ILayoutManager _layoutManager;
 		public ILayoutManager LayoutManager => _layoutManager ??= CreateLayoutManager();
@@ -152,5 +153,7 @@ Thickness IPaddingElement.PaddingDefaultValueCreator()
 		{
 			return new Thickness(0);
 		}
+
+		IReadOnlyList<IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => Children.Cast<IVisualTreeElement>().ToList().AsReadOnly();
 	}
 }

src/Controls/src/Core/Shell/ShellContent.cs

@@ -14,7 +14,7 @@
 namespace Microsoft.Maui.Controls
 {
 	[ContentProperty(nameof(Content))]
-	public class ShellContent : BaseShellItem, IShellContentController
+	public class ShellContent : BaseShellItem, IShellContentController, IVisualTreeElement
 	{
 		static readonly BindablePropertyKey MenuItemsPropertyKey =
 			BindableProperty.CreateReadOnly(nameof(MenuItems), typeof(MenuItemCollection), typeof(ShellContent), null,
@@ -309,5 +309,8 @@ static void ApplyQueryAttributes(object content, IDictionary<string, string> que
 				}
 			}
 		}
+
+		IReadOnlyList<Maui.IVisualTreeElement> GetVisualChildren() => new List<Maui.IVisualTreeElement> { ((IShellContentController)this).Page }.AsReadOnly();
+
 	}
 }

src/Controls/src/Core/Shell/ShellItem.cs

@@ -38,7 +38,7 @@ public TabBar()
 
 	[ContentProperty(nameof(Items))]
 	[EditorBrowsable(EditorBrowsableState.Never)]
-	public class ShellItem : ShellGroupItem, IShellItemController, IElementConfiguration<ShellItem>, IPropertyPropagationController
+	public class ShellItem : ShellGroupItem, IShellItemController, IElementConfiguration<ShellItem>, IPropertyPropagationController, IVisualTreeElement
 	{
 		#region PropertyKeys
 
@@ -310,5 +310,7 @@ internal override void SendDisappearing()
 				CurrentItem.SendDisappearing();
 			}
 		}
+
+		IReadOnlyList<Maui.IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => Items.ToList().AsReadOnly();
 	}
 }

src/Controls/src/Core/Shell/ShellSection.cs

@@ -16,7 +16,7 @@ public class Tab : ShellSection
 
 	[ContentProperty(nameof(Items))]
 	[EditorBrowsable(EditorBrowsableState.Never)]
-	public class ShellSection : ShellGroupItem, IShellSectionController, IPropertyPropagationController
+	public class ShellSection : ShellGroupItem, IShellSectionController, IPropertyPropagationController, IVisualTreeElement
 	{
 		#region PropertyKeys
 
@@ -1205,5 +1205,7 @@ ShellNavigationState GetUpdatedStatus(IReadOnlyList<Page> stack)
 				return ShellNavigationManager.GetNavigationState(shellItem, shellSection, shellContent, stack, modalStack);
 			}
 		}
+
+		IReadOnlyList<Maui.IVisualTreeElement> IVisualTreeElement.GetVisualChildren() => AllChildren.ToList().AsReadOnly();
 	}
 }

src/Controls/tests/Core.UnitTests/ShellTests.cs

@@ -1318,5 +1318,17 @@ public void SendStructureChangedFiresWhenAddingItems()
 			Assert.Greater(count, previousCount, "StructureChanged not fired when adding Shell Content");
 		}
 
+		[Test]
+		public void VisualTreeHelperCount()
+		{
+			Shell shell = new Shell();
+			shell.Items.Add(CreateShellItem());
+			shell.CurrentItem.Items.Add(CreateShellSection());
+			shell.CurrentItem.CurrentItem.Items.Add(CreateShellContent());
+			var shellCount = (shell as IVisualTreeElement).GetVisualChildren();
+			var shellItemCount = (shell.CurrentItem as IVisualTreeElement).GetVisualChildren();
+			Assert.AreEqual(shellCount.Count, 1);
+			Assert.AreEqual(shellItemCount.Count, 2);
+		}
 	}
 }

src/Controls/tests/Core.UnitTests/VisualTreeHelperTests.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.Maui.Graphics;
+using NSubstitute;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Core.UnitTests
+{
+	[TestFixture]
+	public class VisualTreeHelperTests : BaseTestFixture
+	{
+		[Test]
+		public void VerticalStackLayoutChildren()
+		{
+			var verticalStackLayout = new VerticalStackLayout() { IsPlatformEnabled = true };
+			verticalStackLayout.Children.Add(new Label());
+			verticalStackLayout.Children.Add(new Button());
+			Assert.AreEqual(verticalStackLayout.Children.Count, (verticalStackLayout as IVisualTreeElement).GetVisualChildren().Count);
+		}
+
+		[Test]
+		public void HorizontalStackLayoutChildren()
+		{
+			var horizontalStackLayout = new HorizontalStackLayout() { IsPlatformEnabled = true };
+			horizontalStackLayout.Children.Add(new Label());
+			horizontalStackLayout.Children.Add(new Button());
+			Assert.AreEqual(horizontalStackLayout.Children.Count, (horizontalStackLayout as IVisualTreeElement).GetVisualChildren().Count);
+		}
+
+		[Test]
+		public void StackLayoutChildren()
+		{
+			var stackLayout = new StackLayout() { IsPlatformEnabled = true };
+			stackLayout.Children.Add(new Label());
+			stackLayout.Children.Add(new Button());
+			Assert.AreEqual(stackLayout.Children.Count, (stackLayout as IVisualTreeElement).GetVisualChildren().Count);
+		}
+
+		[Test]
+		public void ApplicationChildren()
+		{
+			var app = new Application();
+			var iapp = app as IApplication;
+			var page = new ContentPage();
+
+			app.MainPage = page;
+
+			var window = iapp.CreateWindow(null);
+
+			var appChildren = (app as IVisualTreeElement).GetVisualChildren();
+			var windowChildren = (window as IVisualTreeElement).GetVisualChildren();
+			Assert.Greater(appChildren.Count, 0);
+			Assert.IsTrue(appChildren[0] is IWindow);
+			Assert.AreEqual(windowChildren.Count, 1);
+		}
+
+		[Test]
+		public void VisualElementParent()
+		{
+			var app = new Application();
+			var iapp = app as IApplication;
+			var page = new ContentPage();
+			app.MainPage = page;
+			var window = iapp.CreateWindow(null);
+			var appParent = (app as IVisualTreeElement).GetVisualParent();
+			var windowParent = (window as IVisualTreeElement).GetVisualParent();
+			Assert.IsNull(appParent);
+			Assert.IsNotNull(windowParent);
+		}
+	}
+}

src/Core/src/Core/IVisualTreeElement.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+
+namespace Microsoft.Maui
+{
+	public interface IVisualTreeElement
+	{
+		IReadOnlyList<IVisualTreeElement> GetVisualChildren();
+
+		IVisualTreeElement? GetVisualParent();
+	}
+}