Weak subscription to CanExecuteChange events (#29837)

* Weak subscription to CanExecuteChange events

* - add tests

* - fix lifecycle of CanExecute subscription

* - cleanup code based on Pictos recommendations

* - add a netstandard_20 version of DependentHandle

* - add more

* - fix missing null check on textcell

Author: PureWeen

src/Controls/src/Core/Button/Button.cs

@@ -616,5 +616,7 @@ private protected override string GetDebuggerDisplay()
 			var commandText = DebuggerDisplayHelpers.GetDebugText(nameof(Command), Command, false);
 			return $"{base.GetDebuggerDisplay()}, {textString}, {commandText}";
 		}
+
+		WeakCommandSubscription ICommandElement.CleanupTracker { get; set; }
 	}
 }

src/Controls/src/Core/Cells/TextCell.cs

@@ -1,42 +1,27 @@
 #nullable disable
 using System;
 using System.Windows.Input;
+using Microsoft.Maui.Controls.Internals;
 using Microsoft.Maui.Graphics;
 
 namespace Microsoft.Maui.Controls
 {
 	/// <include file="../../../docs/Microsoft.Maui.Controls/TextCell.xml" path="Type[@FullName='Microsoft.Maui.Controls.TextCell']/Docs/*" />
-	public class TextCell : Cell
+	public class TextCell : Cell, ICommandElement
 	{
 		/// <summary>Bindable property for <see cref="Command"/>.</summary>
-		public static readonly BindableProperty CommandProperty = BindableProperty.Create(nameof(Command), typeof(ICommand), typeof(TextCell), default(ICommand),
-			propertyChanging: (bindable, oldvalue, newvalue) =>
-			{
-				var textCell = (TextCell)bindable;
-				var oldcommand = (ICommand)oldvalue;
-				if (oldcommand != null)
-					oldcommand.CanExecuteChanged -= textCell.OnCommandCanExecuteChanged;
-			}, propertyChanged: (bindable, oldvalue, newvalue) =>
-			{
-				var textCell = (TextCell)bindable;
-				var newcommand = (ICommand)newvalue;
-				if (newcommand != null)
-				{
-					textCell.IsEnabled = newcommand.CanExecute(textCell.CommandParameter);
-					newcommand.CanExecuteChanged += textCell.OnCommandCanExecuteChanged;
-				}
-			});
+		public static readonly BindableProperty CommandProperty =
+			BindableProperty.Create(nameof(Command), typeof(ICommand), typeof(TextCell),
+			propertyChanging: CommandElement.OnCommandChanging,
+			propertyChanged: CommandElement.OnCommandChanged);
 
 		/// <summary>Bindable property for <see cref="CommandParameter"/>.</summary>
-		public static readonly BindableProperty CommandParameterProperty = BindableProperty.Create(nameof(CommandParameter), typeof(object), typeof(TextCell), default(object),
-			propertyChanged: (bindable, oldvalue, newvalue) =>
-			{
-				var textCell = (TextCell)bindable;
-				if (textCell.Command != null)
-				{
-					textCell.IsEnabled = textCell.Command.CanExecute(newvalue);
-				}
-			});
+		public static readonly BindableProperty CommandParameterProperty =
+			BindableProperty.Create(nameof(CommandParameter),
+				typeof(object),
+				typeof(TextCell),
+				null,
+				propertyChanged: CommandElement.OnCommandParameterChanged);
 
 		/// <summary>Bindable property for <see cref="Text"/>.</summary>
 		public static readonly BindableProperty TextProperty = BindableProperty.Create(nameof(Text), typeof(string), typeof(TextCell), default(string));
@@ -104,9 +89,14 @@ protected internal override void OnTapped()
 			Command?.Execute(CommandParameter);
 		}
 
-		void OnCommandCanExecuteChanged(object sender, EventArgs eventArgs)
+		void ICommandElement.CanExecuteChanged(object sender, EventArgs eventArgs)
 		{
+			if (Command is null)
+				return;
+
 			IsEnabled = Command.CanExecute(CommandParameter);
 		}
+
+		WeakCommandSubscription ICommandElement.CleanupTracker { get; set; }
 	}
 }

src/Controls/src/Core/CommandElement.cs

@@ -10,15 +10,24 @@ static class CommandElement
 		public static void OnCommandChanging(BindableObject bo, object o, object n)
 		{
 			var commandElement = (ICommandElement)bo;
-			if (o is ICommand oldCommand)
-				oldCommand.CanExecuteChanged -= commandElement.CanExecuteChanged;
+			commandElement.CleanupTracker?.Dispose();
+			commandElement.CleanupTracker = null;
 		}
 
 		public static void OnCommandChanged(BindableObject bo, object o, object n)
 		{
 			var commandElement = (ICommandElement)bo;
-			if (n is ICommand newCommand)
-				newCommand.CanExecuteChanged += commandElement.CanExecuteChanged;
+
+			if (n is null)
+			{
+				commandElement.CleanupTracker?.Dispose();
+				commandElement.CleanupTracker = null;
+			}
+			else
+			{
+				commandElement.CleanupTracker = new WeakCommandSubscription(bo, (ICommand)n, commandElement.CanExecuteChanged);
+			}
+
 			commandElement.CanExecuteChanged(bo, EventArgs.Empty);
 		}
 
@@ -36,4 +45,4 @@ public static bool GetCanExecute(ICommandElement commandElement)
 			return commandElement.Command.CanExecute(commandElement.CommandParameter);
 		}
 	}
-}
+}

src/Controls/src/Core/DepdendentHandle.cs

@@ -0,0 +1,96 @@
+#nullable enable
+
+#if NETSTANDARD2_0 || NETSTANDARD2_1
+using System;
+using System.Runtime;
+using System.Runtime.CompilerServices;
+using System.Windows.Input;
+
+namespace System.Runtime;
+
+/// <summary>
+/// A wrapper around ConditionalWeakTable that replicates DependentHandle behavior.
+/// Creates a dependency between a primary object and a dependent object where
+/// the dependent object becomes eligible for collection when the primary is collected.
+/// </summary>
+internal class DependentHandle : IDisposable
+{
+    private readonly ConditionalWeakTable<object, object> _table;
+    private readonly WeakReference<object> _primaryRef;
+    private bool _disposed;
+
+    /// <summary>
+    /// Initializes a new instance of DependentHandle with a primary and dependent object.
+    /// </summary>
+    /// <param name="primary">The primary object that controls the lifetime of the dependent object.</param>
+    /// <param name="dependent">The dependent object that will be collected when primary is collected.</param>
+    public DependentHandle(object primary, object? dependent)
+    {
+        _table = new ConditionalWeakTable<object, object>();
+        _primaryRef = new WeakReference<object>(primary);
+
+        // Store the dependent object in the table, keyed by the primary object
+        if (dependent is not null)
+        {
+            _table.Add(primary, dependent);
+        }
+    }
+
+    /// <summary>
+    /// Gets the primary object if it's still alive, otherwise returns null.
+    /// </summary>
+    public object? Target
+    {
+        get
+        {
+            if (_disposed)
+                return null;
+
+            return _primaryRef.TryGetTarget(out var target) ? target : null;
+        }
+    }
+
+    /// <summary>
+    /// Gets the dependent object if the primary object is still alive, otherwise returns null.
+    /// </summary>
+    public object? Dependent
+    {
+        get
+        {
+            if (_disposed)
+                return null;
+
+            if (_primaryRef.TryGetTarget(out var primary) &&
+                _table.TryGetValue(primary, out var dependent))
+            {
+                return dependent;
+            }
+
+            return null;
+        }
+    }
+
+    /// <summary>
+    /// Checks if both primary and dependent objects are still alive.
+    /// </summary>
+    public bool IsAllocated => Target is not null && Dependent is not null;
+
+    /// <summary>
+    /// Disposes the DependentHandleCWT, clearing all references.
+    /// </summary>
+    public void Dispose()
+    {
+        if (_disposed)
+            return;
+
+        _disposed = true;
+
+        // Clear the table - this will allow dependent objects to be collected
+        // even if the primary object is still alive
+        if (_primaryRef.TryGetTarget(out var primary))
+        {
+            _table.Remove(primary);
+        }
+    }
+}
+#endif

src/Controls/src/Core/ICommandElement.cs

@@ -12,5 +12,7 @@ interface ICommandElement
 
 		// implement these explicitly
 		void CanExecuteChanged(object? sender, EventArgs e);
+
+		WeakCommandSubscription? CleanupTracker { get; set; }
 	}
 }

src/Controls/src/Core/ImageButton/ImageButton.cs

@@ -288,5 +288,13 @@ void IButton.Released()
 		Color IButtonStroke.StrokeColor => (Color)GetValue(BorderColorProperty);
 
 		int IButtonStroke.CornerRadius => (int)GetValue(CornerRadiusProperty);
+
+
+		WeakCommandSubscription ICommandElement.CleanupTracker
+		{
+			get;
+			set;
+		}
+
 	}
 }

src/Controls/src/Core/Menu/MenuItem.cs

@@ -198,5 +198,11 @@ void OnImageSourceSourceChanged(object sender, EventArgs e)
 		{
 			OnPropertyChanged(IconImageSourceProperty.PropertyName);
 		}
+
+		WeakCommandSubscription ICommandElement.CleanupTracker
+		{
+			get;
+			set;
+		}
 	}
 }

src/Controls/src/Core/RefreshView/RefreshView.cs

@@ -156,5 +156,11 @@ private protected override string GetDebuggerDisplay()
 			var debugText = DebuggerDisplayHelpers.GetDebugText(nameof(Command), Command, nameof(IsRefreshing), IsRefreshing, false);
 			return $"{base.GetDebuggerDisplay()}, {debugText}";
 		}
+
+		WeakCommandSubscription ICommandElement.CleanupTracker
+		{
+			get;
+			set;
+		}
 	}
 }

src/Controls/src/Core/SearchBar/SearchBar.cs

@@ -160,5 +160,11 @@ private protected override string GetDebuggerDisplay()
 			var debugText = DebuggerDisplayHelpers.GetDebugText(nameof(SearchCommand), SearchCommand);
 			return $"{base.GetDebuggerDisplay()}, {debugText}";
 		}
+
+		WeakCommandSubscription ICommandElement.CleanupTracker
+		{
+			get;
+			set;
+		}
 	}
 }

src/Controls/src/Core/Shell/SearchHandler.cs

@@ -666,16 +666,16 @@ void ClearPlaceholderCanExecuteChanged(object sender, EventArgs e)
 			ClearPlaceholderEnabledCore = ClearPlaceholderCommand.CanExecute(ClearPlaceholderCommandParameter);
 		}
 
+		internal WeakCommandSubscription ClearPlaceholderCommandSubscription { get; set; }
+
 		void OnClearPlaceholderCommandChanged(ICommand oldCommand, ICommand newCommand)
 		{
-			if (oldCommand != null)
-			{
-				oldCommand.CanExecuteChanged -= ClearPlaceholderCanExecuteChanged;
-			}
+			ClearPlaceholderCommandSubscription?.Dispose();
+			ClearPlaceholderCommandSubscription = null;
 
 			if (newCommand != null)
 			{
-				newCommand.CanExecuteChanged += ClearPlaceholderCanExecuteChanged;
+				ClearPlaceholderCommandSubscription = new WeakCommandSubscription(this, newCommand, ClearPlaceholderCanExecuteChanged);
 				ClearPlaceholderEnabledCore = ClearPlaceholderCommand.CanExecute(ClearPlaceholderCommandParameter);
 			}
 			else
@@ -690,16 +690,16 @@ void OnClearPlaceholderCommandParameterChanged()
 				ClearPlaceholderEnabledCore = ClearPlaceholderCommand.CanExecute(CommandParameter);
 		}
 
+		internal WeakCommandSubscription CommandSubscription { get; set; }
+
 		void OnCommandChanged(ICommand oldCommand, ICommand newCommand)
 		{
-			if (oldCommand != null)
-			{
-				oldCommand.CanExecuteChanged -= CanExecuteChanged;
-			}
+			CommandSubscription?.Dispose();
+			CommandSubscription = null;
 
-			if (newCommand != null)
+			if (newCommand is not null)
 			{
-				newCommand.CanExecuteChanged += CanExecuteChanged;
+				CommandSubscription = new WeakCommandSubscription(this, newCommand, CanExecuteChanged);
 				IsSearchEnabledCore = Command.CanExecute(CommandParameter);
 			}
 			else