Fix crash using BackButtonBehavior and navigating to root (#11438) Fixes

 #11401 Fixes #10514

* Fix crash using BackButtonBehavior and navigating to root

* Added Device Test

* - add unit test and fixup ShellToolbar to unhook from previous command

* - remove extra call to OnBackButtonCommandPropertyChanged

* - remove excess code from ShellToolbar

Co-authored-by: Shane Neuville <shneuvil@microsoft.com>

src/Controls/src/Core/ShellToolbar.cs

@@ -1,5 +1,8 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Windows.Input;
@@ -10,8 +13,8 @@ namespace Microsoft.Maui.Controls
  internal class ShellToolbar : Toolbar
  {
  Shell _shell;
- Page _currentPage;
- BackButtonBehavior _backButtonBehavior;
+ Page? _currentPage;
+ BackButtonBehavior? _backButtonBehavior;
  ToolbarTracker _toolbarTracker = new ToolbarTracker();
 #if WINDOWS
  MenuBarTracker _menuBarTracker = new MenuBarTracker();
@@ -77,7 +80,7 @@ internal void ApplyChanges()
  _menuBarTracker.Target = _shell;
 #endif
 
- Page previousPage = null;
+ Page? previousPage = null;
  if (stack.Count > 1)
  previousPage = stack[stack.Count - 1];
 
@@ -95,13 +98,6 @@ internal void ApplyChanges()
  BackButtonVisible = backButtonVisible && stack.Count > 1;
  BackButtonEnabled = _backButtonBehavior?.IsEnabled ?? true;
 
- if (_backButtonBehavior?.Command != null)
- {
- BackButtonEnabled =
- BackButtonEnabled &&
- _backButtonBehavior.Command.CanExecute(_backButtonBehavior.CommandParameter);
- }
-
  UpdateTitle();
 
  if (_currentPage != null &&
@@ -131,7 +127,6 @@ internal void ApplyChanges()
  DynamicOverflowEnabled = PlatformConfiguration.WindowsSpecific.Page.GetToolbarDynamicOverflowEnabled(currentPage);
  }
 
- ICommand _backButtonCommand;
  void UpdateBackbuttonBehavior()
  {
  var bbb = Shell.GetBackButtonBehavior(_currentPage);
@@ -140,37 +135,20 @@ void UpdateBackbuttonBehavior()
  return;
 
  if (_backButtonBehavior != null)
- _backButtonBehavior.PropertyChanged -= OnBackButtonPropertyChanged;
+ _backButtonBehavior.PropertyChanged -= OnBackButtonCommandPropertyChanged;
 
  _backButtonBehavior = bbb;
 
  if (_backButtonBehavior != null)
- _backButtonBehavior.PropertyChanged += OnBackButtonPropertyChanged;
-
- void OnBackButtonPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
- {
- ApplyChanges();
-
- if (_backButtonBehavior.Command == _backButtonCommand)
- return;
-
- if (_backButtonCommand != null)
- _backButtonCommand.CanExecuteChanged -= OnBackButtonCanExecuteChanged;
-
- _backButtonCommand = _backButtonBehavior.Command;
-
- if (_backButtonCommand != null)
- _backButtonCommand.CanExecuteChanged += OnBackButtonCanExecuteChanged;
- }
+ _backButtonBehavior.PropertyChanged += OnBackButtonCommandPropertyChanged;
+ }
 
- void OnBackButtonCanExecuteChanged(object sender, EventArgs e)
- {
- BackButtonEnabled =
- _backButtonCommand.CanExecute(_backButtonBehavior.CommandParameter);
- }
+ void OnBackButtonCommandPropertyChanged(object? sender, PropertyChangedEventArgs e)
+ {
+ ApplyChanges();
  }
 
- void OnCurrentPagePropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
+ void OnCurrentPagePropertyChanged(object? sender, System.ComponentModel.PropertyChangedEventArgs e)
  {
  if (e.Is(Page.TitleProperty))
  UpdateTitle();
@@ -195,7 +173,7 @@ internal void UpdateTitle()
  return;
  }
 
- Page currentPage = _shell.GetCurrentShellPage() as Page;
+ Page? currentPage = _shell.GetCurrentShellPage() as Page;
  if (currentPage?.IsSet(Page.TitleProperty) == true)
  {
  Title = currentPage.Title ?? String.Empty;

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

@@ -101,6 +101,50 @@ public async Task BackButtonDisabledWhenCommandDisabled()
  Assert.True(testShell.Toolbar.BackButtonEnabled);
  }
 
+ [Fact]
+ public async Task BackButtonBehaviorCommandFromPoppedPageIsCorrectlyUnsubscribedFrom()
+ {
+ var firstPage = new ContentPage();
+ var secondPage = new ContentPage();
+ bool canExecute = true;
+ var backButtonBehavior = new BackButtonBehavior();
+ TestShell testShell = new TestShell(firstPage);
+
+ await testShell.Navigation.PushAsync(secondPage);
+
+ Shell.SetBackButtonBehavior(secondPage, backButtonBehavior);
+
+ backButtonBehavior.Command = new Command(() => { }, () => canExecute);
+
+ await testShell.Navigation.PopAsync();
+
+ canExecute = false;
+ (backButtonBehavior.Command as Command).ChangeCanExecute();
+
+ Assert.True(testShell.Toolbar.BackButtonEnabled);
+ }
+
+ [Fact]
+ public async Task BackButtonUpdatesWhenSetToNewCommand()
+ {
+ var firstPage = new ContentPage();
+ var secondPage = new ContentPage();
+ bool canExecute = true;
+ var backButtonBehavior = new BackButtonBehavior();
+ TestShell testShell = new TestShell(firstPage);
+
+ await testShell.Navigation.PushAsync(secondPage);
+
+ Shell.SetBackButtonBehavior(secondPage, backButtonBehavior);
+
+ backButtonBehavior.Command = new Command(() => { }, () => true);
+ Assert.True(testShell.Toolbar.BackButtonEnabled);
+ backButtonBehavior.Command = new Command(() => { }, () => false);
+ Assert.False(testShell.Toolbar.BackButtonEnabled);
+ backButtonBehavior.Command = null;
+ Assert.True(testShell.Toolbar.BackButtonEnabled);
+ }
+
  [Fact]
  public async Task ShellToolbarUpdatesFromNewBackButtonBehavior()
  {

src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.cs

@@ -596,6 +596,45 @@ await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
  });
  }
 
+ [Fact(DisplayName = "Navigate to Root with BackButtonBehavior no Crash")]
+ public async Task NavigateToRootWithBackButtonBehaviorNoCrash()
+ {
+ SetupBuilder();
+
+ var shell = await CreateShellAsync(shell =>
+ {
+ shell.CurrentItem = new ContentPage();
+ });
+
+ await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
+ {
+ Assert.False(IsBackButtonVisible(shell.Handler));
+ var secondPage = new ContentPage();
+ await shell.Navigation.PushAsync(secondPage);
+ Assert.True(IsBackButtonVisible(shell.Handler));
+
+ bool canExecute = true;
+ Command command = new Command(() => { }, () => canExecute);
+ BackButtonBehavior behavior = new BackButtonBehavior()
+ {
+ IsVisible = false,
+ Command = command
+ };
+
+ Shell.SetBackButtonBehavior(secondPage, behavior);
+ Assert.False(IsBackButtonVisible(shell.Handler));
+ behavior.IsVisible = true;
+ NavigationPage.SetHasBackButton(shell.CurrentPage, true);
+
+ await shell.Navigation.PopToRootAsync(false);
+ await Task.Delay(100);
+ canExecute = false;
+
+ command.ChangeCanExecute();
+ Assert.NotNull(shell.Handler);
+ });
+ }
+
  protected Task<Shell> CreateShellAsync(Action<Shell> action) =>
  InvokeOnMainThreadAsync(() =>
  {