Optimize the shell handler

Author: Tamilarasan-Paranthaman

src/Controls/src/Core/Handlers/Shell/ShellItemHandler.Android.cs

@@ -1,7 +1,6 @@
 #nullable enable
 using System;
 using System.Collections.Generic;
-using System.Threading.Tasks;
 using Android.OS;
 using Android.Views;
 using AndroidX.CoordinatorLayout.Widget;
@@ -298,10 +297,15 @@ internal void SwitchToShellItem(ShellItem newItem)
             // That causes SetSelectedTab on the old BNV, firing OnNavigationItemSelected which
             // poisons the old ShellItem's CurrentItem via ProposeSection.
             _switchingShellItem = true;
-            SetVirtualView(newItem);
-            _switchingShellItem = false;
-
-            _preserveFragmentResources = false;
+            try
+            {
+                SetVirtualView(newItem);
+            }
+            finally
+            {
+                _switchingShellItem = false;
+                _preserveFragmentResources = false;
+            }
 
             // Rebuild ViewPager2 adapter for new ShellItem's sections
             SetupViewPagerAdapter();
@@ -638,37 +642,15 @@ void NotifyTopTabsForSectionSwitch(ShellSection? oldSection, ShellSection? newSe
         }
 
         /// <summary>
-        /// Handles the back button press. Returns true if navigation was handled, false otherwise.
-        /// Back navigation is delegated to the current section's StackNavigationManager.
+        /// Handles the back button press by delegating to Shell's full navigation pipeline.
+        /// Shell.SendBackButtonPressed() handles BackButtonBehavior commands, page overrides,
+        /// navigation stack pops, modal dismissal, and ShellNavigatingEventArgs cancellation.
+        /// Returns true if Shell handled it, false if system should handle (app exit).
         /// </summary>
         internal bool OnBackButtonPressed()
         {
-            if (_shellSection is null)
-            {
-                return false;
-            }
-
-            var stack = _shellSection.Stack;
-
-            // If we're at the root page, don't handle back - let the system handle it
-            if (stack.Count <= 1)
-            {
-                return false;
-            }
-
-            // We have pages in the stack, so we can pop
-            Task.Run(async () =>
-            {
-                try
-                {
-                    await _shellSection.Navigation.PopAsync();
-                }
-                catch (Exception)
-                {
-                }
-            });
-
-            return true; // We handled the back press
+            var shell = VirtualView?.FindParentOfType<Shell>();
+            return shell?.SendBackButtonPressed() ?? false;
         }
 
         #endregion Navigation Support
@@ -1156,10 +1138,17 @@ public void Dispose()
         /// </summary>
         class ShellItemWrapperFragment : Fragment
         {
-            readonly ShellItemHandler _handler;
+            readonly ShellItemHandler? _handler;
             CoordinatorLayout? _rootLayout;
             ShellBackPressedCallback? _backPressedCallback;
 
+            // Default constructor required by Android's FragmentManager for fragment restoration.
+            // Without this, FragmentManager.instantiate() crashes on process-death restoration.
+            public ShellItemWrapperFragment()
+            {
+                _handler = null;
+            }
+
             public ShellItemWrapperFragment(ShellItemHandler handler)
             {
                 _handler = handler;
@@ -1169,6 +1158,13 @@ public ShellItemWrapperFragment(ShellItemHandler handler)
 
             public override AView OnCreateView(LayoutInflater inflater, ViewGroup? container, Bundle? savedInstanceState)
             {
+                // If restored without proper handler reference, return empty view.
+                // The Shell infrastructure will recreate proper fragments after reconnecting.
+                if (_handler is null)
+                {
+                    return new global::Android.Widget.FrameLayout(inflater.Context!);
+                }
+
                 // Inflate from XML layout — consistent with NavigationViewHandler/FlyoutViewHandler pattern
                 var rootView = inflater.Inflate(Resource.Layout.shellitemlayout, container, false)
                     ?? throw new InvalidOperationException("shellitemlayout inflation failed");
@@ -1195,8 +1191,14 @@ public override void OnViewCreated(AView view, Bundle? savedInstanceState)
             {
                 base.OnViewCreated(view, savedInstanceState);
 
+                // Skip setup if restored without handler (parameterless constructor path)
+                if (_handler is null)
+                {
+                    return;
+                }
+
                 // Setup back button handling
-                _backPressedCallback = new ShellBackPressedCallback(_handler);
+                _backPressedCallback = new ShellBackPressedCallback(_handler, this);
                 RequireActivity().OnBackPressedDispatcher.AddCallback(ViewLifecycleOwner, _backPressedCallback);
 
                 // Setup the shared toolbar
@@ -1255,20 +1257,33 @@ protected override void Dispose(bool disposing)
             sealed class ShellBackPressedCallback : AndroidX.Activity.OnBackPressedCallback
             {
                 readonly ShellItemHandler _handler;
+                readonly Fragment _fragment;
 
-                public ShellBackPressedCallback(ShellItemHandler handler) : base(true)
+                public ShellBackPressedCallback(ShellItemHandler handler, Fragment fragment) : base(true)
                 {
                     _handler = handler;
+                    _fragment = fragment;
                 }
 
                 public override void HandleOnBackPressed()
                 {
-                    // Let the handler try to handle the back press
+                    // Route through Shell's full back navigation pipeline.
+                    // Shell.SendBackButtonPressed() handles:
+                    //   - BackButtonBehavior.Command execution
+                    //   - Page.OnBackButtonPressed() overrides
+                    //   - Navigation stack pops (via Shell.OnBackButtonPressed)
+                    //   - Modal stack dismissal
+                    //   - ShellNavigatingEventArgs cancellation
+                    // This matches the old renderer behavior where the lifecycle chain
+                    // (Activity → Window → Shell) naturally invoked the full pipeline.
                     if (!_handler.OnBackButtonPressed())
                     {
-                        // Handler didn't handle it (we're at root), let system handle it
+                        // Shell didn't handle it (at root, no stack to pop, not cancelled).
+                        // Forward to system by temporarily disabling this callback so the
+                        // dispatcher falls through to the next handler in the chain
+                        // (e.g., the Activity's default that finishes the app).
                         this.Enabled = false;
-                        // The system will handle app exit
+                        _fragment.RequireActivity().OnBackPressedDispatcher.OnBackPressed();
                         this.Enabled = true;
                     }
                 }
@@ -1389,7 +1404,7 @@ public override Fragment CreateFragment(int position)
                 return observableFragment.Fragment;
             }
 
-            throw new InvalidOperationException($"ShellSectionRenderer for {section.Title} is not an IShellObservableFragment");
+            throw new InvalidOperationException($"ShellSectionHandler for {section.Title} is not an IShellObservableFragment");
         }
 
         public IShellSectionRenderer? GetRenderer(int position)

src/Controls/src/Core/Handlers/Shell/ShellSectionHandler.Android.cs

@@ -36,6 +36,7 @@ public partial class ShellSectionHandler : ElementHandler<ShellSection, AView>,
         ShellContentFragmentAdapter? _adapter;
         TabbedViewManager? _tabbedViewManager;
         ShellSectionTabbedViewAdapter? _shellSectionAdapter;
+        ViewPagerPageChangeCallback? _pageChangedCallback;
 
         /// <summary>
         /// Gets the toolbar tracker from the parent ShellItemHandler.
@@ -229,9 +230,9 @@ internal void SetupViewPagerAdapter()
             };
             _tabbedViewManager.SetElement(_shellSectionAdapter);
 
-            // Register page change callback
-            var pageChangedCallback = new ViewPagerPageChangeCallback(this);
-            _viewPager.RegisterOnPageChangeCallback(pageChangedCallback);
+            // Register page change callback (stored in field for cleanup in DisconnectHandler)
+            _pageChangedCallback = new ViewPagerPageChangeCallback(this);
+            _viewPager.RegisterOnPageChangeCallback(_pageChangedCallback);
 
             // Update TabLayout visibility based on item count
             UpdateTabLayoutVisibility();
@@ -409,6 +410,13 @@ protected override void DisconnectHandler(AView platformView)
             _tabbedViewManager = null;
             _shellSectionAdapter = null;
 
+            // Unregister page change callback
+            if (_pageChangedCallback is not null && _viewPager is not null)
+            {
+                _viewPager.UnregisterOnPageChangeCallback(_pageChangedCallback);
+                _pageChangedCallback = null;
+            }
+
             // Cleanup adapter
             _adapter = null;
             _viewPager?.Adapter = null;
@@ -459,7 +467,7 @@ public static void MapCurrentItem(ShellSectionHandler handler, ShellSection shel
 
         void OnItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            if (_adapter is null)
+            if (_adapter is null || _viewPager is null || _parentFragment is null || VirtualView is null || MauiContext is null)
             {
                 return;
             }
@@ -477,7 +485,7 @@ void OnItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e
                 // Replace with a fresh adapter to avoid stale state restoration
                 _viewPager?.Adapter = null;
 
-                _adapter = new ShellContentFragmentAdapter(VirtualView, _parentFragment!, MauiContext!.MakeScoped(fragmentManager: _parentFragment!.ChildFragmentManager))
+                _adapter = new ShellContentFragmentAdapter(VirtualView, _parentFragment, MauiContext.MakeScoped(fragmentManager: _parentFragment.ChildFragmentManager))
                 {
                     Handler = this
                 };
@@ -657,6 +665,8 @@ internal class ShellContentFragmentAdapter : FragmentStateAdapter
         readonly ShellSection _shellSection;
         readonly IMauiContext _mauiContext;
         IList<ShellContent>? _visibleItems;
+        long _itemIdCounter;
+        readonly Dictionary<ShellContent, long> _contentIds = new Dictionary<ShellContent, long>();
         IShellSectionController SectionController => (IShellSectionController)_shellSection;
 
         public ShellContentFragmentAdapter(ShellSection shellSection, Fragment parentFragment, IMauiContext mauiContext)
@@ -673,10 +683,24 @@ public ShellContentFragmentAdapter(ShellSection shellSection, Fragment parentFra
 
         /// <summary>
         /// Refreshes the visible items list. Called when items collection changes (add/remove/visibility).
+        /// Removes stale IDs for items no longer present so ContainsItem returns false,
+        /// causing FragmentStateAdapter to remove their fragments.
         /// </summary>
         public void OnItemsCollectionChanged()
         {
-            _visibleItems = SectionController.GetItems();
+            var newItems = SectionController.GetItems();
+
+            // Remove IDs for items that are no longer visible
+            var toRemove = new List<ShellContent>();
+            foreach (var kvp in _contentIds)
+            {
+                if (!newItems.Contains(kvp.Key))
+                    toRemove.Add(kvp.Key);
+            }
+            foreach (var item in toRemove)
+                _contentIds.Remove(item);
+
+            _visibleItems = newItems;
         }
 
         public override Fragment CreateFragment(int position)
@@ -690,13 +714,24 @@ public override Fragment CreateFragment(int position)
             return new ShellContentNavigationFragment(shellContent, _mauiContext, Handler);
         }
 
+        /// <summary>
+        /// Returns a stable ID for each ShellContent. Uses an incrementing counter
+        /// with a dictionary to avoid hash collisions (GetHashCode is not guaranteed unique).
+        /// Same pattern as ShellSectionFragmentAdapter.GetItemId.
+        /// </summary>
         public override long GetItemId(int position)
         {
             if (_visibleItems is null || position >= _visibleItems.Count)
             {
                 return -1;
             }
-            return _visibleItems[position].GetHashCode();
+            var content = _visibleItems[position];
+            if (!_contentIds.TryGetValue(content, out var id))
+            {
+                id = ++_itemIdCounter;
+                _contentIds[content] = id;
+            }
+            return id;
         }
 
         public override bool ContainsItem(long itemId)
@@ -705,12 +740,10 @@ public override bool ContainsItem(long itemId)
             {
                 return false;
             }
-            foreach (var item in _visibleItems)
+            foreach (var kvp in _contentIds)
             {
-                if (item.GetHashCode() == itemId)
-                {
+                if (kvp.Value == itemId && _visibleItems.Contains(kvp.Key))
                     return true;
-                }
             }
             return false;
         }
@@ -853,7 +886,7 @@ public override AView OnCreateView(LayoutInflater inflater, ViewGroup? container
             _rootPage = ((IShellContentController)_shellContent)?.GetOrCreateContent();
             if (_rootPage is null)
             {
-                return null!;
+                return new FrameLayout(inflater.Context!);
             }
 
             // Subscribe to navigation events EARLY - before anything else

src/Controls/src/Core/Platform/Android/TabbedViewManager.cs

@@ -36,7 +36,7 @@ internal class TabbedViewManager
 {
     Fragment _tabLayoutFragment;
     ColorStateList _originalTabTextColors;
-    ColorStateList _orignalTabIconColors;
+    ColorStateList _originalTabIconColors;
     ColorStateList _newTabTextColors;
     ColorStateList _newTabIconColors;
     FragmentManager _fragmentManager;
@@ -481,6 +481,7 @@ protected virtual void OnTabsCollectionChanged(object sender, NotifyCollectionCh
 
                 UpdateTabIcons();
 #pragma warning disable CS0618 // Type or member is obsolete
+                tabs.RemoveOnTabSelectedListener(_listeners);
                 tabs.AddOnTabSelectedListener(_listeners);
 #pragma warning restore CS0618 // Type or member is obsolete
             }
@@ -986,17 +987,17 @@ internal virtual ColorStateList GetItemIconTintColorState(int tabIndex)
             return null;
         }
 
-        if (_orignalTabIconColors is null)
+        if (_originalTabIconColors is null)
         {
-            _orignalTabIconColors = IsBottomTabPlacement ? _bottomNavigationView.ItemIconTintList : _tabLayout.TabIconTint;
+            _originalTabIconColors = IsBottomTabPlacement ? _bottomNavigationView.ItemIconTintList : _tabLayout.TabIconTint;
         }
 
         Color barItemColor = BarItemColor;
         Color barSelectedItemColor = BarSelectedItemColor;
 
         if (barItemColor is null && barSelectedItemColor is null)
         {
-            return _orignalTabIconColors;
+            return _originalTabIconColors;
         }
 
         if (_newTabIconColors is not null)