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[Android] Shell: Defer tab infrastructure for single-page startup #37321

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Merged
kubaflo merged 3 commits into from
Aug 14, 2026
Merged

[Android] Shell: Defer tab infrastructure for single-page startup #37321

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413fb9f
[Android] Defer unused Shell tab infrastructure
jonathanpeppers Aug 11, 2026
d6437ef
Fix deferred Shell tab lifecycle
jonathanpeppers Aug 12, 2026
0d9d391
Fix deferred Shell tab state replay
jonathanpeppers Aug 13, 2026
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82 changes: 55 additions & 27 deletions src/Controls/src/Core/Handlers/Shell/ShellItemHandler.Android.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -173,14 +173,15 @@ internal void SetupViewPagerAdapter()
/// </summary>
internal void SetupTabbedViewManager()
{
if (_viewPager is null || VirtualView is null || MauiContext is null)
if (_tabbedViewManager is not null || _viewPager is null || VirtualView is null || MauiContext is null)
{
return;
}

var shellSections = ((IShellItemController)VirtualView).GetItems();
var shellItemController = (IShellItemController)VirtualView;
var shellSections = shellItemController.GetItems();

if (shellSections is null || shellSections.Count == 0)
if (shellSections is null || shellSections.Count == 0 || !shellItemController.ShowTabs)
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{
return;
}
Expand Down Expand Up @@ -217,6 +218,22 @@ internal void SetupTabbedViewManager()

// Get BNV reference for appearance tracker
_bottomNavigationView = _tabbedViewManager.BottomNavigationView;
RecreateBottomNavigationAppearanceTracker();
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// Initial setup registers the appearance observer immediately afterward. When
// setup was deferred, replay state for the newly-created bottom tabs.
if (_registeredShell is not null && _displayedPage is not null)
{
UpdateAllBadges();
((IShellController)_registeredShell).AppearanceChanged(_displayedPage, false);
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}
}

void RecreateBottomNavigationAppearanceTracker()
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{
_appearanceTracker?.Dispose();
_shellContext ??= GetShellContext();
_appearanceTracker = _shellContext.CreateBottomNavViewAppearanceTracker(VirtualView);
}

/// <summary>
Expand All @@ -236,6 +253,15 @@ internal void RebuildBottomNavigation()

// Update BNV reference (SetElement creates a new BNV)
_bottomNavigationView = _tabbedViewManager.BottomNavigationView;
RecreateBottomNavigationAppearanceTracker();
}

void RemoveBottomNavigationInfrastructure()
{
_tabbedViewManager?.SetElement(null);
_tabbedViewManager = null;
_shellItemAdapter = null;
_bottomNavigationView = null;
}

/// <summary>
Expand Down Expand Up @@ -342,22 +368,26 @@ internal void SwitchToShellItem(ShellItem newItem)
// Rebuild ViewPager2 adapter for new ShellItem's sections
SetupViewPagerAdapter();

// Rebuild bottom navigation for new ShellItem's sections via TabbedViewManager
RebuildBottomNavigation();

// Apply badges to the rebuilt bottom navigation
UpdateAllBadges();

// Update tab visibility for new ShellItem (may need to show/hide bottom tabs)
var showTabs = ((IShellItemController)newItem).ShowTabs;

if (showTabs)
{
if (_tabbedViewManager is null)
{
SetupTabbedViewManager();
}
else
{
// Rebuild bottom navigation for new ShellItem's sections via TabbedViewManager
RebuildBottomNavigation();
}

UpdateAllBadges();
_tabbedViewManager?.SetTabLayout();
}
else
{
_tabbedViewManager?.RemoveTabs();
RemoveBottomNavigationInfrastructure();
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}

// Re-register appearance observer with new ShellItem
Expand Down Expand Up @@ -616,7 +646,7 @@ void UpdateDisplayedPage(Page page)

void UpdateTabBarVisibility()
{
if (_tabbedViewManager is null || _displayedPage is null || ((ElementHandler)this).VirtualView is null)
if (_switchingShellItem || _displayedPage is null || ((ElementHandler)this).VirtualView is null)
{
return;
}
Expand All @@ -625,11 +655,12 @@ void UpdateTabBarVisibility()

if (showTabs)
{
_tabbedViewManager.SetTabLayout();
SetupTabbedViewManager();
_tabbedViewManager?.SetTabLayout();
}
else
{
_tabbedViewManager.RemoveTabs();
_tabbedViewManager?.RemoveTabs();
}
}

Expand Down Expand Up @@ -727,9 +758,9 @@ protected override void ConnectHandler(ViewPager2 platformView)
((IShellItemController)VirtualView).ItemsCollectionChanged += OnShellItemsChanged;
}

// Initialize shell context and appearance tracker early
// Initialize the Shell context early. The bottom navigation appearance tracker
// is created with the deferred TabbedViewManager when tabs are actually needed.
_shellContext ??= GetShellContext();
_appearanceTracker = _shellContext.CreateBottomNavViewAppearanceTracker(VirtualView);

// NOTE: Appearance observer registration is deferred to RegisterAppearanceObserver()
// called from OnViewCreated in the wrapper fragment. At ConnectHandler time,
Expand Down Expand Up @@ -776,11 +807,14 @@ void OnShellItemsChanged(object? sender, System.Collections.Specialized.NotifyCo
// 0→N transition: adapter/manager were not created during initial setup
// because there were no sections. Now that sections exist, create them.
SetupViewPagerAdapter();
SetupTabbedViewManager();
}

// Rebuild the bottom navigation menu for the updated sections via TabbedViewManager
_tabbedViewManager?.RefreshTabs();
var existingTabbedViewManager = _tabbedViewManager;
SetupTabbedViewManager();

// Rebuild an existing bottom navigation menu. A newly-created manager was
// already populated by SetElement in SetupTabbedViewManager.
existingTabbedViewManager?.RefreshTabs();
UpdateTabBarVisibility();

// Signal that the adapter was just rebuilt. The next SwitchToSection call
Expand Down Expand Up @@ -837,7 +871,7 @@ protected override void DisconnectHandler(ViewPager2 platformView)
_registeredShell = null;
}

// Dispose per-item appearance tracker (ConnectHandler recreates for new item)
// Dispose per-item appearance tracker; tab setup or rebuild recreates it as needed.
_appearanceTracker?.Dispose();
_appearanceTracker = null;

Expand All @@ -851,13 +885,7 @@ protected override void DisconnectHandler(ViewPager2 platformView)
if (!_preserveFragmentResources)
{
// Full disconnect: fragment is being destroyed — clean everything
if (_tabbedViewManager is not null)
{
_tabbedViewManager.RemoveTabs();
_tabbedViewManager.SetElement(null);
_tabbedViewManager = null;
}
_shellItemAdapter = null;
RemoveBottomNavigationInfrastructure();

_toolbarAppearanceTracker?.Dispose();
_toolbarAppearanceTracker = null;
Expand Down
84 changes: 51 additions & 33 deletions src/Controls/src/Core/Handlers/Shell/ShellSectionHandler.Android.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -149,21 +149,6 @@ protected override AView CreatePlatformElement()
_rootLayout = rootView.FindViewById<LinearLayout>(Resource.Id.shellsection_coordinator);
_viewPager = rootView.FindViewById<ViewPager2>(Resource.Id.shellsection_viewpager);

// Create TabLayout programmatically (no longer from XML layout).
// It will be placed into navigationlayout_toptabs via PlaceTopTabs().
var context = MauiContext?.Context
?? throw new InvalidOperationException("MauiContext.Context cannot be null");

int actionBarHeight = context.GetActionBarHeight();

_contentTabLayout = new TabLayout(context)
{
Id = AView.GenerateViewId(),
LayoutParameters = new LP(LP.MatchParent, actionBarHeight),
Visibility = ViewStates.Gone, // Hidden by default (shown when > 1 tab)
TabMode = TabLayout.ModeScrollable
};

return rootView;
}

Expand Down Expand Up @@ -253,18 +238,7 @@ internal void SetupViewPagerAdapter()
var visibleItems = SectionController.GetItems();
_viewPager.OffscreenPageLimit = Math.Max(visibleItems.Count, 1);

// Setup TabbedViewManager for top tab management.
// Pre-assign Shell's TabLayout (specific sizing) before SetElement.
_shellSectionAdapter = new ShellSectionTabbedViewAdapter(VirtualView);
_tabbedViewManager = new TabbedViewManager(MauiContext, _viewPager)
{
TabLayout = _contentTabLayout
};
_tabbedViewManager.SetElement(_shellSectionAdapter);

// Register page change callback (stored in field for cleanup in DisconnectHandler)
_pageChangedCallback = new ViewPagerPageChangeCallback(this);
_viewPager.RegisterOnPageChangeCallback(_pageChangedCallback);
SetupTabbedViewManager();

// Update TabLayout visibility based on item count
UpdateTabLayoutVisibility();
Expand All @@ -275,10 +249,6 @@ internal void SetupViewPagerAdapter()
// Set initial position
SetInitialPosition();

// Setup TabLayout appearance tracker
_tabLayoutAppearanceTracker = _shellContext.CreateTabLayoutAppearanceTracker(VirtualView);

// Register as appearance observer for TabLayout updates
var shell = VirtualView.FindParentOfType<Shell>();
if (shell is not null)
{
Expand All @@ -305,6 +275,53 @@ internal void SetupViewPagerAdapter()
}
}

void SetupTabbedViewManager()
{
if (_tabbedViewManager is not null || VirtualView is null || _viewPager is null || MauiContext is null || _shellContext is null)
{
return;
}

if (SectionController.GetItems().Count <= 1)
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{
return;
}

var context = MauiContext.Context
?? throw new InvalidOperationException("MauiContext.Context cannot be null");

_contentTabLayout = new TabLayout(context)
{
Id = AView.GenerateViewId(),
LayoutParameters = new LP(LP.MatchParent, context.GetActionBarHeight()),
Visibility = ViewStates.Gone,
TabMode = TabLayout.ModeScrollable
};

_shellSectionAdapter = new ShellSectionTabbedViewAdapter(VirtualView);
_tabbedViewManager = new TabbedViewManager(MauiContext, _viewPager)
{
TabLayout = _contentTabLayout
};
_tabbedViewManager.SetElement(_shellSectionAdapter);

_pageChangedCallback = new ViewPagerPageChangeCallback(this);
_viewPager.RegisterOnPageChangeCallback(_pageChangedCallback);

_tabLayoutAppearanceTracker = _shellContext.CreateTabLayoutAppearanceTracker(VirtualView);

// Initial setup registers the appearance observer immediately afterward. When
// setup was deferred, replay the appearance for the newly-created TabLayout.
if (_registeredShell is not null && IsCurrentlyActiveSection() && VirtualView.CurrentItem is ShellContent currentContent)
{
var page = ((IShellContentController)currentContent).GetOrCreateContent();

@MauiBot MauiBot Aug 13, 2026

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🔍 AI-Generated Review (multi-model)

[moderate] Logic and Correctness — The deferred-setup appearance replay uses the section's root content page (CurrentItem → GetOrCreateContent()) as the AppearanceChanged source, not the page currently displayed by this section. IShellController.AppearanceChanged(source, false) calls UpdateToolbarAppearanceFeatures(source, null) with that source directly (Shell.cs), so the toolbar features are recomputed from a page that may not be on screen. Concrete scenario: an active ShellSection with one ShellContent where the app has pushed a detail page onto the section's stack, and then adds a second ShellContent (the deferred-creation trigger). The replay fires with the section root ContentPage while the pushed detail page is displayed, so Shell-level toolbar appearance is resolved against the wrong page. The sibling ShellItemHandler.SetupTabbedViewManager replay avoids this by using _displayedPage; this path should use the section's currently displayed page (navigation-stack top) and skip the replay when it cannot be resolved.

if (page is not null)
{
((IShellController)_registeredShell).AppearanceChanged(page, false);
}
}
}

void SetInitialPosition()
{
if (VirtualView?.CurrentItem is null || _viewPager is null)
Expand All @@ -322,7 +339,7 @@ void SetInitialPosition()

void UpdateTabLayoutVisibility()
{
if (_tabbedViewManager is null || VirtualView is null)
if (VirtualView is null)
{
return;
}
Expand Down Expand Up @@ -646,6 +663,7 @@ void OnItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e
var visibleCount = SectionController.GetItems().Count;
_viewPager?.OffscreenPageLimit = Math.Max(visibleCount, 1);

SetupTabbedViewManager();
UpdateTabLayoutVisibility();
UpdateViewPagerUserInput();
}
Expand Down Expand Up @@ -1017,7 +1035,7 @@ public override void OnPageSelected(int position)
toolbarTracker?.Page = page;

// Update CurrentItem
virtualView.CurrentItem = newCurrentItem;
virtualView.SetValueFromRenderer(ShellSection.CurrentItemProperty, newCurrentItem);
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// Trigger appearance update
if (shell is not null)
Expand Down
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