Handle CupertinoTabScaffold rebuilds with deleted tabs (#27576)

packages/flutter/lib/src/cupertino/bottom_tab_bar.dart

@@ -54,7 +54,10 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
       ),
     ),
   }) : assert(items != null),
-       assert(items.length >= 2),
+       assert(
+         items.length >= 2,
+         "Tabs need at least 2 items to conform to Apple's HIG",
+       ),
        assert(currentIndex != null),
        assert(0 <= currentIndex && currentIndex < items.length),
        assert(iconSize != null),
@@ -75,7 +78,8 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
 
   /// The index into [items] of the current active item.
   ///
-  /// Must not be null.
+  /// Must not be null and must inclusively be between 0 and the number of tabs
+  /// minus 1.
   final int currentIndex;
 
   /// The background color of the tab bar. If it contains transparency, the

packages/flutter/lib/src/cupertino/tab_scaffold.dart

@@ -167,6 +167,16 @@ class _CupertinoTabScaffoldState extends State<CupertinoTabScaffold> {
   @override
   void didUpdateWidget(CupertinoTabScaffold oldWidget) {
     super.didUpdateWidget(oldWidget);
+    if (_currentPage >= widget.tabBar.items.length) {
+      // Clip down to an acceptable range.
+      _currentPage = widget.tabBar.items.length - 1;
+      // Sanity check, since CupertinoTabBar.items's minimum length is 2.
+      assert(
+        _currentPage >= 0,
+        'CupertinoTabBar is expected to keep at least 2 tabs after updating',
+      );
+    }
+    // The user can still specify an exact desired index.
     if (widget.tabBar.currentIndex != oldWidget.tabBar.currentIndex) {
       _currentPage = widget.tabBar.currentIndex;
     }

packages/flutter/test/cupertino/tab_scaffold_test.dart

@@ -395,6 +395,82 @@ void main() {
     expect(tester.getRect(find.byType(Placeholder)), Rect.fromLTWH(0, 0, 800, 400));
     expect(MediaQuery.of(innerContext).padding.bottom, 0);
   });
+
+  testWidgets('Deleting tabs after selecting them works', (WidgetTester tester) async {
+    final List<int> tabsBuilt = <int>[];
+
+    BottomNavigationBarItem tabGenerator(int index) {
+      return BottomNavigationBarItem(
+        icon: const ImageIcon(TestImageProvider(24, 24)),
+        title: Text('Tab ${index + 1}'),
+      );
+    }
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabScaffold(
+          tabBar: CupertinoTabBar(
+            items: List<BottomNavigationBarItem>.generate(4, tabGenerator),
+            onTap: (int newTab) => selectedTabs.add(newTab),
+          ),
+          tabBuilder: (BuildContext context, int index) {
+            tabsBuilt.add(index);
+            return Text('Page ${index + 1}');
+          },
+        ),
+      ),
+    );
+
+    expect(tabsBuilt, <int>[0]);
+    // selectedTabs list is appended to on onTap callbacks. We didn't tap
+    // any tabs yet.
+    expect(selectedTabs, <int>[]);
+    tabsBuilt.clear();
+
+    await tester.tap(find.text('Tab 4'));
+    await tester.pump();
+
+    // Tabs 1 and 4 are built but only one is onstage.
+    expect(tabsBuilt, <int>[0, 3]);
+    expect(selectedTabs, <int>[3]);
+    expect(find.text('Page 1', skipOffstage: false), isOffstage);
+    expect(find.text('Page 4'), findsOneWidget);
+    tabsBuilt.clear();
+
+    // Delete 2 tabs.
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabScaffold(
+          tabBar: CupertinoTabBar(
+            items: List<BottomNavigationBarItem>.generate(2, tabGenerator),
+            onTap: (int newTab) => selectedTabs.add(newTab),
+          ),
+          tabBuilder: (BuildContext context, int index) {
+            tabsBuilt.add(index);
+            // Change the builder too.
+            return Text('Different page ${index + 1}');
+          },
+        ),
+      ),
+    );
+
+    expect(tabsBuilt, <int>[0, 1]);
+    // We didn't tap on any additional tabs to invoke the onTap callback. We
+    // just deleted a tab.
+    expect(selectedTabs, <int>[3]);
+    // Tab 1 was previously built so it's rebuilt again, albeit offstage.
+    expect(find.text('Different page 1', skipOffstage: false), isOffstage);
+    // Since all the tabs after tab 2 are deleted, tab 2 is now the last tab and
+    // the actively shown tab.
+    expect(find.text('Different page 2'), findsOneWidget);
+    // No more tab 4 since it's deleted.
+    expect(find.text('Different page 4', skipOffstage: false), findsNothing);
+    // We also changed the builder so no tabs should be built with the old
+    // builder.
+    expect(find.text('Page 1', skipOffstage: false), findsNothing);
+    expect(find.text('Page 2', skipOffstage: false), findsNothing);
+    expect(find.text('Page 4', skipOffstage: false), findsNothing);
+  });
 }
 
 CupertinoTabBar _buildTabBar({ int selectedTab = 0 }) {