Fix CupertinoTabScaffold index out of range (#33624)

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

@@ -120,6 +120,10 @@ class CupertinoTabController extends ChangeNotifier {
 /// pages as there are [tabBar.items]. Inactive tabs will be moved [Offstage]
 /// and their animations disabled.
 ///
+/// Adding/removing tabs, or changing the order of tabs is supported but not
+/// recommended. Doing so is against the iOS human interface guidelines, and
+/// [CupertinoTabScaffold] may lose some tabs' state in the process.
+///
 /// Use [CupertinoTabView] as the root widget of each tab to support tabs with
 /// parallel navigation state and history. Since each [CupertinoTabView] contains
 /// a [Navigator], rebuilding the [CupertinoTabView] with a different
@@ -193,6 +197,7 @@ class CupertinoTabController extends ChangeNotifier {
 ///  * [CupertinoPageRoute], a route hosting modal pages with iOS style transitions.
 ///  * [CupertinoPageScaffold], typical contents of an iOS modal page implementing
 ///    layout with a navigation bar on top.
+///  * [iOS human interface guidelines](https://developer.apple.com/design/human-interface-guidelines/ios/bars/tab-bars/).
 class CupertinoTabScaffold extends StatefulWidget {
   /// Creates a layout for applications with a tab bar at the bottom.
   ///
@@ -448,17 +453,13 @@ class _TabSwitchingView extends StatefulWidget {
 }
 
 class _TabSwitchingViewState extends State<_TabSwitchingView> {
-  List<Widget> tabs;
+  List<bool> shouldBuildTab;
   List<FocusScopeNode> tabFocusNodes;
 
   @override
   void initState() {
     super.initState();
-    tabs = List<Widget>(widget.tabNumber);
-    tabFocusNodes = List<FocusScopeNode>.generate(
-      widget.tabNumber,
-      (int index) => FocusScopeNode(debugLabel: 'Tab Focus Scope $index'),
-    );
+    shouldBuildTab = List<bool>.filled(widget.tabNumber, false, growable: true);
   }
 
   @override
@@ -470,10 +471,28 @@ class _TabSwitchingViewState extends State<_TabSwitchingView> {
   @override
   void didUpdateWidget(_TabSwitchingView oldWidget) {
     super.didUpdateWidget(oldWidget);
+
+    // Only partially invalidate the tabs cache to avoid breaking the current
+    // behavior. We assume that the only possible change is either:
+    // - new tabs are appended to the tab list, or
+    // - some trailing tabs are removed.
+    // If the above assumption is not true, some tabs may lose their state.
+    final int lengthDiff = widget.tabNumber - shouldBuildTab.length;
+    if (lengthDiff > 0) {
+      shouldBuildTab.addAll(List<bool>.filled(lengthDiff, false));
+    } else if (lengthDiff < 0) {
+      shouldBuildTab.removeRange(widget.tabNumber, shouldBuildTab.length);
+    }
     _focusActiveTab();
   }
 
   void _focusActiveTab() {
+    if (tabFocusNodes?.length != widget.tabNumber) {
+      tabFocusNodes = List<FocusScopeNode>.generate(
+        widget.tabNumber,
+        (int index) => FocusScopeNode(debugLabel: 'Tab Focus Scope $index'),
+      );
+    }
     FocusScope.of(context).setFirstFocus(tabFocusNodes[widget.currentTabIndex]);
   }
 
@@ -491,10 +510,7 @@ class _TabSwitchingViewState extends State<_TabSwitchingView> {
       fit: StackFit.expand,
       children: List<Widget>.generate(widget.tabNumber, (int index) {
         final bool active = index == widget.currentTabIndex;
-
-        if (active || tabs[index] != null) {
-          tabs[index] = widget.tabBuilder(context, index);
-        }
+        shouldBuildTab[index] = active || shouldBuildTab[index];
 
         return Offstage(
           offstage: !active,
@@ -502,7 +518,9 @@ class _TabSwitchingViewState extends State<_TabSwitchingView> {
             enabled: active,
             child: FocusScope(
               node: tabFocusNodes[index],
-              child: tabs[index] ?? Container(),
+              child: shouldBuildTab[index]
+                ? widget.tabBuilder(context, index)
+                : Container(),
             ),
           ),
         );

packages/flutter/test/cupertino/tab_scaffold_test.dart

@@ -66,7 +66,7 @@ void main() {
       ),
     );
 
-    expect(tabsPainted, <int>[0]);
+    expect(tabsPainted, const <int>[0]);
     RichText tab1 = tester.widget(find.descendant(
       of: find.text('Tab 1'),
       matching: find.byType(RichText),
@@ -81,7 +81,7 @@ void main() {
     await tester.tap(find.text('Tab 2'));
     await tester.pump();
 
-    expect(tabsPainted, <int>[0, 1]);
+    expect(tabsPainted, const <int>[0, 1]);
     tab1 = tester.widget(find.descendant(
       of: find.text('Tab 1'),
       matching: find.byType(RichText),
@@ -96,9 +96,9 @@ void main() {
     await tester.tap(find.text('Tab 1'));
     await tester.pump();
 
-    expect(tabsPainted, <int>[0, 1, 0]);
+    expect(tabsPainted, const <int>[0, 1, 0]);
     // CupertinoTabBar's onTap callbacks are passed on.
-    expect(selectedTabs, <int>[1, 0]);
+    expect(selectedTabs, const <int>[1, 0]);
   });
 
   testWidgets('Tabs are lazy built and moved offstage when inactive', (WidgetTester tester) async {
@@ -116,7 +116,7 @@ void main() {
       ),
     );
 
-    expect(tabsBuilt, <int>[0]);
+    expect(tabsBuilt, const <int>[0]);
     expect(find.text('Page 1'), findsOneWidget);
     expect(find.text('Page 2'), findsNothing);
 
@@ -124,14 +124,14 @@ void main() {
     await tester.pump();
 
     // Both tabs are built but only one is onstage.
-    expect(tabsBuilt, <int>[0, 0, 1]);
+    expect(tabsBuilt, const <int>[0, 0, 1]);
     expect(find.text('Page 1', skipOffstage: false), isOffstage);
     expect(find.text('Page 2'), findsOneWidget);
 
     await tester.tap(find.text('Tab 1'));
     await tester.pump();
 
-    expect(tabsBuilt, <int>[0, 0, 1, 0, 1]);
+    expect(tabsBuilt, const <int>[0, 0, 1, 0, 1]);
     expect(find.text('Page 1'), findsOneWidget);
     expect(find.text('Page 2', skipOffstage: false), isOffstage);
   });
@@ -256,12 +256,12 @@ void main() {
       ),
     );
 
-    expect(tabsPainted, <int>[1]);
+    expect(tabsPainted, const <int>[1]);
 
     controller.index = 0;
     await tester.pump();
 
-    expect(tabsPainted, <int>[1, 0]);
+    expect(tabsPainted, const <int>[1, 0]);
     // onTap is not called when changing tabs programmatically.
     expect(selectedTabs, isEmpty);
 
@@ -269,8 +269,8 @@ void main() {
     await tester.tap(find.text('Tab 2'));
     await tester.pump();
 
-    expect(tabsPainted, <int>[1, 0, 1]);
-    expect(selectedTabs, <int>[1]);
+    expect(tabsPainted, const <int>[1, 0, 1]);
+    expect(selectedTabs, const <int>[1]);
   });
 
   testWidgets('Programmatic tab switching by passing in a new controller', (WidgetTester tester) async {
@@ -292,7 +292,7 @@ void main() {
       ),
     );
 
-    expect(tabsPainted, <int>[0]);
+    expect(tabsPainted, const <int>[0]);
 
     await tester.pumpWidget(
       CupertinoApp(
@@ -311,7 +311,7 @@ void main() {
       ),
     );
 
-    expect(tabsPainted, <int>[0, 1]);
+    expect(tabsPainted, const <int>[0, 1]);
     // onTap is not called when changing tabs programmatically.
     expect(selectedTabs, isEmpty);
 
@@ -319,8 +319,8 @@ void main() {
     await tester.tap(find.text('Tab 1'));
     await tester.pump();
 
-    expect(tabsPainted, <int>[0, 1, 0]);
-    expect(selectedTabs, <int>[0]);
+    expect(tabsPainted, const <int>[0, 1, 0]);
+    expect(selectedTabs, const <int>[0]);
   });
 
   testWidgets('Tab bar respects themes', (WidgetTester tester) async {
@@ -482,18 +482,18 @@ void main() {
       ),
     );
 
-    expect(tabsBuilt, <int>[0]);
+    expect(tabsBuilt, const <int>[0]);
     // selectedTabs list is appended to on onTap callbacks. We didn't tap
     // any tabs yet.
-    expect(selectedTabs, <int>[]);
+    expect(selectedTabs, const <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(tabsBuilt, const <int>[0, 3]);
+    expect(selectedTabs, const <int>[3]);
     expect(find.text('Page 1', skipOffstage: false), isOffstage);
     expect(find.text('Page 4'), findsOneWidget);
     tabsBuilt.clear();
@@ -515,10 +515,10 @@ void main() {
       )
     );
 
-    expect(tabsBuilt, <int>[0, 1]);
+    expect(tabsBuilt, const <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]);
+    expect(selectedTabs, const <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
@@ -533,6 +533,61 @@ void main() {
     expect(find.text('Page 4', skipOffstage: false), findsNothing);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/33455
+  testWidgets('Adding new tabs does not crash the app', (WidgetTester tester) async {
+    final List<int> tabsPainted = <int>[];
+    final CupertinoTabController controller = CupertinoTabController(initialIndex: 0);
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabScaffold(
+          tabBar: CupertinoTabBar(
+            items: List<BottomNavigationBarItem>.generate(10, tabGenerator),
+          ),
+          controller: controller,
+          tabBuilder: (BuildContext context, int index) {
+            return CustomPaint(
+              child: Text('Page ${index + 1}'),
+              painter: TestCallbackPainter(
+                onPaint: () { tabsPainted.add(index); }
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    expect(tabsPainted, const <int> [0]);
+
+    // Increase the num of tabs to 20.
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoTabScaffold(
+          tabBar: CupertinoTabBar(
+            items: List<BottomNavigationBarItem>.generate(20, tabGenerator),
+          ),
+          controller: controller,
+          tabBuilder: (BuildContext context, int index) {
+            return CustomPaint(
+              child: Text('Page ${index + 1}'),
+              painter: TestCallbackPainter(
+                onPaint: () { tabsPainted.add(index); }
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    expect(tabsPainted, const <int> [0, 0]);
+
+    await tester.tap(find.text('Tab 19'));
+    await tester.pump();
+
+    // Tapping the tabs should still work.
+    expect(tabsPainted, const <int>[0, 0, 18]);
+  });
+
   testWidgets('If a controller is initially provided then the parent stops doing so for rebuilds, '
               'a new instance of CupertinoTabController should be created and used by the widget, '
               "while preserving the previous controller's tab index",
@@ -554,12 +609,12 @@ void main() {
                   onPaint: () { tabsPainted.add(index); }
                 ),
               );
-            }
+            },
           ),
         )
       );
 
-      expect(tabsPainted, <int> [0]);
+      expect(tabsPainted, const <int> [0]);
 
       await tester.pumpWidget(
         CupertinoApp(
@@ -576,24 +631,24 @@ void main() {
                   onPaint: () { tabsPainted.add(index); }
                 ),
               );
-            }
+            },
           ),
         )
       );
 
-      expect(tabsPainted, <int> [0, 0]);
+      expect(tabsPainted, const <int> [0, 0]);
 
       await tester.tap(find.text('Tab 2'));
       await tester.pump();
 
       // Tapping the tabs should still work.
-      expect(tabsPainted, <int>[0, 0, 1]);
+      expect(tabsPainted, const <int>[0, 0, 1]);
 
       oldController.index = 10;
       await tester.pump();
 
       // Changing [index] of the oldController should not work.
-      expect(tabsPainted, <int> [0, 0, 1]);
+      expect(tabsPainted, const <int> [0, 0, 1]);
   });
 
   testWidgets('Do not call dispose on a controller that we do not own'
@@ -610,7 +665,7 @@ void main() {
             controller: mockController,
             tabBuilder: (BuildContext context, int index) => const Placeholder(),
           ),
-        )
+        ),
       );
 
       expect(mockController.numOfListeners, 1);
@@ -625,7 +680,7 @@ void main() {
             controller: null,
             tabBuilder: (BuildContext context, int index) => const Placeholder(),
           ),
-        )
+        ),
       );
 
       expect(mockController.numOfListeners, 0);
@@ -644,7 +699,7 @@ void main() {
           controller: controller,
           tabBuilder: (BuildContext context, int index) => const Placeholder()
         ),
-      )
+      ),
     );
     expect(find.text('Tab 1'), findsOneWidget);
     expect(find.text('Tab 2'), findsOneWidget);
@@ -661,7 +716,7 @@ void main() {
           controller: controller,
           tabBuilder: (BuildContext context, int index) => const Placeholder()
         ),
-      )
+      ),
     );
 
     // Should not crash here.
@@ -691,9 +746,9 @@ void main() {
                     return CustomPaint(
                       painter: TestCallbackPainter(
                         onPaint: () => tabsPainted0.add(index)
-                      )
+                      ),
                     );
-                  }
+                  },
                 ),
                 CupertinoTabScaffold(
                   tabBar: CupertinoTabBar(
@@ -704,14 +759,14 @@ void main() {
                     return CustomPaint(
                       painter: TestCallbackPainter(
                         onPaint: () => tabsPainted1.add(index)
-                      )
+                      ),
                     );
-                  }
+                  },
                 ),
-              ]
-            )
-          )
-        )
+              ],
+            ),
+          ),
+        ),
       );
       expect(tabsPainted0, const <int>[2]);
       expect(tabsPainted1, const <int>[2]);
@@ -738,14 +793,14 @@ void main() {
                     return CustomPaint(
                       painter: TestCallbackPainter(
                         onPaint: () => tabsPainted0.add(index)
-                      )
+                      ),
                     );
-                  }
+                  },
                 ),
-              ]
-            )
-          )
-        )
+              ],
+            ),
+          ),
+        ),
       );
 
       expect(tabsPainted0, const <int>[2, 0, 1]);