Fix null issue with dynamically updating from zero tabs for TabBar (#69005)

packages/flutter/lib/src/material/tabs.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' show lerpDouble;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter/gestures.dart' show DragStartBehavior;
@@ -331,8 +332,12 @@ class _IndicatorPainter extends CustomPainter {
   final TabBarIndicatorSize? indicatorSize;
   final List<GlobalKey> tabKeys;
 
-  late List<double> _currentTabOffsets;
-  late TextDirection _currentTextDirection;
+  // _currentTabOffsets and _currentTextDirection are set each time TabBar
+  // layout is completed. These values can be null when TabBar contains no
+  // tabs, since there are nothing to lay out.
+  List<double>? _currentTabOffsets;
+  TextDirection? _currentTextDirection;
+
   Rect? _currentRect;
   BoxPainter? _painter;
   bool _needsPaint = false;
@@ -344,38 +349,38 @@ class _IndicatorPainter extends CustomPainter {
     _painter?.dispose();
   }
 
-  void saveTabOffsets(List<double> tabOffsets, TextDirection textDirection) {
+  void saveTabOffsets(List<double>? tabOffsets, TextDirection? textDirection) {
     _currentTabOffsets = tabOffsets;
     _currentTextDirection = textDirection;
   }
 
   // _currentTabOffsets[index] is the offset of the start edge of the tab at index, and
   // _currentTabOffsets[_currentTabOffsets.length] is the end edge of the last tab.
-  int get maxTabIndex => _currentTabOffsets.length - 2;
+  int get maxTabIndex => _currentTabOffsets!.length - 2;
 
   double centerOf(int tabIndex) {
     assert(_currentTabOffsets != null);
-    assert(_currentTabOffsets.isNotEmpty);
+    assert(_currentTabOffsets!.isNotEmpty);
     assert(tabIndex >= 0);
     assert(tabIndex <= maxTabIndex);
-    return (_currentTabOffsets[tabIndex] + _currentTabOffsets[tabIndex + 1]) / 2.0;
+    return (_currentTabOffsets![tabIndex] + _currentTabOffsets![tabIndex + 1]) / 2.0;
   }
 
   Rect indicatorRect(Size tabBarSize, int tabIndex) {
     assert(_currentTabOffsets != null);
     assert(_currentTextDirection != null);
-    assert(_currentTabOffsets.isNotEmpty);
+    assert(_currentTabOffsets!.isNotEmpty);
     assert(tabIndex >= 0);
     assert(tabIndex <= maxTabIndex);
     double tabLeft, tabRight;
-    switch (_currentTextDirection) {
+    switch (_currentTextDirection!) {
       case TextDirection.rtl:
-        tabLeft = _currentTabOffsets[tabIndex + 1];
-        tabRight = _currentTabOffsets[tabIndex];
+        tabLeft = _currentTabOffsets![tabIndex + 1];
+        tabRight = _currentTabOffsets![tabIndex];
         break;
       case TextDirection.ltr:
-        tabLeft = _currentTabOffsets[tabIndex];
-        tabRight = _currentTabOffsets[tabIndex + 1];
+        tabLeft = _currentTabOffsets![tabIndex];
+        tabRight = _currentTabOffsets![tabIndex + 1];
         break;
     }
 
@@ -426,25 +431,13 @@ class _IndicatorPainter extends CustomPainter {
     _painter!.paint(canvas, _currentRect!.topLeft, configuration);
   }
 
-  static bool _tabOffsetsEqual(List<double> a, List<double> b) {
-    // TODO(shihaohong): The following null check should be replaced when a fix
-    // for https://github.com/flutter/flutter/issues/40014 is available.
-    if (a == null || b == null || a.length != b.length)
-      return false;
-    for (int i = 0; i < a.length; i += 1) {
-      if (a[i] != b[i])
-        return false;
-    }
-    return true;
-  }
-
   @override
   bool shouldRepaint(_IndicatorPainter old) {
     return _needsPaint
         || controller != old.controller
         || indicator != old.indicator
         || tabKeys.length != old.tabKeys.length
-        || (!_tabOffsetsEqual(_currentTabOffsets, old._currentTabOffsets))
+        || (!listEquals(_currentTabOffsets, old._currentTabOffsets))
         || _currentTextDirection != old._currentTextDirection;
   }
 }

packages/flutter/test/material/tabs_test.dart

@@ -2571,6 +2571,81 @@ void main() {
     expect(find.text('No tabs'), findsOneWidget);
   });
 
+  testWidgets('TabBar - updating to and from zero tabs', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/68962.
+    final List<String> tabTitles = <String>[];
+    final List<Widget> tabContents = <Widget>[];
+    TabController _tabController = TabController(length: tabContents.length, vsync: const TestVSync());
+
+    void _onTabAdd(StateSetter setState) {
+      setState(() {
+        tabTitles.add('Tab ${tabTitles.length + 1}');
+        tabContents.add(
+          Container(
+            color: Colors.red,
+            height: 200,
+            width: 200,
+          ),
+        );
+        _tabController = TabController(length: tabContents.length, vsync: const TestVSync());
+      });
+    }
+
+    void _onTabRemove(StateSetter setState) {
+      setState(() {
+        tabTitles.removeLast();
+        tabContents.removeLast();
+        _tabController = TabController(length: tabContents.length, vsync: const TestVSync());
+      });
+    }
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return Scaffold(
+              appBar: AppBar(
+                actions: <Widget>[
+                  FlatButton(
+                    key: const Key('Add tab'),
+                    child: const Text('Add tab'),
+                    onPressed: () => _onTabAdd(setState),
+                  ),
+                  FlatButton(
+                    key: const Key('Remove tab'),
+                    child: const Text('Remove tab'),
+                    onPressed: () => _onTabRemove(setState),
+                  ),
+                ],
+                bottom: PreferredSize(
+                  preferredSize: const Size.fromHeight(40.0),
+                  child: Expanded(
+                    child: TabBar(
+                      controller: _tabController,
+                      tabs: tabTitles
+                        .map((String title) => Tab(text: title))
+                        .toList(),
+                    ),
+                  ),
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    expect(find.text('Tab 1'), findsNothing);
+    expect(find.text('Add tab'), findsOneWidget);
+    await tester.tap(find.byKey(const Key('Add tab')));
+    await tester.pumpAndSettle();
+    expect(find.text('Tab 1'), findsOneWidget);
+
+    await tester.tap(find.byKey(const Key('Remove tab')));
+    await tester.pumpAndSettle();
+    expect(find.text('Tab 1'), findsNothing);
+  });
+
    testWidgets('TabBar expands vertically to accommodate the Icon and child Text() pair the same amount it would expand for Icon and text pair.', (WidgetTester tester) async {
     const double indicatorWeight = 2.0;