Allow TabBars, TabBarViews, TabControllers, with zero or one tabs (#10608)

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

@@ -16,6 +16,8 @@ import 'constants.dart';
 /// A stateful widget that builds a [TabBar] or a [TabBarView] can create
 /// a TabController and share it directly.
 ///
+/// ## Sample code
+///
 /// ```dart
 /// class _MyDemoState extends State<MyDemo> with SingleTickerProviderStateMixin {
 ///   final List<Tab> myTabs = <Tab>[
@@ -62,12 +64,18 @@ import 'constants.dart';
 /// inherited widget.
 class TabController extends ChangeNotifier {
   /// Creates an object that manages the state required by [TabBar] and a [TabBarView].
+  ///
+  /// The [length] cannot be null or negative. Typically its a value greater than one, i.e.
+  /// typically there are two or more tabs.
+  ///
+  /// The `initialIndex` must be valid given [length] and cannot be null. If [length] is
+  /// zero, then `initialIndex` must be 0 (the default).
   TabController({ int initialIndex: 0, @required this.length, @required TickerProvider vsync })
-    : assert(length != null && length > 1),
-      assert(initialIndex != null && initialIndex >= 0 && initialIndex < length),
+    : assert(length != null && length >= 0),
+      assert(initialIndex != null && initialIndex >= 0 && (length == 0 || initialIndex < length)),
       _index = initialIndex,
       _previousIndex = initialIndex,
-      _animationController = new AnimationController(
+      _animationController = length < 2 ? null : new AnimationController(
         value: initialIndex.toDouble(),
         upperBound: (length - 1).toDouble(),
         vsync: vsync
@@ -81,18 +89,21 @@ class TabController extends ChangeNotifier {
   /// selected tab is changed, the animation's value equals [index]. The
   /// animation's value can be [offset] by +/- 1.0 to reflect [TabBarView]
   /// drag scrolling.
-  Animation<double> get animation => _animationController.view;
+  ///
+  /// If length is zero or one, [index] animations don't happen and the value
+  /// of this property is [kAlwaysCompleteAnimation].
+  Animation<double> get animation => _animationController?.view ?? kAlwaysCompleteAnimation;
   final AnimationController _animationController;
 
-  /// The total number of tabs. Must be greater than one.
+  /// The total number of tabs. Typically greater than one.
   final int length;
 
   void _changeIndex(int value, { Duration duration, Curve curve }) {
     assert(value != null);
-    assert(value >= 0 && value < length);
+    assert(value >= 0 && (value < length || length == 0));
     assert(duration == null ? curve == null : true);
     assert(_indexIsChangingCount >= 0);
-    if (value == _index)
+    if (value == _index || length < 2)
       return;
     _previousIndex = index;
     _index = value;
@@ -118,6 +129,9 @@ class TabController extends ChangeNotifier {
   /// [indexIsChanging] to false, and notifies listeners.
   ///
   /// To change the currently selected tab and play the [animation] use [animateTo].
+  ///
+  /// The value of [index] must be valid given [length]. If [length] is zero,
+  /// then [index] will also be zero.
   int get index => _index;
   int _index;
   set index(int value) {
@@ -148,8 +162,9 @@ class TabController extends ChangeNotifier {
   /// drags left or right. A value between -1.0 and 0.0 implies that the
   /// TabBarView has been dragged to the left. Similarly a value between
   /// 0.0 and 1.0 implies that the TabBarView has been dragged to the right.
-  double get offset => _animationController.value - _index.toDouble();
+  double get offset => length > 1 ? _animationController.value - _index.toDouble() : 0.0;
   set offset(double value) {
+    assert(length > 1);
     assert(value != null);
     assert(value >= -1.0 && value <= 1.0);
     assert(!indexIsChanging);
@@ -160,7 +175,7 @@ class TabController extends ChangeNotifier {
 
   @override
   void dispose() {
-    _animationController.dispose();
+    _animationController?.dispose();
     super.dispose();
   }
 }
@@ -220,7 +235,7 @@ class _TabControllerScope extends InheritedWidget {
 class DefaultTabController extends StatefulWidget {
   /// Creates a default tab controller for the given [child] widget.
   ///
-  /// The [length] argument must be great than one.
+  /// The [length] argument is typically greater than one.
   ///
   /// The [initialIndex] argument must not be null.
   const DefaultTabController({
@@ -231,7 +246,7 @@ class DefaultTabController extends StatefulWidget {
   }) : assert(initialIndex != null),
        super(key: key);
 
-  /// The total number of tabs. Must be greater than one.
+  /// The total number of tabs. Typically greater than one.
   final int length;
 
   /// The initial index of the selected tab.

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

@@ -389,22 +389,23 @@ class _TabBarScrollController extends ScrollController {
 
 /// A material design widget that displays a horizontal row of tabs.
 ///
-/// Typically created as part of an [AppBar] and in conjuction with a
-/// [TabBarView].
+/// Typically created as the [AppBar.bottom] part of an [AppBar] and in
+/// conjuction with a [TabBarView].
 ///
 /// If a [TabController] is not provided, then there must be a
-/// [DefaultTabController] ancestor.
+/// [DefaultTabController] ancestor. The tab controller's [TabController.length]
+/// must equal the length of the [tabs] list.
 ///
 /// Requires one of its ancestors to be a [Material] widget.
 ///
 /// See also:
 ///
-///  * [TabBarView], which displays the contents that the tab bar is selecting
-///    between.
+///  * [TabBarView], which displays page views that correspond to each tab.
 class TabBar extends StatefulWidget implements PreferredSizeWidget {
   /// Creates a material design tab bar.
   ///
-  /// The [tabs] argument must not be null and must have more than one widget.
+  /// The [tabs] argument cannot be null and its length must match the [controller]'s
+  /// [TabController.length].
   ///
   /// If a [TabController] is not provided, then there must be a
   /// [DefaultTabController] ancestor.
@@ -424,13 +425,15 @@ class TabBar extends StatefulWidget implements PreferredSizeWidget {
     this.labelStyle,
     this.unselectedLabelColor,
     this.unselectedLabelStyle,
-  }) : assert(tabs != null && tabs.length > 1),
+  }) : assert(tabs != null),
        assert(isScrollable != null),
        assert(indicatorWeight != null && indicatorWeight > 0.0),
        assert(indicatorPadding != null),
        super(key: key);
 
-  /// Typically a list of [Tab] widgets.
+  /// Typically a list of two or more [Tab] widgets.
+  ///
+  /// The length of this list must match the [controller]'s [TabController.length].
   final List<Widget> tabs;
 
   /// This widget's selection and animation state.
@@ -667,6 +670,12 @@ class _TabBarState extends State<TabBar> {
 
   @override
   Widget build(BuildContext context) {
+    if (_controller.length == 0) {
+      return new Container(
+        height: _kTabHeight + widget.indicatorWeight,
+      );
+    }
+
     final List<Widget> wrappedTabs = new List<Widget>.from(widget.tabs, growable: false);
 
     // If the controller was provided by DefaultTabController and we're part
@@ -774,8 +783,7 @@ class TabBarView extends StatefulWidget {
     Key key,
     @required this.children,
     this.controller,
-  }) : assert(children != null && children.length > 1),
-       super(key: key);
+  }) : assert(children != null), super(key: key);
 
   /// This widget's selection and animation state.
   ///

packages/flutter/test/material/tabs_test.dart

@@ -900,4 +900,99 @@ void main() {
       rect: new Rect.fromLTRB(tabLeft + padLeft, height, tabRight - padRight, height + weight)
     ));
   });
+
+  testWidgets('TabBar etc with zero tabs', (WidgetTester tester) async {
+    final TabController controller = new TabController(
+      vsync: const TestVSync(),
+      length: 0,
+    );
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Column(
+          children: <Widget>[
+            new TabBar(
+              controller: controller,
+              tabs: const <Widget>[],
+            ),
+            new Flexible(
+              child: new TabBarView(
+                controller: controller,
+                children: const <Widget>[],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(controller.index, 0);
+    expect(tester.getSize(find.byType(TabBar)), const Size(800.0, 48.0));
+    expect(tester.getSize(find.byType(TabBarView)), const Size(800.0, 600.0 - 48.0));
+
+    // A fling in the TabBar or TabBarView, shouldn't do anything.
+
+    await(tester.fling(find.byType(TabBar), const Offset(-100.0, 0.0), 5000.0));
+    await(tester.pumpAndSettle());
+
+    await(tester.fling(find.byType(TabBarView), const Offset(100.0, 0.0), 5000.0));
+    await(tester.pumpAndSettle());
+
+    expect(controller.index, 0);
+  });
+
+  testWidgets('TabBar etc with one tab', (WidgetTester tester) async {
+    final TabController controller = new TabController(
+      vsync: const TestVSync(),
+      length: 1,
+    );
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Column(
+          children: <Widget>[
+            new TabBar(
+              controller: controller,
+              tabs: const <Widget>[const Tab(text: 'TAB')],
+            ),
+            new Flexible(
+              child: new TabBarView(
+                controller: controller,
+                children: const <Widget>[const Text('PAGE')],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(controller.index, 0);
+    expect(find.text('TAB'), findsOneWidget);
+    expect(find.text('PAGE'), findsOneWidget);
+    expect(tester.getSize(find.byType(TabBar)), const Size(800.0, 48.0));
+    expect(tester.getSize(find.byType(TabBarView)), const Size(800.0, 600.0 - 48.0));
+
+    // The one tab spans the app's width
+    expect(tester.getTopLeft(find.widgetWithText(Tab, 'TAB')).dx, 0);
+    expect(tester.getTopRight(find.widgetWithText(Tab, 'TAB')).dx, 800);
+
+    // A fling in the TabBar or TabBarView, shouldn't move the tab.
+
+    await(tester.fling(find.byType(TabBar), const Offset(-100.0, 0.0), 5000.0));
+    await(tester.pump(const Duration(milliseconds: 50)));
+    expect(tester.getTopLeft(find.widgetWithText(Tab, 'TAB')).dx, 0);
+    expect(tester.getTopRight(find.widgetWithText(Tab, 'TAB')).dx, 800);
+    await(tester.pumpAndSettle());
+
+    await(tester.fling(find.byType(TabBarView), const Offset(100.0, 0.0), 5000.0));
+    await(tester.pump(const Duration(milliseconds: 50)));
+    expect(tester.getTopLeft(find.widgetWithText(Tab, 'TAB')).dx, 0);
+    expect(tester.getTopRight(find.widgetWithText(Tab, 'TAB')).dx, 800);
+    await(tester.pumpAndSettle());
+
+    expect(controller.index, 0);
+    expect(find.text('TAB'), findsOneWidget);
+    expect(find.text('PAGE'), findsOneWidget);
+  });
+
 }