Reverts "Remove hack from PageView." (#141479)

Reverts flutter/flutter#141138
Initiated by: itsjustkevin
This change reverts the following previous change:
Original Description:
Fixes https://github.com/flutter/flutter/issues/141119

The change is breaking, because now controller is nullable.

Migration path: https://github.com/flutter/website/pull/10033

Packages to fix:

examples/api/test/widgets/page_view/page_view.0_test.dart

@@ -49,7 +49,7 @@ void main() {
 
     // Verify that page view index is also updated with same index to page indicator.
     final PageView pageView = tester.widget<PageView>(find.byType(PageView));
-    expect(pageView.controller!.page, 1);
+    expect(pageView.controller.page, 1);
 
     // Tap backward button on page indicator area.
     await tester.tap(find.byIcon(Icons.arrow_left_rounded));

packages/flutter/lib/src/widgets/page_view.dart

@@ -576,6 +576,11 @@ class PageScrollPhysics extends ScrollPhysics {
   bool get allowImplicitScrolling => false;
 }
 
+// Having this global (mutable) page controller is a bit of a hack. We need it
+// to plumb in the factory for _PagePosition, but it will end up accumulating
+// a large list of scroll positions. As long as you don't try to actually
+// control the scroll positions, everything should be fine.
+final PageController _defaultPageController = PageController();
 const PageScrollPhysics _kPagePhysics = PageScrollPhysics();
 
 /// A scrollable list that works page by page.
@@ -640,7 +645,7 @@ class PageView extends StatefulWidget {
     super.key,
     this.scrollDirection = Axis.horizontal,
     this.reverse = false,
-    this.controller,
+    PageController? controller,
     this.physics,
     this.pageSnapping = true,
     this.onPageChanged,
@@ -651,7 +656,8 @@ class PageView extends StatefulWidget {
     this.clipBehavior = Clip.hardEdge,
     this.scrollBehavior,
     this.padEnds = true,
-  }) : childrenDelegate = SliverChildListDelegate(children);
+  }) : controller = controller ?? _defaultPageController,
+       childrenDelegate = SliverChildListDelegate(children);
 
   /// Creates a scrollable list that works page by page using widgets that are
   /// created on demand.
@@ -682,7 +688,7 @@ class PageView extends StatefulWidget {
     super.key,
     this.scrollDirection = Axis.horizontal,
     this.reverse = false,
-    this.controller,
+    PageController? controller,
     this.physics,
     this.pageSnapping = true,
     this.onPageChanged,
@@ -695,7 +701,8 @@ class PageView extends StatefulWidget {
     this.clipBehavior = Clip.hardEdge,
     this.scrollBehavior,
     this.padEnds = true,
-  }) : childrenDelegate = SliverChildBuilderDelegate(
+  }) : controller = controller ?? _defaultPageController,
+       childrenDelegate = SliverChildBuilderDelegate(
          itemBuilder,
          findChildIndexCallback: findChildIndexCallback,
          childCount: itemCount,
@@ -712,11 +719,11 @@ class PageView extends StatefulWidget {
   /// {@end-tool}
   ///
   /// {@macro flutter.widgets.PageView.allowImplicitScrolling}
-  const PageView.custom({
+  PageView.custom({
     super.key,
     this.scrollDirection = Axis.horizontal,
     this.reverse = false,
-    this.controller,
+    PageController? controller,
     this.physics,
     this.pageSnapping = true,
     this.onPageChanged,
@@ -727,7 +734,7 @@ class PageView extends StatefulWidget {
     this.clipBehavior = Clip.hardEdge,
     this.scrollBehavior,
     this.padEnds = true,
-  });
+  }) : controller = controller ?? _defaultPageController;
 
   /// Controls whether the widget's pages will respond to
   /// [RenderObject.showOnScreen], which will allow for implicit accessibility
@@ -769,7 +776,7 @@ class PageView extends StatefulWidget {
 
   /// An object that can be used to control the position to which this page
   /// view is scrolled.
-  final PageController? controller;
+  final PageController controller;
 
   /// How the page view should respond to user input.
   ///
@@ -841,37 +848,10 @@ class PageView extends StatefulWidget {
 class _PageViewState extends State<PageView> {
   int _lastReportedPage = 0;
 
-  late PageController _controller;
-
   @override
   void initState() {
     super.initState();
-    _initController();
-    _lastReportedPage = _controller.initialPage;
-  }
-
-  @override
-  void dispose() {
-    if (widget.controller == null) {
-      _controller.dispose();
-    }
-    super.dispose();
-  }
-
-
-  void _initController() {
-    _controller = widget.controller ?? PageController();
-  }
-
-  @override
-  void didUpdateWidget(PageView oldWidget) {
-    if (oldWidget.controller != widget.controller) {
-      if (oldWidget.controller == null) {
-        _controller.dispose();
-      }
-      _initController();
-    }
-    super.didUpdateWidget(oldWidget);
+    _lastReportedPage = widget.controller.initialPage;
   }
 
   AxisDirection _getDirection(BuildContext context) {
@@ -912,7 +892,7 @@ class _PageViewState extends State<PageView> {
       child: Scrollable(
         dragStartBehavior: widget.dragStartBehavior,
         axisDirection: axisDirection,
-        controller: _controller,
+        controller: widget.controller,
         physics: physics,
         restorationId: widget.restorationId,
         scrollBehavior: widget.scrollBehavior ?? ScrollConfiguration.of(context).copyWith(scrollbars: false),
@@ -928,7 +908,7 @@ class _PageViewState extends State<PageView> {
             clipBehavior: widget.clipBehavior,
             slivers: <Widget>[
               SliverFillViewport(
-                viewportFraction: _controller.viewportFraction,
+                viewportFraction: widget.controller.viewportFraction,
                 delegate: widget.childrenDelegate,
                 padEnds: widget.padEnds,
               ),
@@ -944,7 +924,7 @@ class _PageViewState extends State<PageView> {
     super.debugFillProperties(description);
     description.add(EnumProperty<Axis>('scrollDirection', widget.scrollDirection));
     description.add(FlagProperty('reverse', value: widget.reverse, ifTrue: 'reversed'));
-    description.add(DiagnosticsProperty<PageController>('controller', _controller, showName: false));
+    description.add(DiagnosticsProperty<PageController>('controller', widget.controller, showName: false));
     description.add(DiagnosticsProperty<ScrollPhysics>('physics', widget.physics, showName: false));
     description.add(FlagProperty('pageSnapping', value: widget.pageSnapping, ifFalse: 'snapping disabled'));
     description.add(FlagProperty('allowImplicitScrolling', value: widget.allowImplicitScrolling, ifTrue: 'allow implicit scrolling'));

packages/flutter/test/material/tabs_test.dart

@@ -1482,7 +1482,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -1531,7 +1531,7 @@ void main() {
     expect(tabController.index, 0);
 
     final PageView pageView = tester.widget<PageView>(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     expect(position.pixels, 0.0);
@@ -1592,7 +1592,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
 
     // The TabView was initialized with viewportFraction as 0.8
     // So it's expected the PageView inside would obtain the same viewportFraction
@@ -1635,7 +1635,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
 
     // The TabView was initialized with default viewportFraction
     // So it's expected the PageView inside would obtain the value 1
@@ -1680,13 +1680,13 @@ void main() {
 
     await tester.pumpWidget(buildFrame(0.8));
     PageView pageView = tester.widget(find.byType(PageView));
-    PageController pageController = pageView.controller!;
+    PageController pageController = pageView.controller;
     expect(pageController.viewportFraction, 0.8);
 
     // Rebuild with a different viewport fraction.
     await tester.pumpWidget(buildFrame(0.5));
     pageView = tester.widget(find.byType(PageView));
-    pageController = pageView.controller!;
+    pageController = pageView.controller;
     expect(pageController.viewportFraction, 0.5);
   });
 
@@ -1861,7 +1861,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -1905,7 +1905,7 @@ void main() {
     expect(tabController.index, 0);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -1950,7 +1950,7 @@ void main() {
     expect(tabController.index, 0);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -1999,7 +1999,7 @@ void main() {
     expect(tabController.index, 0);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -2050,7 +2050,7 @@ void main() {
     expect(tabController.index, 0);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -2246,7 +2246,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -2432,7 +2432,7 @@ void main() {
     expect(tabController.index, 1);
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
@@ -4208,7 +4208,7 @@ void main() {
 
     await tester.pumpWidget(buildFrame(15));
     PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController1 = pageView.controller!;
+    final PageController pageController1 = pageView.controller;
     TabController tabController = DefaultTabController.of(tester.element(find.text('Page 14')));
     expect(tabController.index, 14);
     expect(pageController1.page, 14);
@@ -4216,7 +4216,7 @@ void main() {
     // Rebuild with a new default tab controller with more tabs.
     await tester.pumpWidget(buildFrame(10));
     pageView = tester.widget(find.byType(PageView));
-    final PageController pageController2 = pageView.controller!;
+    final PageController pageController2 = pageView.controller;
     tabController = DefaultTabController.of(tester.element(find.text('Page 9')));
     expect(tabController.index, 9);
     expect(pageController2.page, 9);
@@ -5065,7 +5065,7 @@ void main() {
     double expectedIndicatorLeft = canvas.indicatorRect.left;
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     void pageControllerListener() {
       // Whenever TabBarView scrolls due to changing TabController's index,
       // check if indicator stays idle in its expectedIndicatorLeft
@@ -5225,7 +5225,7 @@ void main() {
     ));
 
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     final ScrollPosition position = pageController.position;
 
     expect(tabController.index, 0);
@@ -5715,7 +5715,7 @@ void main() {
 
     await tester.pumpWidget(buildFrame(controller1, showLast: true));
     final PageView pageView = tester.widget(find.byType(PageView));
-    final PageController pageController = pageView.controller!;
+    final PageController pageController = pageView.controller;
     await tester.tap(find.text('three'));
     await tester.pumpAndSettle();
     expect(controller1.index, 2);
@@ -5782,7 +5782,7 @@ void main() {
 
     await tester.pumpWidget(buildFrame(controller1, showLast: true));
     PageView pageView = tester.widget(find.byType(PageView));
-    PageController pageController = pageView.controller!;
+    PageController pageController = pageView.controller;
     await tester.tap(find.text('three'));
     await tester.pumpAndSettle();
     expect(controller1.index, 2);
@@ -5792,7 +5792,7 @@ void main() {
     await tester.pumpWidget(buildFrame(controller2, showLast: false));
     await tester.pumpAndSettle();
     pageView = tester.widget(find.byType(PageView));
-    pageController = pageView.controller!;
+    pageController = pageView.controller;
     expect(controller2.index, 0);
     expect(pageController.page, 0);
 
@@ -5800,7 +5800,7 @@ void main() {
     await tester.pumpWidget(buildFrame(controller1, showLast: true));
     await tester.pumpAndSettle();
     pageView = tester.widget(find.byType(PageView));
-    pageController = pageView.controller!;
+    pageController = pageView.controller;
     expect(controller1.index, 2);
     expect(pageController.page, 2);
   });

packages/flutter/test/widgets/page_view_test.dart

@@ -1303,77 +1303,4 @@ void main() {
     expect(attach, 1);
     expect(detach, 1);
   });
-
-  group('$PageView handles change of controller', () {
-    final GlobalKey key = GlobalKey();
-
-    Widget createPageView(PageController? controller) {
-      return MaterialApp(
-        home: Scaffold(
-          body: PageView(
-            key: key,
-            controller: controller,
-            children: const <Widget>[
-              Center(child: Text('0')),
-              Center(child: Text('1')),
-              Center(child: Text('2')),
-            ],
-          ),
-        ),
-      );
-    }
-
-    Future<void> testPageViewWithController(PageController controller, WidgetTester tester, bool controls) async  {
-      int curentVisiblePage() {
-        return int.parse(tester.widgetList(find.byType(Text)).whereType<Text>().first.data!);
-      }
-
-      final int initialPageInView = curentVisiblePage();
-
-      for (int i = 0; i < 3; i++) {
-        if (controls) {
-          controller.jumpToPage(i);
-          await tester.pumpAndSettle();
-          expect(curentVisiblePage(), i);
-        } else {
-          expect(()=> controller.jumpToPage(i), throwsAssertionError);
-          expect(curentVisiblePage(), initialPageInView);
-        }
-      }
-    }
-
-    testWidgets('null to value', (WidgetTester tester) async {
-      final PageController controller = PageController();
-      addTearDown(controller.dispose);
-      await tester.pumpWidget(createPageView(null));
-      await tester.pumpWidget(createPageView(controller));
-      await testPageViewWithController(controller, tester, true);
-    });
-
-    testWidgets('value to value', (WidgetTester tester) async {
-      final PageController controller1 = PageController();
-      addTearDown(controller1.dispose);
-      final PageController controller2 = PageController();
-      addTearDown(controller2.dispose);
-      await tester.pumpWidget(createPageView(controller1));
-      await testPageViewWithController(controller1, tester, true);
-      await tester.pumpWidget(createPageView(controller2));
-      await testPageViewWithController(controller1, tester, false);
-      await testPageViewWithController(controller2, tester, true);
-    });
-
-    testWidgets('value to null', (WidgetTester tester) async {
-      final PageController controller = PageController();
-      addTearDown(controller.dispose);
-      await tester.pumpWidget(createPageView(controller));
-      await testPageViewWithController(controller, tester, true);
-      await tester.pumpWidget(createPageView(null));
-      await testPageViewWithController(controller, tester, false);
-    });
-
-    testWidgets('null to null', (WidgetTester tester) async {
-      await tester.pumpWidget(createPageView(null));
-      await tester.pumpWidget(createPageView(null));
-    });
-  });
 }