Assert for valid ScrollController in Scrollbar gestures (#81278)

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

@@ -278,6 +278,9 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///    section when there are many actions.
   final ScrollController? scrollController;
 
+  ScrollController get _effectiveScrollController =>
+    scrollController ?? ScrollController();
+
   /// A scroll controller that can be used to control the scrolling of the
   /// actions in the dialog.
   ///
@@ -289,6 +292,9 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///    section when it is long.
   final ScrollController? actionScrollController;
 
+  ScrollController get _effectiveActionScrollController =>
+    actionScrollController ?? ScrollController();
+
   /// {@macro flutter.material.dialog.insetAnimationDuration}
   final Duration insetAnimationDuration;
 
@@ -305,7 +311,7 @@ class CupertinoAlertDialog extends StatelessWidget {
           child: _CupertinoAlertContentSection(
             title: title,
             message: content,
-            scrollController: scrollController,
+            scrollController: _effectiveScrollController,
             titlePadding: EdgeInsets.only(
               left: _kDialogEdgePadding,
               right: _kDialogEdgePadding,
@@ -344,7 +350,7 @@ class CupertinoAlertDialog extends StatelessWidget {
     );
     if (actions.isNotEmpty) {
       actionSection = _CupertinoAlertActionSection(
-        scrollController: actionScrollController,
+        scrollController: _effectiveActionScrollController,
         isActionSheet: false,
         children: actions,
       );
@@ -591,12 +597,18 @@ class CupertinoActionSheet extends StatelessWidget {
   /// short.
   final ScrollController? messageScrollController;
 
+  ScrollController get _effectiveMessageScrollController =>
+    messageScrollController ?? ScrollController();
+
   /// A scroll controller that can be used to control the scrolling of the
   /// [actions] in the action sheet.
   ///
   /// This attribute is typically not needed.
   final ScrollController? actionScrollController;
 
+  ScrollController get _effectiveActionScrollController =>
+    actionScrollController ?? ScrollController();
+
   /// The optional cancel button that is grouped separately from the other
   /// actions.
   ///
@@ -609,7 +621,7 @@ class CupertinoActionSheet extends StatelessWidget {
       final Widget titleSection = _CupertinoAlertContentSection(
         title: title,
         message: message,
-        scrollController: messageScrollController,
+        scrollController: _effectiveMessageScrollController,
         titlePadding: const EdgeInsets.only(
           left: _kActionSheetContentHorizontalPadding,
           right: _kActionSheetContentHorizontalPadding,
@@ -650,7 +662,7 @@ class CupertinoActionSheet extends StatelessWidget {
       );
     }
     return _CupertinoAlertActionSection(
-      scrollController: actionScrollController,
+      scrollController: _effectiveActionScrollController,
       hasCancelButton: cancelButton != null,
       isActionSheet: true,
       children: actions!,
@@ -1501,6 +1513,7 @@ class _CupertinoAlertContentSection extends StatelessWidget {
     }
 
     return CupertinoScrollbar(
+      controller: scrollController,
       child: SingleChildScrollView(
         controller: scrollController,
         child: Column(
@@ -1565,6 +1578,7 @@ class _CupertinoAlertActionSectionState
     }
 
     return CupertinoScrollbar(
+      controller: widget.scrollController,
       child: SingleChildScrollView(
         controller: widget.scrollController,
         child: _CupertinoDialogActionsRenderWidget(

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

@@ -1026,19 +1026,43 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   // A scroll event is required in order to paint the thumb.
   void _maybeTriggerScrollbar() {
     WidgetsBinding.instance!.addPostFrameCallback((Duration duration) {
+      final ScrollController? scrollController = widget.controller ?? PrimaryScrollController.of(context);
       if (showScrollbar) {
         _fadeoutTimer?.cancel();
         // Wait one frame and cause an empty scroll event.  This allows the
         // thumb to show immediately when isAlwaysShown is true. A scroll
         // event is required in order to paint the thumb.
+        _checkForValidScrollPosition();
+        scrollController!.position.didUpdateScrollPositionBy(0);
+      }
+
+      // Interactive scrollbars need to be properly configured.
+      // If there is no scroll controller, there will not be gestures at all.
+      if (scrollController != null && enableGestures) {
+        _checkForValidScrollPosition();
+      }
+    });
+  }
+
+  void _checkForValidScrollPosition() {
     final ScrollController? scrollController = widget.controller ?? PrimaryScrollController.of(context);
     final bool tryPrimary = widget.controller == null;
     final String controllerForError = tryPrimary
       ? 'provided ScrollController'
       : 'PrimaryScrollController';
+
+    String when = '';
+    if (showScrollbar) {
+      when = 'Scrollbar.isAlwaysShown is true';
+    } else if (enableGestures) {
+      when = 'the scrollbar is interactive';
+    } else {
+      when = 'using the Scrollbar';
+    }
+
     assert(
       scrollController != null,
-          'A ScrollController is required when Scrollbar.isAlwaysShown is true. '
+      'A ScrollController is required when $when. '
       '${tryPrimary ? 'The Scrollbar was not provided a ScrollController, '
       'and attempted to use the PrimaryScrollController, but none was found.' :''}',
     );
@@ -1083,7 +1107,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
             'The Scrollbar requires a single ScrollPosition in order to be painted.',
           ),
           ErrorHint(
-                'When Scrollbar.isAlwaysShown is true, the associated Scrollable '
+            'When $when, the associated Scrollable '
             'widgets must have unique ScrollControllers. '
             '${tryPrimary
               ? 'The PrimaryScrollController is used by default for '
@@ -1099,9 +1123,6 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       }
       return true;
     }());
-        scrollController!.position.didUpdateScrollPositionBy(0);
-      }
-    });
   }
 
   /// This method is responsible for configuring the [scrollbarPainter]
@@ -1191,6 +1212,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @protected
   @mustCallSuper
   void handleThumbPress() {
+    _checkForValidScrollPosition();
     if (getScrollbarDirection() == null) {
       return;
     }
@@ -1203,6 +1225,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @protected
   @mustCallSuper
   void handleThumbPressStart(Offset localPosition) {
+    _checkForValidScrollPosition();
     _currentController = widget.controller ?? PrimaryScrollController.of(context);
     final Axis? direction = getScrollbarDirection();
     if (direction == null) {
@@ -1219,6 +1242,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @protected
   @mustCallSuper
   void handleThumbPressUpdate(Offset localPosition) {
+    _checkForValidScrollPosition();
     final Axis? direction = getScrollbarDirection();
     if (direction == null) {
       return;
@@ -1231,9 +1255,11 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   @protected
   @mustCallSuper
   void handleThumbPressEnd(Offset localPosition, Velocity velocity) {
+    _checkForValidScrollPosition();
     final Axis? direction = getScrollbarDirection();
-    if (direction == null)
+    if (direction == null) {
       return;
+    }
     _maybeStartFadeoutTimer();
     _dragScrollbarAxisOffset = null;
     _currentController = null;
@@ -1241,6 +1267,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
 
   void _handleTrackTapDown(TapDownDetails details) {
     // The Scrollbar should page towards the position of the tap on the track.
+    _checkForValidScrollPosition();
     _currentController = widget.controller ?? PrimaryScrollController.of(context);
 
     double scrollIncrement;

packages/flutter/test/cupertino/action_sheet_test.dart

@@ -389,6 +389,36 @@ void main() {
     expect(tester.getSize(find.byType(CupertinoActionSheet)).height, screenHeight);
   });
 
+  testWidgets('CupertinoActionSheet scrollbars controllers should be different', (WidgetTester tester) async {
+    // https://github.com/flutter/flutter/pull/81278
+    await tester.pumpWidget(
+      createAppWithButtonThatLaunchesActionSheet(
+        CupertinoActionSheet(
+            title: const Text('The title'),
+            message: Text('Very long content' * 200),
+            actions: <Widget>[
+              CupertinoActionSheetAction(
+                child: const Text('One'),
+                onPressed: () { },
+              ),
+            ],
+          )
+        ),
+    );
+
+    await tester.tap(find.text('Go'));
+    await tester.pump();
+
+    final List<CupertinoScrollbar> scrollbars =
+      find.descendant(
+        of: find.byType(CupertinoActionSheet),
+        matching: find.byType(CupertinoScrollbar),
+      ).evaluate().map((Element e) => e.widget as CupertinoScrollbar).toList();
+
+    expect(scrollbars.length, 2);
+    expect(scrollbars[0].controller != scrollbars[1].controller, isTrue);
+  });
+
   testWidgets('Tap on button calls onPressed', (WidgetTester tester) async {
     bool wasPressed = false;
     await tester.pumpWidget(

packages/flutter/test/cupertino/dialog_test.dart

@@ -1286,6 +1286,32 @@ void main() {
     // one for the content.
     expect(find.byType(CupertinoScrollbar), findsNWidgets(2));
   }, variant: TargetPlatformVariant.all());
+
+  testWidgets('CupertinoAlertDialog scrollbars controllers should be different', (WidgetTester tester) async {
+    // https://github.com/flutter/flutter/pull/81278
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: MediaQuery(
+          data: MediaQueryData(viewInsets: EdgeInsets.zero),
+          child: CupertinoAlertDialog(
+            actions: <Widget>[
+              CupertinoDialogAction(child: Text('OK')),
+            ],
+            content: Placeholder(fallbackHeight: 200.0),
+          ),
+        ),
+      ),
+    );
+
+    final List<CupertinoScrollbar> scrollbars =
+      find.descendant(
+        of: find.byType(CupertinoAlertDialog),
+        matching: find.byType(CupertinoScrollbar),
+      ).evaluate().map((Element e) => e.widget as CupertinoScrollbar).toList();
+
+    expect(scrollbars.length, 2);
+    expect(scrollbars[0].controller != scrollbars[1].controller, isTrue);
+  });
 }
 
 RenderBox findActionButtonRenderBoxByTitle(WidgetTester tester, String title) {

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -934,6 +934,41 @@ void main() {
     );
   });
 
+  testWidgets('Interactive scrollbars should have a valid scroll controller', (WidgetTester tester) async {
+    final ScrollController primaryScrollController = ScrollController();
+    final ScrollController scrollController = ScrollController();
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: primaryScrollController,
+              child: CupertinoScrollbar(
+                child: SingleChildScrollView(
+                controller: scrollController,
+                child: const SizedBox(
+                  height: 1000.0,
+                  width: 1000.0,
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect(
+      (exception as AssertionError).message,
+      contains("The Scrollbar's ScrollController has no ScrollPosition attached."),
+    );
+  });
+
   testWidgets('Simultaneous dragging and pointer scrolling does not cause a crash', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/70105
     final ScrollController scrollController = ScrollController();

packages/flutter/test/material/scrollbar_test.dart

@@ -1051,8 +1051,9 @@ void main() {
           ),
           child: Scrollbar(
             controller: controller,
-            child: const SingleChildScrollView(
+            child: SingleChildScrollView(
-              child: SizedBox(width: 4000.0, height: 4000.0),
+              controller: controller,
+              child: const SizedBox(width: 4000.0, height: 4000.0),
             ),
           ),
         ),

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1171,6 +1171,41 @@ void main() {
     );
   });
 
+  testWidgets('Interactive scrollbars should have a valid scroll controller', (WidgetTester tester) async {
+    final ScrollController primaryScrollController = ScrollController();
+    final ScrollController scrollController = ScrollController();
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: primaryScrollController,
+              child: RawScrollbar(
+                child: SingleChildScrollView(
+                controller: scrollController,
+                child: const SizedBox(
+                  height: 1000.0,
+                  width: 1000.0,
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect(
+      (exception as AssertionError).message,
+      contains("The Scrollbar's ScrollController has no ScrollPosition attached."),
+    );
+  });
+
   testWidgets('Simultaneous dragging and pointer scrolling does not cause a crash', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/70105
     final ScrollController scrollController = ScrollController();