Apply PrimaryScrollController updates to SingleChildScrollView (#106430)

examples/api/lib/widgets/scroll_position/scroll_metrics_notification.0.dart

@@ -46,6 +46,7 @@ class ScrollMetricsDemoState extends State<ScrollMetricsDemo> {
               height: windowSize,
               width: double.infinity,
               child: const SingleChildScrollView(
+                primary: true,
                 child: FlutterLogo(
                   size: 300.0,
                 ),

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

@@ -142,7 +142,7 @@ class SingleChildScrollView extends StatelessWidget {
     this.scrollDirection = Axis.vertical,
     this.reverse = false,
     this.padding,
-    bool? primary,
+    this.primary,
     this.physics,
     this.controller,
     this.child,
@@ -153,11 +153,12 @@ class SingleChildScrollView extends StatelessWidget {
   }) : assert(scrollDirection != null),
        assert(dragStartBehavior != null),
        assert(clipBehavior != null),
-       assert(!(controller != null && (primary ?? false)),
-          'Primary ScrollViews obtain their ScrollController via inheritance from a PrimaryScrollController widget. '
-          'You cannot both set primary to true and pass an explicit controller.',
-       ),
-       primary = primary ?? controller == null && identical(scrollDirection, Axis.vertical);
+       assert(
+         !(controller != null && (primary ?? false)),
+         'Primary ScrollViews obtain their ScrollController via inheritance '
+         'from a PrimaryScrollController widget. You cannot both set primary to '
+         'true and pass an explicit controller.',
+       );
 
   /// The axis along which the scroll view scrolls.
   ///
@@ -195,20 +196,8 @@ class SingleChildScrollView extends StatelessWidget {
   /// [ScrollController.animateTo]).
   final ScrollController? controller;
 
-  /// Whether this is the primary scroll view associated with the parent
-  /// [PrimaryScrollController].
-  ///
-  /// When true, the scroll view is used for default [ScrollAction]s. If a
-  /// ScrollAction is not handled by an otherwise focused part of the application,
-  /// the ScrollAction will be evaluated using this scroll view, for example,
-  /// when executing [Shortcuts] key events like page up and down.
-  ///
-  /// On iOS, this identifies the scroll view that will scroll to top in
-  /// response to a tap in the status bar.
-  ///
-  /// Defaults to true when [scrollDirection] is vertical and [controller] is
-  /// not specified.
-  final bool primary;
+  /// {@macro flutter.widgets.scroll_view.primary}
+  final bool? primary;
 
   /// How the scroll view should respond to user input.
   ///
@@ -248,9 +237,13 @@ class SingleChildScrollView extends StatelessWidget {
     if (padding != null) {
       contents = Padding(padding: padding!, child: contents);
     }
-    final ScrollController? scrollController = primary
+    final bool effectivePrimary = primary
+        ?? controller == null && PrimaryScrollController.shouldInherit(context, scrollDirection);
+
+    final ScrollController? scrollController = effectivePrimary
         ? PrimaryScrollController.of(context)
         : controller;
+
     Widget scrollable = Scrollable(
       dragStartBehavior: dragStartBehavior,
       axisDirection: axisDirection,
@@ -280,7 +273,9 @@ class SingleChildScrollView extends StatelessWidget {
       );
     }
 
-    return primary && scrollController != null
+    return effectivePrimary && scrollController != null
+      // Further descendant ScrollViews will not inherit the same
+      // PrimaryScrollController
       ? PrimaryScrollController.none(child: scrollable)
       : scrollable;
   }

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1079,6 +1079,7 @@ void main() {
                 isAlwaysShown: true,
                 controller: scrollController,
                 child: const SingleChildScrollView(
+                  primary: true,
                   child: SizedBox(width: 4000.0, height: 4000.0),
                 ),
               ),

packages/flutter/test/widgets/single_child_scroll_view_test.dart

@@ -34,6 +34,19 @@ class TestScrollController extends ScrollController {
   }
 }
 
+Widget primaryScrollControllerBoilerplate({ required Widget child, required ScrollController controller }) {
+  return Directionality(
+    textDirection: TextDirection.ltr,
+    child: MediaQuery(
+      data: const MediaQueryData(),
+      child: PrimaryScrollController(
+        controller: controller,
+        child: child,
+      ),
+    ),
+  );
+}
+
 void main() {
   testWidgets('SingleChildScrollView overflow and clipRect test', (WidgetTester tester) async {
     // the test widowSize is Size(800.0, 600.0)
@@ -210,23 +223,41 @@ void main() {
     ));
   });
 
-  testWidgets('Vertical SingleChildScrollViews are primary by default', (WidgetTester tester) async {
+  testWidgets('Vertical SingleChildScrollViews are not primary by default', (WidgetTester tester) async {
     const SingleChildScrollView view = SingleChildScrollView();
-    expect(view.primary, isTrue);
+    expect(view.primary, isNull);
   });
 
-  testWidgets('Horizontal SingleChildScrollViews are non-primary by default', (WidgetTester tester) async {
+  testWidgets('Horizontal SingleChildScrollViews are not primary by default', (WidgetTester tester) async {
     const SingleChildScrollView view = SingleChildScrollView(scrollDirection: Axis.horizontal);
-    expect(view.primary, isFalse);
+    expect(view.primary, isNull);
   });
 
-  testWidgets('SingleChildScrollViews with controllers are non-primary by default', (WidgetTester tester) async {
+  testWidgets('SingleChildScrollViews with controllers are not primary by default', (WidgetTester tester) async {
     final SingleChildScrollView view = SingleChildScrollView(
       controller: ScrollController(),
     );
-    expect(view.primary, isFalse);
+    expect(view.primary, isNull);
   });
 
+  testWidgets('Vertical SingleChildScrollViews use PrimaryScrollController by default on mobile', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(primaryScrollControllerBoilerplate(
+      child: const SingleChildScrollView(),
+      controller: controller,
+    ));
+    expect(controller.hasClients, isTrue);
+  }, variant: TargetPlatformVariant.mobile());
+
+  testWidgets("Vertical SingleChildScrollViews don't use PrimaryScrollController by default on desktop", (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    await tester.pumpWidget(primaryScrollControllerBoilerplate(
+      child: const SingleChildScrollView(),
+      controller: controller,
+    ));
+    expect(controller.hasClients, isFalse);
+  }, variant: TargetPlatformVariant.desktop());
+
   testWidgets('Nested scrollables have a null PrimaryScrollController', (WidgetTester tester) async {
     const Key innerKey = Key('inner');
     final ScrollController primaryScrollController = ScrollController();