Swap scope with gesture (#21157)

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

@@ -524,21 +524,29 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
   @override
   Widget build(BuildContext context) {
     assert(position != null);
-    // TODO(ianh): Having all these global keys is sad.
-    Widget result = RawGestureDetector(
-      key: _gestureDetectorKey,
-      gestures: _gestureRecognizers,
-      behavior: HitTestBehavior.opaque,
-      excludeFromSemantics: widget.excludeFromSemantics,
-      child: Semantics(
-        explicitChildNodes: !widget.excludeFromSemantics,
-        child: IgnorePointer(
-          key: _ignorePointerKey,
-          ignoring: _shouldIgnorePointer,
-          ignoringSemantics: false,
-          child: _ScrollableScope(
-            scrollable: this,
-            position: position,
+    // _ScrollableScope must be placed above the BuildContext returned by notificationContext
+    // so that we can get this ScrollableState by doing the following:
+    //
+    // ScrollNotification notification;
+    // Scrollable.of(notification.context)
+    //
+    // Since notificationContext is pointing to _gestureDetectorKey.context, _ScrollableScope
+    // must be placed above the widget using it: RawGestureDetector
+    Widget result = _ScrollableScope(
+      scrollable: this,
+      position: position,
+      // TODO(ianh): Having all these global keys is sad.
+      child: RawGestureDetector(
+        key: _gestureDetectorKey,
+        gestures: _gestureRecognizers,
+        behavior: HitTestBehavior.opaque,
+        excludeFromSemantics: widget.excludeFromSemantics,
+        child: Semantics(
+          explicitChildNodes: !widget.excludeFromSemantics,
+          child: IgnorePointer(
+            key: _ignorePointerKey,
+            ignoring: _shouldIgnorePointer,
+            ignoringSemantics: false,
             child: widget.viewportBuilder(context, position),
           ),
         ),

packages/flutter/test/widgets/scrollable_of_test.dart

@@ -82,4 +82,24 @@ void main() {
     controller.jumpTo(400.0);
     expect(logValue, 'listener 400.0');
   });
+
+  testWidgets('Scrollable.of() is possible using ScrollNotification context', (WidgetTester tester) async {
+    ScrollNotification notification;
+
+    await tester.pumpWidget(NotificationListener<ScrollNotification>(
+      onNotification: (ScrollNotification value) {
+        notification = value;
+        return false;
+      },
+      child: SingleChildScrollView(
+        child: const SizedBox(height: 1200.0)
+      )
+    ));
+
+    await tester.startGesture(const Offset(100.0, 100.0));
+    await tester.pump(const Duration(seconds: 1));
+
+    final StatefulElement scrollableElement = find.byType(Scrollable).evaluate().first;
+    expect(Scrollable.of(notification.context), equals(scrollableElement.state));
+  });
 }