Fix DraggableScrollableSheet crash when switching out scrollables (#105549)

Author: Piinks

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

@@ -631,7 +631,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   @override
   void didUpdateWidget(covariant DraggableScrollableSheet oldWidget) {
     super.didUpdateWidget(oldWidget);
-    _replaceExtent();
+    _replaceExtent(oldWidget);
   }
 
   @override
@@ -671,7 +671,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
     super.dispose();
   }
 
-  void _replaceExtent() {
+  void _replaceExtent(covariant DraggableScrollableSheet oldWidget) {
     final _DraggableSheetExtent previousExtent = _extent;
     _extent.dispose();
     _extent = _extent.copyWith(
@@ -688,13 +688,21 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
     // If an external facing controller was provided, let it know that the
     // extent has been replaced.
     widget.controller?._onExtentReplaced(previousExtent);
-    if (widget.snap) {
-      // Trigger a snap in case snap or snapSizes has changed. We put this in a
-      // post frame callback so that `build` can update `_extent.availablePixels`
-      // before this runs-we can't use the previous extent's available pixels as
-      // it may have changed when the widget was updated.
+    if (widget.snap
+        && (widget.snap != oldWidget.snap || widget.snapSizes != oldWidget.snapSizes)
+        && _scrollController.hasClients
+    ) {
+      // Trigger a snap in case snap or snapSizes has changed and there is a
+      // scroll position currently attached. We put this in a post frame
+      // callback so that `build` can update `_extent.availablePixels` before
+      // this runs-we can't use the previous extent's available pixels as it may
+      // have changed when the widget was updated.
       WidgetsBinding.instance.addPostFrameCallback((Duration timeStamp) {
-        _scrollController.position.goBallistic(0);
+        for (int index = 0; index < _scrollController.positions.length; index++) {
+          final _DraggableScrollableSheetScrollPosition position =
+            _scrollController.positions.elementAt(index) as _DraggableScrollableSheetScrollPosition;
+          position.goBallistic(0);
+        }
       });
     }
   }

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -745,6 +745,63 @@ void main() {
     await tester.pumpAndSettle();
   }, variant: TargetPlatformVariant.all());
 
+  testWidgets('Transitioning between scrollable children sharing a scroll controller will not throw', (WidgetTester tester) async {
+    int s = 0;
+    await tester.pumpWidget(MaterialApp(
+      home: StatefulBuilder(
+        builder: (BuildContext context, StateSetter setState) {
+          return Scaffold(
+            body: DraggableScrollableSheet(
+              initialChildSize: 0.25,
+              snap: true,
+              snapSizes: const <double>[0.25, 0.5, 1.0],
+              builder: (BuildContext context, ScrollController scrollController) {
+                return PrimaryScrollController(
+                  controller: scrollController,
+                  child: AnimatedSwitcher(
+                    duration: const Duration(milliseconds: 500),
+                    child: (s.isEven)
+                      ? ListView(
+                        children: <Widget>[
+                          ElevatedButton(
+                            onPressed: () => setState(() => ++s),
+                            child: const Text('Switch to 2'),
+                          ),
+                          Container(
+                            height: 400,
+                            color: Colors.blue,
+                          ),
+                        ],
+                      )
+                      : SingleChildScrollView(
+                        child: Column(
+                          children: <Widget>[
+                            ElevatedButton(
+                              onPressed: () => setState(() => ++s),
+                              child: const Text('Switch to 1'),
+                            ),
+                            Container(
+                              height: 400,
+                              color: Colors.blue,
+                            ),
+                          ],
+                        )
+                      ),
+                  ),
+                );
+              },
+            ),
+          );
+        },
+      ),
+    ));
+
+    // Trigger the AnimatedSwitcher between ListViews
+    await tester.tap(find.text('Switch to 2'));
+    await tester.pump();
+    // Completes without throwing
+  });
+
   testWidgets('ScrollNotification correctly dispatched when flung without covering its container', (WidgetTester tester) async {
     final List<Type> notificationTypes = <Type>[];
     await tester.pumpWidget(boilerplateWidget(