SliverList works with changing children that are keyed (#16724)

Fixes #14590.

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

@@ -738,6 +738,20 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
     });
   }
 
+  @override
+  Element updateChild(Element child, Widget newWidget, dynamic newSlot) {
+    final SliverMultiBoxAdaptorParentData oldParentData = child?.renderObject?.parentData;
+    final Element newChild = super.updateChild(child, newWidget, newSlot);
+    final SliverMultiBoxAdaptorParentData newParentData = newChild?.renderObject?.parentData;
+
+    // Preserve the old layoutOffset if the renderObject was swapped out.
+    if (oldParentData != newParentData && oldParentData != null && newParentData != null) {
+      newParentData.layoutOffset = oldParentData.layoutOffset;
+    }
+
+    return newChild;
+  }
+
   @override
   void forgetChild(Element child) {
     assert(child != null);

packages/flutter/test/widgets/sliver_list_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('SliverList reverse children (with keys)', (WidgetTester tester) async {
+    final List<int> items = new List<int>.generate(20, (int i) => i);
+    const double itemHeight = 300.0;
+    const double viewportHeight = 500.0;
+
+    const double scrollPosition = 18 * itemHeight;
+    final ScrollController controller = new ScrollController(initialScrollOffset: scrollPosition);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items,
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, scrollPosition);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 19'), findsOneWidget);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items.reversed.toList(),
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    final int frames = await tester.pumpAndSettle();
+    expect(frames, 1); // ensures that there is no (animated) bouncing of the scrollable
+
+    expect(controller.offset, scrollPosition);
+    expect(find.text('Tile 19'), findsNothing);
+    expect(find.text('Tile 18'), findsNothing);
+    expect(find.text('Tile 1'), findsOneWidget);
+    expect(find.text('Tile 0'), findsOneWidget);
+
+    controller.jumpTo(0.0);
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, 0.0);
+    expect(find.text('Tile 19'), findsOneWidget);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 0'), findsNothing);
+  });
+
+  testWidgets('SliverList replace children (with keys)', (WidgetTester tester) async {
+    final List<int> items = new List<int>.generate(20, (int i) => i);
+    const double itemHeight = 300.0;
+    const double viewportHeight = 500.0;
+
+    const double scrollPosition = 18 * itemHeight;
+    final ScrollController controller = new ScrollController(initialScrollOffset: scrollPosition);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items,
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, scrollPosition);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 19'), findsOneWidget);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items.map((int i) => i + 100).toList(),
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    final int frames = await tester.pumpAndSettle();
+    expect(frames, 1); // ensures that there is no (animated) bouncing of the scrollable
+
+    expect(controller.offset, scrollPosition);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 18'), findsNothing);
+    expect(find.text('Tile 19'), findsNothing);
+
+    expect(find.text('Tile 100'), findsNothing);
+    expect(find.text('Tile 101'), findsNothing);
+    expect(find.text('Tile 118'), findsOneWidget);
+    expect(find.text('Tile 119'), findsOneWidget);
+
+    controller.jumpTo(0.0);
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, 0.0);
+    expect(find.text('Tile 100'), findsOneWidget);
+    expect(find.text('Tile 101'), findsOneWidget);
+    expect(find.text('Tile 118'), findsNothing);
+    expect(find.text('Tile 119'), findsNothing);
+  });
+
+  testWidgets('SliverList replace with shorter children list (with keys)', (WidgetTester tester) async {
+    final List<int> items = new List<int>.generate(20, (int i) => i);
+    const double itemHeight = 300.0;
+    const double viewportHeight = 500.0;
+
+    final double scrollPosition = items.length * itemHeight - viewportHeight;
+    final ScrollController controller = new ScrollController(initialScrollOffset: scrollPosition);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items,
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    await tester.pumpAndSettle();
+
+    expect(controller.offset, scrollPosition);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 17'), findsNothing);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 19'), findsOneWidget);
+
+    await tester.pumpWidget(_buildSliverList(
+      items: items.sublist(0, items.length - 1),
+      controller: controller,
+      itemHeight: itemHeight,
+      viewportHeight: viewportHeight,
+    ));
+    final int frames = await tester.pumpAndSettle();
+    expect(frames, greaterThan(1)); // ensure animation to bring tile17 into view
+
+    expect(controller.offset, scrollPosition - itemHeight);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 1'), findsNothing);
+    expect(find.text('Tile 17'), findsOneWidget);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 19'), findsNothing);
+  });
+}
+
+Widget _buildSliverList({
+  List<int> items: const <int>[],
+  ScrollController controller,
+  double itemHeight: 500.0,
+  double viewportHeight: 300.0,
+}) {
+  return new Directionality(
+    textDirection: TextDirection.ltr,
+    child: new Center(
+      child: new Container(
+        height: viewportHeight,
+        child: new CustomScrollView(
+          controller: controller,
+          slivers: <Widget>[
+            new SliverList(
+              delegate: new SliverChildBuilderDelegate(
+                    (BuildContext context, int i) {
+                  return new Container(
+                    key: new ValueKey<int>(items[i]),
+                    height: itemHeight,
+                    child: new Text('Tile ${items[i]}'),
+                  );
+                },
+                childCount: items.length,
+              ),
+            ),
+          ],
+        ),
+      ),
+    ),
+  );
+}