[H] Expose "center" on CustomScrollView (#27424)

* Expose "center" on CustomScrollView

* Also support anchor

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

@@ -51,6 +51,12 @@ abstract class ScrollView extends StatelessWidget {
   /// Creates a widget that scrolls.
   ///
   /// If the [primary] argument is true, the [controller] must be null.
+  ///
+  /// If the [shrinkWrap] argument is true, the [center] argument must be null.
+  ///
+  /// The [scrollDirection], [reverse], and [shrinkWrap] arguments must not be null.
+  ///
+  /// The [anchor] argument must be non-null and in the range 0.0 to 1.0.
   const ScrollView({
     Key key,
     this.scrollDirection = Axis.vertical,
@@ -59,16 +65,22 @@ abstract class ScrollView extends StatelessWidget {
     bool primary,
     ScrollPhysics physics,
     this.shrinkWrap = false,
+    this.center,
+    this.anchor = 0.0,
     this.cacheExtent,
     this.semanticChildCount,
     this.dragStartBehavior = DragStartBehavior.down,
-  }) : assert(reverse != null),
+  }) : assert(scrollDirection != null),
+       assert(reverse != null),
        assert(shrinkWrap != null),
        assert(dragStartBehavior != null),
        assert(!(controller != null && primary == true),
            'Primary ScrollViews obtain their ScrollController via inheritance from a PrimaryScrollController widget. '
            'You cannot both set primary to true and pass an explicit controller.'
        ),
+       assert(!shrinkWrap || center == null),
+       assert(anchor != null),
+       assert(anchor >= 0.0 && anchor <= 1.0),
        primary = primary ?? controller == null && identical(scrollDirection, Axis.vertical),
        physics = physics ?? (primary == true || (primary == null && controller == null && identical(scrollDirection, Axis.vertical)) ? const AlwaysScrollableScrollPhysics() : null),
        super(key: key);
@@ -172,6 +184,32 @@ abstract class ScrollView extends StatelessWidget {
   /// Defaults to false.
   final bool shrinkWrap;
 
+  /// The first child in the [GrowthDirection.forward] growth direction.
+  ///
+  /// Children after [center] will be placed in the [axisDirection] relative to
+  /// the [center]. Children before [center] will be placed in the opposite of
+  /// the [axisDirection] relative to the [center].
+  ///
+  /// The [center] must be the key of one of the slivers built by [buildSlivers].
+  ///
+  /// Of the built-in subclasses of [ScrollView], only [CustomScrollView]
+  /// supports [center]; for that class, the given key must be the key of one of
+  /// the slivers in the [CustomScrollView.slivers] list.
+  ///
+  /// See also:
+  ///
+  ///  * [anchor], which controls where the [center] as aligned in the viewport.
+  final Key center;
+
+  /// The relative position of the zero scroll offset.
+  ///
+  /// For example, if [anchor] is 0.5 and the [axisDirection] is
+  /// [AxisDirection.down] or [AxisDirection.up], then the zero scroll offset is
+  /// vertically centered within the viewport. If the [anchor] is 1.0, and the
+  /// [axisDirection] is [AxisDirection.right], then the zero scroll offset is
+  /// on the left edge of the viewport.
+  final double anchor;
+
   /// {@macro flutter.rendering.viewport.cacheExtent}
   final double cacheExtent;
 
@@ -221,6 +259,14 @@ abstract class ScrollView extends StatelessWidget {
   /// Subclasses may override this method to change how the viewport is built.
   /// The default implementation uses a [ShrinkWrappingViewport] if [shrinkWrap]
   /// is true, and a regular [Viewport] otherwise.
+  ///
+  /// The `offset` argument is the value obtained from
+  /// [Scrollable.viewportBuilder].
+  ///
+  /// The `axisDirection` argument is the value obtained from [getDirection],
+  /// which by default uses [scrollDirection] and [reverse].
+  ///
+  /// The `slivers` argument is the value obtained from [buildSlivers].
   @protected
   Widget buildViewport(
     BuildContext context,
@@ -240,6 +286,8 @@ abstract class ScrollView extends StatelessWidget {
       offset: offset,
       slivers: slivers,
       cacheExtent: cacheExtent,
+      center: center,
+      anchor: anchor,
     );
   }
 
@@ -392,7 +440,7 @@ abstract class ScrollView extends StatelessWidget {
 class CustomScrollView extends ScrollView {
   /// Creates a [ScrollView] that creates custom scroll effects using slivers.
   ///
-  /// If the [primary] argument is true, the [controller] must be null.
+  /// See the [new ScrollView] constructor for more details on these arguments.
   const CustomScrollView({
     Key key,
     Axis scrollDirection = Axis.vertical,
@@ -401,6 +449,8 @@ class CustomScrollView extends ScrollView {
     bool primary,
     ScrollPhysics physics,
     bool shrinkWrap = false,
+    Key center,
+    double anchor = 0.0,
     double cacheExtent,
     this.slivers = const <Widget>[],
     int semanticChildCount,
@@ -413,6 +463,8 @@ class CustomScrollView extends ScrollView {
     primary: primary,
     physics: physics,
     shrinkWrap: shrinkWrap,
+    center: center,
+    anchor: anchor,
     cacheExtent: cacheExtent,
     semanticChildCount: semanticChildCount,
     dragStartBehavior: dragStartBehavior,

packages/flutter/test/widgets/custom_scroll_view_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/widgets.dart';
+
+void main() {
+  testWidgets('CustomScrollView.center', (WidgetTester tester) async {
+    await tester.pumpWidget(const Directionality(
+      textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        slivers: <Widget>[
+          SliverToBoxAdapter(key: Key('a'), child: SizedBox(height: 100.0)),
+          SliverToBoxAdapter(key: Key('b'), child: SizedBox(height: 100.0)),
+        ],
+        center: Key('a'),
+      ),
+    ));
+    await tester.pumpAndSettle();
+    expect(tester.getRect(find.descendant(of: find.byKey(const Key('a')), matching: find.byType(SizedBox))),
+           Rect.fromLTRB(0.0, 0.0, 800.0, 100.0));
+    expect(tester.getRect(find.descendant(of: find.byKey(const Key('b')), matching: find.byType(SizedBox))),
+           Rect.fromLTRB(0.0, 100.0, 800.0, 200.0));
+  });
+
+  testWidgets('CustomScrollView.center', (WidgetTester tester) async {
+    await tester.pumpWidget(const Directionality(
+      textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        slivers: <Widget>[
+          SliverToBoxAdapter(key: Key('a'), child: SizedBox(height: 100.0)),
+          SliverToBoxAdapter(key: Key('b'), child: SizedBox(height: 100.0)),
+        ],
+        center: Key('b'),
+      ),
+    ));
+    await tester.pumpAndSettle();
+    expect(
+      tester.getRect(
+        find.descendant(
+          of: find.byKey(const Key('a'), skipOffstage: false),
+          matching: find.byType(SizedBox, skipOffstage: false),
+        ),
+      ),
+      Rect.fromLTRB(0.0, -100.0, 800.0, 0.0),
+    );
+    expect(
+      tester.getRect(
+        find.descendant(
+          of: find.byKey(const Key('b')),
+          matching: find.byType(SizedBox),
+        ),
+      ),
+      Rect.fromLTRB(0.0, 0.0, 800.0, 100.0),
+    );
+  });
+
+  testWidgets('CustomScrollView.anchor', (WidgetTester tester) async {
+    await tester.pumpWidget(const Directionality(
+      textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        slivers: <Widget>[
+          SliverToBoxAdapter(key: Key('a'), child: SizedBox(height: 100.0)),
+          SliverToBoxAdapter(key: Key('b'), child: SizedBox(height: 100.0)),
+        ],
+        center: Key('b'),
+        anchor: 1.0,
+      ),
+    ));
+    await tester.pumpAndSettle();
+    expect(
+      tester.getRect(
+        find.descendant(
+          of: find.byKey(const Key('a')),
+          matching: find.byType(SizedBox),
+        ),
+      ),
+      Rect.fromLTRB(0.0, 500.0, 800.0, 600.0),
+    );
+    expect(
+      tester.getRect(
+        find.descendant(
+          of: find.byKey(const Key('b'), skipOffstage: false),
+          matching: find.byType(SizedBox, skipOffstage: false),
+        ),
+      ),
+      Rect.fromLTRB(0.0, 600.0, 800.0, 700.0),
+    );
+  });
+}