Add padEnds option to SliverFillViewport (#48207)

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

@@ -30,9 +30,11 @@ class SliverFillViewport extends StatelessWidget {
     Key key,
     @required this.delegate,
     this.viewportFraction = 1.0,
+    this.padEnds = true,
   }) : assert(viewportFraction != null),
-      assert(viewportFraction > 0.0),
-      super(key: key);
+       assert(viewportFraction > 0.0),
+       assert(padEnds != null),
+       super(key: key);
 
   /// The fraction of the viewport that each child should fill in the main axis.
   ///
@@ -41,13 +43,26 @@ class SliverFillViewport extends StatelessWidget {
   /// the viewport in the main axis.
   final double viewportFraction;
 
+  /// Whether to add padding to both ends of the list.
+  ///
+  /// If this is set to true and [viewportFraction] < 1.0, padding will be added
+  /// such that the first and last child slivers will be in the center of
+  /// the viewport when scrolled all the way to the start or end, respectively.
+  /// You may want to set this to false if this [SliverFillViewport] is not the only
+  /// widget along this main axis, such as in a [CustomScrollView] with multiple
+  /// children.
+  ///
+  /// This option cannot be [null]. If [viewportFraction] >= 1.0, this option has no
+  /// effect. Defaults to [true].
+  final bool padEnds;
+
   /// {@macro flutter.widgets.sliverMultiBoxAdaptor.delegate}
   final SliverChildDelegate delegate;
 
   @override
   Widget build(BuildContext context) {
     return _SliverFractionalPadding(
-      viewportFraction: (1 - viewportFraction).clamp(0, 1) / 2,
+      viewportFraction: padEnds ? (1 - viewportFraction).clamp(0, 1) / 2 : 0,
       sliver: _SliverFillViewportRenderObjectWidget(
         viewportFraction: viewportFraction,
         delegate: delegate,

packages/flutter/test/widgets/sliver_fill_viewport_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -145,4 +146,50 @@ void main() {
       ),
     );
   });
+
+  testWidgets('SliverFillViewport padding test', (WidgetTester tester) async {
+    final SliverChildListDelegate delegate = SliverChildListDelegate(
+      <Widget>[
+        Container(child: const Text('0')),
+      ],
+      addAutomaticKeepAlives: false,
+      addSemanticIndexes: false,
+    );
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          slivers: <Widget>[
+            SliverFillViewport(
+              padEnds: true,
+              viewportFraction: 0.5,
+              delegate: delegate,
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final RenderSliver boxWithPadding = tester.renderObject<RenderSliver>(find.byType(SliverFillViewport));
+    expect(boxWithPadding.geometry.paintExtent, equals(600.0));
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          slivers: <Widget>[
+            SliverFillViewport(
+              padEnds: false,
+              viewportFraction: 0.5,
+              delegate: delegate,
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final RenderSliver boxWithoutPadding = tester.renderObject<RenderSliver>(find.byType(SliverFillViewport));
+    expect(boxWithoutPadding.geometry.paintExtent, equals(300.0));
+  });
 }