Added ListView.separated() constructor (#18619)

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

diff --git a/packages/flutter/lib/src/widgets/scroll_view.dart b/packages/flutter/lib/src/widgets/scroll_view.dart
index 32d9ba2..2dad164 100644
--- a/packages/flutter/lib/src/widgets/scroll_view.dart
+++ b/packages/flutter/lib/src/widgets/scroll_view.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
@@ -681,6 +683,91 @@
     cacheExtent: cacheExtent
   );
 
+  /// Creates a fixed-length scrollable linear array of list "items" separated
+  /// by list item "separators".
+  ///
+  /// This constructor is appropriate for list views with a large number of
+  /// item and separator children because the builders are only called for
+  /// the children that are actually visible.
+  ///
+  /// The `itemBuilder` callback will be called with indices greater than
+  /// or equal to zero and less than `itemCount`.
+  ///
+  /// Separators only appear between list items: separator 0 appears after item
+  /// 0 and the last separator appears before the last item.
+  ///
+  /// The `separatorBuilder` callback will be called with indices greater than
+  /// or equal to zero and less than `itemCount - 1`.
+  ///
+  /// The `itemBuilder` and `separatorBuilder` callbacks should actually create
+  /// widget instances when called. Avoid using a builder that returns a
+  /// previously-constructed widget; if the list view's children are created in
+  /// advance, or all at once when the [ListView] itself is created, it is more
+  /// efficient to use [new ListView].
+  ///
+  /// ## Sample code
+  ///
+  /// This example shows how to create [ListView] whose [ListTile] list items
+  /// are separated by [Divider]s.
+  ///
+  /// ```dart
+  /// new ListView.separated(
+  ///   itemCount: 25,
+  ///   separatorBuilder: (BuildContext context, int index) => new Divider(),
+  ///   itemBuilder: (BuildContext context, int index) {
+  ///     return new ListTile(
+  ///       title: new Text('item $index'),
+  ///     );
+  ///   },
+  /// )
+  /// ```
+  ///
+  /// The `addAutomaticKeepAlives` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addAutomaticKeepAlives] property. The
+  /// `addRepaintBoundaries` argument corresponds to the
+  /// [SliverChildBuilderDelegate.addRepaintBoundaries] property. Both must not
+  /// be null.
+  ListView.separated({
+    Key key,
+    Axis scrollDirection = Axis.vertical,
+    bool reverse = false,
+    ScrollController controller,
+    bool primary,
+    ScrollPhysics physics,
+    bool shrinkWrap = false,
+    EdgeInsetsGeometry padding,
+    @required IndexedWidgetBuilder itemBuilder,
+    @required IndexedWidgetBuilder separatorBuilder,
+    @required int itemCount,
+    bool addAutomaticKeepAlives = true,
+    bool addRepaintBoundaries = true,
+    double cacheExtent,
+  }) : assert(itemBuilder != null),
+       assert(separatorBuilder != null),
+       assert(itemCount != null && itemCount >= 0),
+       itemExtent = null,
+       childrenDelegate = new SliverChildBuilderDelegate(
+         (BuildContext context, int index) {
+           final int itemIndex = index ~/ 2;
+           return (index == 0 || index.isEven)
+             ? itemBuilder(context, itemIndex)
+             : separatorBuilder(context, itemIndex);
+         },
+         childCount: math.max(0, itemCount * 2 - 1),
+         addAutomaticKeepAlives: addAutomaticKeepAlives,
+         addRepaintBoundaries: addRepaintBoundaries,
+       ), super(
+    key: key,
+    scrollDirection: scrollDirection,
+    reverse: reverse,
+    controller: controller,
+    primary: primary,
+    physics: physics,
+    shrinkWrap: shrinkWrap,
+    padding: padding,
+    cacheExtent: cacheExtent
+  );
+
   /// Creates a scrollable, linear array of widgets with a custom child model.
   ///
   /// For example, a custom child model can control the algorithm used to

packages/flutter/test/widgets/list_view_builder_test.dart

diff --git a/packages/flutter/test/widgets/list_view_builder_test.dart b/packages/flutter/test/widgets/list_view_builder_test.dart
index 4d59b39..38ec803 100644
--- a/packages/flutter/test/widgets/list_view_builder_test.dart
+++ b/packages/flutter/test/widgets/list_view_builder_test.dart
@@ -261,6 +261,49 @@
     callbackTracker.clear();
   });
 
+  testWidgets('ListView.separated', (WidgetTester tester) async {
+    Widget buildFrame({ int itemCount }) {
+      return new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new ListView.separated(
+          itemCount: itemCount,
+          itemBuilder: (BuildContext context, int index) {
+            return new SizedBox(
+              height: 100.0,
+              child: new Text('i$index'),
+            );
+          },
+          separatorBuilder: (BuildContext context, int index) {
+            return new SizedBox(
+              height: 10.0,
+              child: new Text('s$index'),
+            );
+          },
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(itemCount: 0));
+    expect(find.text('i0'), findsNothing);
+    expect(find.text('s0'), findsNothing);
+
+    await tester.pumpWidget(buildFrame(itemCount: 1));
+    expect(find.text('i0'), findsOneWidget);
+    expect(find.text('s0'), findsNothing);
+
+    await tester.pumpWidget(buildFrame(itemCount: 2));
+    expect(find.text('i0'), findsOneWidget);
+    expect(find.text('s0'), findsOneWidget);
+    expect(find.text('i1'), findsOneWidget);
+    expect(find.text('s1'), findsNothing);
+
+    // ListView's height is 600, so items i0-i5 and s0-s4 fit.
+    await tester.pumpWidget(buildFrame(itemCount: 25));
+    for(String s in <String>['i0', 's0', 'i1', 's1', 'i2', 's2', 'i3', 's3', 'i4', 's4', 'i5'])
+      expect(find.text(s), findsOneWidget);
+    expect(find.text('s5'), findsNothing);
+    expect(find.text('i6'), findsNothing);
+  });
 }
 
 void check({List<int> visible = const <int>[], List<int> hidden = const <int>[]}) {