Copybara sync (#3)

* Initial commit of LinkedScrollController

PiperOrigin-RevId: 220688858

* Correct typos in readme.md

PiperOrigin-RevId: 220697252

* Fix travis.sh

Closes #4

PiperOrigin-RevId: 220704036

.travis.yml

@@ -1,6 +1,6 @@
-language: dart
-sudo: false
-dart:
-  - stable
-  - dev
+os: linux
+before_script:
+  - git clone https://github.com/flutter/flutter.git --depth 1
+  - export PATH=`pwd`/flutter/bin:`pwd`/flutter/bin/cache/dart-sdk/bin:$PATH
+  - flutter doctor
 script: ./tool/travis.sh

gallery/lib/gallery.dart

@@ -6,6 +6,7 @@
 
 import 'package:flutter/material.dart';
 import 'package:meta/meta.dart';
+import 'package:gallery/src/linked_scroll_controller_page.dart';
 import 'package:gallery/src/tagged_text_page.dart';
 import 'package:gallery/src/html_widget_page.dart' as html_latency;
 
@@ -20,6 +21,10 @@ class Gallery extends StatefulWidget {
     ..add(new _GalleryPage(
       title: 'Tagged Text',
       pageBuilder: (context) => new TaggedTextPage(),
+    ))
+    ..add(new _GalleryPage(
+      title: 'Linked Scrollables',
+      pageBuilder: (context) => new LinkedScrollablesPage(),
     ));
 
   @override

gallery/lib/src/linked_scroll_controller_page.dart

@@ -0,0 +1,84 @@
+// Copyright 2018 the Dart project authors.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+import 'package:flutter/material.dart';
+import 'package:flutter_widgets/flutter_widgets.dart';
+
+/// Gallery page for demoing the [LinkedScrollControllerGroup].
+class LinkedScrollablesPage extends StatefulWidget {
+  @override
+  _LinkedScrollablesPageState createState() => _LinkedScrollablesPageState();
+}
+
+class _LinkedScrollablesPageState extends State<LinkedScrollablesPage> {
+  LinkedScrollControllerGroup _controllers;
+  ScrollController _letters;
+  ScrollController _numbers;
+
+  @override
+  void initState() {
+    super.initState();
+    _controllers = LinkedScrollControllerGroup();
+    _letters = _controllers.addAndGet();
+    _numbers = _controllers.addAndGet();
+  }
+
+  @override
+  void dispose() {
+    _letters.dispose();
+    _numbers.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: new AppBar(title: new Text('Linked Scrollables')),
+      body: Row(
+        children: [
+          Expanded(
+            child: ListView(
+              controller: _letters,
+              children: <Widget>[
+                _Tile('Hello A'),
+                _Tile('Hello B'),
+                _Tile('Hello C'),
+                _Tile('Hello D'),
+                _Tile('Hello E'),
+              ],
+            ),
+          ),
+          Expanded(
+            child: ListView(
+              controller: _numbers,
+              children: <Widget>[
+                _Tile('Hello 1'),
+                _Tile('Hello 2'),
+                _Tile('Hello 3'),
+                _Tile('Hello 4'),
+                _Tile('Hello 5'),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _Tile extends StatelessWidget {
+  final String caption;
+
+  _Tile(this.caption);
+
+  @override
+  Widget build(_) => Container(
+        margin: const EdgeInsets.all(8.0),
+        padding: const EdgeInsets.all(8.0),
+        height: 250.0,
+        child: Center(child: Text(caption)),
+      );
+}

gallery/test/widget_loading_test.dart

@@ -20,6 +20,7 @@ void main() {
     // Each test name matches against a tile in the top level of `gallery.dart`.
     // This is necessary because we can't import gallery.dart here.
     const taggedText = 'Tagged Text';
+    const linkedScrollables = 'Linked Scrollables';
 
     /// Factory for test methods against widget loading times.
     ///
@@ -52,5 +53,6 @@ void main() {
     // run of the app.  Make sure to return the app to the entry point after
     // each test.
     test(taggedText, buildWidgetLoadingTest(taggedText));
+    test(linkedScrollables, buildWidgetLoadingTest(linkedScrollables));
   });
 }

lib/src/linked_scroll_controller/README.md

@@ -0,0 +1,92 @@
+# linked_scroll_controller
+
+This package provides a way to set up a set of scrollable widgets whose
+scrolling is synchronized. The set can be stable across the lifetime of the
+containing screen, or can change dynamically (for example, a vertically
+scrolling `ListView.builder()` whose items are Scrollables that scroll
+horizontally in unison).
+
+**If you add controllers dynamically, the corresponding scrollables must be
+given unique keys to avoid the scroll offset going out of sync.**
+
+# Example usage
+
+The code below sets up two side-by-side `ListView`s that scroll in unison.
+
+```dart
+class LinkedScrollables extends StatefulWidget {
+  @override
+  _LinkedScrollablesState createState() => _LinkedScrollablesState();
+}
+
+class _LinkedScrollablesState extends State<LinkedScrollables> {
+  LinkedScrollControllerGroup _controllers;
+  ScrollController _letters;
+  ScrollController _numbers;
+
+  @override
+  void initState() {
+    super.initState();
+    _controllers = LinkedScrollControllerGroup();
+    _letters = _controllers.addAndGet();
+    _numbers = _controllers.addAndGet();
+  }
+
+  @override
+  void dispose() {
+    _letters.dispose();
+    _numbers.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Directionality(
+      textDirection: TextDirection.ltr,
+      child: Row(
+        children: [
+          Expanded(
+            child: ListView(
+              controller: _letters,
+              children: <Widget>[
+                _Tile('Hello A'),
+                _Tile('Hello B'),
+                _Tile('Hello C'),
+                _Tile('Hello D'),
+                _Tile('Hello E'),
+              ],
+            ),
+          ),
+          Expanded(
+            child: ListView(
+              controller: _numbers,
+              children: <Widget>[
+                _Tile('Hello 1'),
+                _Tile('Hello 2'),
+                _Tile('Hello 3'),
+                _Tile('Hello 4'),
+                _Tile('Hello 5'),
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _Tile extends StatelessWidget {
+  final String caption;
+
+  _Tile(this.caption);
+
+  @override
+  Widget build(_) => Container(
+        margin: const EdgeInsets.all(8.0),
+        padding: const EdgeInsets.all(8.0),
+        height: 250.0,
+        child: Center(child: Text(caption)),
+      );
+}
+
+```