add TweenSequence sample (#127)

* Add TweenSequence sample

* rename to TweenSequenceDemo

* add copyright headers

* remote type

* introduce animated builder earlier in basics/

so that other examples can use it

* formatting

* use async / await, increase animation duration

animations/lib/main.dart

@@ -8,8 +8,9 @@ import 'src/basics/01_animated_container.dart';
 import 'src/basics/02_page_route_builder.dart';
 import 'src/basics/03_animation_controller.dart';
 import 'src/basics/04_tweens.dart';
-import 'src/basics/05_custom_tween.dart';
-import 'src/basics/06_animated_builder.dart';
+import 'src/basics/05_animated_builder.dart';
+import 'src/basics/06_custom_tween.dart';
+import 'src/basics/07_tween_sequence.dart';
 import 'src/misc/card_swipe.dart';
 import 'src/misc/carousel.dart';
 import 'src/misc/expand_card.dart';
@@ -35,10 +36,12 @@ final basicDemos = [
   Demo('Animation Controller', AnimationControllerDemo.routeName,
       (context) => AnimationControllerDemo()),
   Demo('Tweens', TweenDemo.routeName, (context) => TweenDemo()),
-  Demo('Custom Tween', CustomTweenDemo.routeName,
-      (context) => CustomTweenDemo()),
   Demo('AnimatedBuilder', AnimatedBuilderDemo.routeName,
       (context) => AnimatedBuilderDemo()),
+  Demo('Custom Tween', CustomTweenDemo.routeName,
+      (context) => CustomTweenDemo()),
+  Demo('Tween Sequences', TweenSequenceDemo.routeName,
+      (context) => TweenSequenceDemo()),
 ];
 
 final miscDemos = [

animations/lib/src/basics/01_animated_container.dart

@@ -1,3 +1,7 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';

animations/lib/src/basics/02_page_route_builder.dart

@@ -1,3 +1,7 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
 
 class PageRouteBuilderDemo extends StatelessWidget {

animations/lib/src/basics/04_tweens.dart

@@ -1,3 +1,7 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
 
 class TweenDemo extends StatefulWidget {

animations/lib/src/basics/05_animated_builder.dart

@@ -1,3 +1,7 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
 
 class AnimatedBuilderDemo extends StatefulWidget {
@@ -32,7 +36,7 @@ class _AnimatedBuilderDemoState extends State<AnimatedBuilderDemo>
       appBar: AppBar(),
       body: Center(
         child: AnimatedBuilder(
-          animation: this.animation,
+          animation: animation,
           builder: (context, child) {
             return MaterialButton(
               color: animation.value,

animations/lib/src/basics/06_custom_tween.dart

@@ -1,3 +1,7 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
 
 class TypewriterTween extends Tween<String> {
@@ -26,11 +30,7 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
   void initState() {
     super.initState();
 
-    controller = AnimationController(vsync: this, duration: _duration)
-      ..addListener(() {
-        // Marks the widget tree as dirty
-        setState(() {});
-      });
+    controller = AnimationController(vsync: this, duration: _duration);
     animation = TypewriterTween(end: message).animate(controller);
   }
 
@@ -70,9 +70,14 @@ class _CustomTweenDemoState extends State<CustomTweenDemo>
               child: Card(
                 child: Container(
                   padding: EdgeInsets.all(8.0),
-                  child: Text('${animation.value}',
-                      style:
-                          TextStyle(fontSize: 16, fontFamily: 'SpecialElite')),
+                  child: AnimatedBuilder(
+                    animation: animation,
+                    builder: (context, child) {
+                      return Text('${animation.value}',
+                          style: TextStyle(
+                              fontSize: 16, fontFamily: 'SpecialElite'));
+                    },
+                  ),
                 ),
               ),
             ),

animations/lib/src/basics/07_tween_sequence.dart

@@ -0,0 +1,74 @@
+// Copyright 2019 The Flutter team. 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/material.dart';
+
+class TweenSequenceDemo extends StatefulWidget {
+  static const String routeName = '/basics/chaining_tweens';
+
+  @override
+  _TweenSequenceDemoState createState() => _TweenSequenceDemoState();
+}
+
+class _TweenSequenceDemoState extends State<TweenSequenceDemo>
+    with SingleTickerProviderStateMixin {
+  static const Duration duration = Duration(seconds: 3);
+  AnimationController controller;
+  Animation<Color> animation;
+
+  static final colors = [
+    Colors.red,
+    Colors.orange,
+    Colors.yellow,
+    Colors.green,
+    Colors.blue,
+    Colors.indigo,
+    Colors.purple,
+  ];
+
+  void initState() {
+    super.initState();
+
+    final sequenceItems = <TweenSequenceItem<Color>>[];
+
+    for (var i = 0; i < colors.length; i++) {
+      final beginColor = colors[i];
+      final endColor = colors[(i + 1) % colors.length];
+      final weight = 1 / colors.length;
+
+      sequenceItems.add(
+        TweenSequenceItem<Color>(
+          tween: ColorTween(begin: beginColor, end: endColor),
+          weight: weight,
+        ),
+      );
+    }
+
+    controller = AnimationController(duration: duration, vsync: this);
+    animation = TweenSequence<Color>(sequenceItems).animate(controller);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: AnimatedBuilder(
+          animation: animation,
+          builder: (context, child) {
+            return MaterialButton(
+              color: animation.value,
+              onPressed: () async {
+                await controller.forward();
+                controller.reset();
+              },
+              child: child,
+            );
+          },
+          child: Text('Animate', style: TextStyle(color: Colors.white)),
+        ),
+      ),
+    );
+  }
+}