Add sample for InheritedNotifier, convert two others to DartPa… (#52349)

This adds a sample for InheritedNotifier, and converts a couple of other samples to be DartPad samples. I also added a new sample template stateful_widget_material_ticker, which adds a TickerProviderStateMixin to the state object so that animation controllers can be created there easily.

dev/snippets/config/templates/stateful_widget_material_ticker.tmpl

+// Flutter code sample for {{element}}
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This Widget is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: MyStatefulWidget(),
+    );
+  }
+}
+
+{{code-preamble}}
+
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with TickerProviderStateMixin {
+  {{code}}
+}

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -38,9 +38,10 @@ import 'observer_list.dart';
 ///
 ///  * [AnimatedBuilder], a widget that uses a builder callback to rebuild
 ///    whenever a given [Listenable] triggers its notifications. This widget is
-///    commonly used with [Animation] subclasses, wherein its name. It is a
-///    subclass of [AnimatedWidget], which can be used to create widgets that
-///    are driven from a [Listenable].
+///    commonly used with [Animation] subclasses, hence its name, but is by no
+///    means limited to animations, as it can be used with any [Listenable]. It
+///    is a subclass of [AnimatedWidget], which can be used to create widgets
+///    that are driven from a [Listenable].
 ///  * [ValueListenableBuilder], a widget that uses a builder callback to
 ///    rebuild whenever a [ValueListenable] object triggers its notifications,
 ///    providing the builder with the value of the object.

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

@@ -28,6 +28,93 @@ import 'framework.dart';
 /// changed. When it returns true, the dependents are marked as needing to be
 /// rebuilt this frame.
 ///
+/// {@tool dartpad --template=stateful_widget_material_ticker}
+///
+/// This example shows three spinning squares that use the value of the notifier
+/// on an ancestor [InheritedNotifier] (`SpinModel`) to give them their
+/// rotation. The [InheritedNotifier] doesn't need to know about the children,
+/// and the `notifier` argument doesn't need to be an animation controller, it
+/// can be anything that implements [Listenable] (like a [ChangeNotifier]).
+///
+/// The `SpinModel` class could just as easily listen to another object (say, a
+/// separate object that keeps the value of an input or data model value) that
+/// is a [Listenable], and get the value from that. The descendants also don't
+/// need to have an instance of the [InheritedNotifier] in order to use it, they
+/// just need to know that there is one in their ancestry. This can help with
+/// decoupling widgets from their models.
+///
+/// ```dart imports
+/// import 'dart:math' as math;
+/// ```
+///
+/// ```dart preamble
+/// class SpinModel extends InheritedNotifier<AnimationController> {
+///   SpinModel({
+///     Key key,
+///     AnimationController notifier,
+///     Widget child,
+///   }) : super(key: key, notifier: notifier, child: child);
+///
+///   static double of(BuildContext context) {
+///     return context.dependOnInheritedWidgetOfExactType<SpinModel>().notifier.value;
+///   }
+/// }
+///
+/// class Spinner extends StatelessWidget {
+///   const Spinner();
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Transform.rotate(
+///       angle: SpinModel.of(context) * 2.0 * math.pi,
+///       child: Container(
+///         width: 100,
+///         height: 100,
+///         color: Colors.green,
+///         child: const Center(
+///           child: Text('Whee!'),
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+///
+/// ```dart
+/// AnimationController _controller;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 10),
+///     vsync: this,
+///   )..repeat();
+/// }
+///
+/// @override
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return SpinModel(
+///     notifier: _controller,
+///     child: Row(
+///       mainAxisAlignment: MainAxisAlignment.spaceAround,
+///       children: const <Widget>[
+///         Spinner(),
+///         Spinner(),
+///         Spinner(),
+///       ],
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [Animation], an implementation of [Listenable] that ticks each frame to

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

@@ -23,41 +23,16 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 /// [AnimatedWidget] is most useful for widgets that are otherwise stateless. To
 /// use [AnimatedWidget], simply subclass it and implement the build function.
 ///
-///{@tool snippet}
+///{@tool dartpad --template=stateful_widget_material_ticker}
 ///
 /// This code defines a widget called `Spinner` that spins a green square
 /// continually. It is built with an [AnimatedWidget].
 ///
-/// ```dart
-/// class Spinner extends StatefulWidget {
-///   @override
-///   _SpinnerState createState() => _SpinnerState();
-/// }
-///
-/// class _SpinnerState extends State<Spinner> with TickerProviderStateMixin {
-///   AnimationController _controller;
-///
-///   @override
-///   void initState() {
-///     super.initState();
-///     _controller = AnimationController(
-///       duration: const Duration(seconds: 10),
-///       vsync: this,
-///     )..repeat();
-///   }
-///
-///   @override
-///   void dispose() {
-///     _controller.dispose();
-///     super.dispose();
-///   }
-///
-///   @override
-///   Widget build(BuildContext context) {
-///     return SpinningContainer(controller: _controller);
-///   }
-/// }
+/// ```dart imports
+/// import 'dart:math' as math;
+/// ```
 ///
+/// ```dart preamble
 /// class SpinningContainer extends AnimatedWidget {
 ///   const SpinningContainer({Key key, AnimationController controller})
 ///       : super(key: key, listenable: controller);
@@ -73,6 +48,30 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 ///   }
 /// }
 /// ```
+///
+/// ```dart
+/// AnimationController _controller;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 10),
+///     vsync: this,
+///   )..repeat();
+/// }
+///
+/// @override
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return SpinningContainer(controller: _controller);
+/// }
+/// ```
 /// {@end-tool}
 ///
 /// For more complex case involving additional state, consider using
@@ -1035,63 +1034,57 @@ class DefaultTextStyleTransition extends AnimatedWidget {
 /// Using this pre-built child is entirely optional, but can improve
 /// performance significantly in some cases and is therefore a good practice.
 ///
-/// {@tool snippet}
+/// {@tool dartpad --template=stateful_widget_material_ticker}
 ///
-/// This code defines a widget called `Spinner` that spins a green square
-/// continually. It is built with an [AnimatedBuilder] and makes use of the
-/// [child] feature to avoid having to rebuild the [Container] each time. The
-/// resulting animation is shown below the code.
+/// This code defines a widget that spins a green square continually. It is
+/// built with an [AnimatedBuilder] and makes use of the [child] feature to
+/// avoid having to rebuild the [Container] each time.
+///
+/// ```dart imports
+/// import 'dart:math' as math;
+/// ```
 ///
 /// ```dart
-/// class Spinner extends StatefulWidget {
-///   @override
-///   _SpinnerState createState() => _SpinnerState();
+/// AnimationController _controller;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 10),
+///     vsync: this,
+///   )..repeat();
 /// }
 ///
-/// class _SpinnerState extends State<Spinner> with SingleTickerProviderStateMixin {
-///   AnimationController _controller;
-///
-///   @override
-///   void initState() {
-///     super.initState();
-///     _controller = AnimationController(
-///       duration: const Duration(seconds: 10),
-///       vsync: this,
-///     )..repeat();
-///   }
-///
-///   @override
-///   void dispose() {
-///     _controller.dispose();
-///     super.dispose();
-///   }
+/// @override
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
 ///
-///   @override
-///   Widget build(BuildContext context) {
-///     return AnimatedBuilder(
-///       animation: _controller,
-///       child: Container(
-///         width: 200.0,
-///         height: 200.0,
-///         color: Colors.green,
-///         child: const Center(
-///           child: Text('Wee'),
-///         ),
+/// @override
+/// Widget build(BuildContext context) {
+///   return AnimatedBuilder(
+///     animation: _controller,
+///     child: Container(
+///       width: 200.0,
+///       height: 200.0,
+///       color: Colors.green,
+///       child: const Center(
+///         child: Text('Whee!'),
 ///       ),
-///       builder: (BuildContext context, Widget child) {
-///         return Transform.rotate(
-///           angle: _controller.value * 2.0 * math.pi,
-///           child: child,
-///         );
-///       },
-///     );
-///   }
+///     ),
+///     builder: (BuildContext context, Widget child) {
+///       return Transform.rotate(
+///         angle: _controller.value * 2.0 * math.pi,
+///         child: child,
+///       );
+///     },
+///   );
 /// }
 /// ```
 /// {@end-tool}
 ///
-/// {@animation 300 300 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_builder.mp4}
-///
 /// See also:
 ///
 ///  * [TweenAnimationBuilder], which animates a property to a target value