More restoration documentation (#63438)

dev/snippets/config/templates/README.md

@@ -87,6 +87,11 @@ the widgets library.
   `stateful_widget` template, with the addition of the `TickerProviderStateMixin`
   class, enabling easy generation of animated samples.
 
+- [`stateful_widget_restoration`](stateful_widget_restoration.tmpl) : Similar to
+  the `stateful_widget` template, but the widget also imports `RestorationMixin`
+  and has a `restorationId` field which it uses to implement the `restorationId`
+  getter on the `State`.
+
 - [`stateless_widget`](stateless_widget.tmpl) : Identical to the
   `stateful_widget` template, except that the default code block is
   inserted as a method (which should be the `build` method) in a

dev/snippets/config/templates/stateful_widget_restoration.tmpl

@@ -0,0 +1,46 @@
+/// Flutter code sample for {{element}}
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This is the main application widget.
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return WidgetsApp(
+      title: 'Flutter Code Sample',
+      home: Center(
+        child: MyStatefulWidget(restorationId: 'main'),
+      ),
+      color: const Color(0xffffffff),
+    );
+  }
+}
+
+{{code-preamble}}
+
+/// This is the stateful widget that the main application instantiates.
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key, this.restorationId}) : super(key: key);
+
+  final String restorationId;
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+/// This is the private State class that goes with MyStatefulWidget.
+/// RestorationProperty objects can be used because of RestorationMixin.
+class _MyStatefulWidgetState extends State<MyStatefulWidget> with RestorationMixin {
+  // In this example, the restoration ID for the mixin is passed in through
+  // the [StatefulWidget]'s constructor.
+  @override
+  String get restorationId => widget.restorationId;
+
+{{code}}
+}