restructured the add to app samples (#698)

add_to_app/prebuilt_module/flutter_module/lib/main.dart

@@ -0,0 +1,161 @@
+// 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';
+import 'package:flutter/services.dart';
+import 'package:provider/provider.dart';
+
+/// The entrypoint for the flutter module.
+void main() {
+  // This call ensures the Flutter binding has been set up before creating the
+  // MethodChannel-based model.
+  WidgetsFlutterBinding.ensureInitialized();
+
+  final model = CounterModel();
+
+  runApp(
+    ChangeNotifierProvider.value(
+      value: model,
+      child: MyApp(),
+    ),
+  );
+}
+
+/// A simple model that uses a [MethodChannel] as the source of truth for the
+/// state of a counter.
+///
+/// Rather than storing app state data within the Flutter module itself (where
+/// the native portions of the app can't access it), this module passes messages
+/// back to the containing app whenever it needs to increment or retrieve the
+/// value of the counter.
+class CounterModel extends ChangeNotifier {
+  CounterModel() {
+    _channel.setMethodCallHandler(_handleMessage);
+    _channel.invokeMethod('requestCounter');
+  }
+
+  final _channel = MethodChannel('dev.flutter.example/counter');
+
+  int _count = 0;
+
+  int get count => _count;
+
+  void increment() {
+    _channel.invokeMethod('incrementCounter');
+  }
+
+  Future<dynamic> _handleMessage(MethodCall call) async {
+    if (call.method == 'reportCounter') {
+      _count = call.arguments as int;
+      notifyListeners();
+    }
+  }
+}
+
+/// The "app" displayed by this module.
+///
+/// It offers two routes, one suitable for displaying as a full screen and
+/// another designed to be part of a larger UI.class MyApp extends StatelessWidget {
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Module Title',
+      routes: {
+        '/': (context) => FullScreenView(),
+        '/mini': (context) => Contents(),
+      },
+    );
+  }
+}
+
+/// Wraps [Contents] in a Material [Scaffold] so it looks correct when displayed
+/// full-screen.
+class FullScreenView extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Full-screen Flutter'),
+      ),
+      body: const Contents(showExit: true),
+    );
+  }
+}
+
+/// The actual content displayed by the module.
+///
+/// This widget displays info about the state of a counter and how much room (in
+/// logical pixels) it's been given. It also offers buttons to increment the
+/// counter and (optionally) close the Flutter view.
+class Contents extends StatelessWidget {
+  final bool showExit;
+
+  const Contents({this.showExit = false});
+
+  @override
+  Widget build(BuildContext context) {
+    final mediaInfo = MediaQuery.of(context);
+
+    return SizedBox.expand(
+      child: Stack(
+        children: [
+          Positioned.fill(
+            child: DecoratedBox(
+              decoration: BoxDecoration(
+                color: Theme.of(context).scaffoldBackgroundColor,
+              ),
+            ),
+          ),
+          Positioned.fill(
+            child: Opacity(
+              opacity: .25,
+              child: FittedBox(
+                fit: BoxFit.cover,
+                child: FlutterLogo(),
+              ),
+            ),
+          ),
+          Center(
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                Text(
+                  'Window is ${mediaInfo.size.width.toStringAsFixed(1)} x '
+                  '${mediaInfo.size.height.toStringAsFixed(1)}',
+                  style: Theme.of(context).textTheme.headline5,
+                ),
+                SizedBox(height: 16),
+                Consumer<CounterModel>(
+                  builder: (context, model, child) {
+                    return Text(
+                      'Taps: ${model.count}',
+                      style: Theme.of(context).textTheme.headline5,
+                    );
+                  },
+                ),
+                SizedBox(height: 16),
+                Consumer<CounterModel>(
+                  builder: (context, model, child) {
+                    return ElevatedButton(
+                      onPressed: () => model.increment(),
+                      child: Text('Tap me!'),
+                    );
+                  },
+                ),
+                if (showExit) ...[
+                  SizedBox(height: 16),
+                  ElevatedButton(
+                    onPressed: () => SystemNavigator.pop(animated: true),
+                    child: Text('Exit this screen'),
+                  ),
+                ],
+              ],
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}