Add a FlutterView level integration example on Android for add-to-app (#632)

add_to_app/flutter_module_using_plugin/lib/cell.dart

@@ -0,0 +1,136 @@
+// 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';
+import 'package:flutter/services.dart';
+import 'package:sensors/sensors.dart';
+
+// This is on alternate entrypoint for this module to display Flutter UI in
+// a (multi-)view integration scenario.
+void main() {
+  runApp(Cell());
+}
+
+class Cell extends StatefulWidget {
+  @override
+  State<StatefulWidget> createState() => new _CellState();
+}
+
+class _CellState extends State<Cell> with WidgetsBindingObserver {
+  static const double gravity = 9.81;
+  static final AccelerometerEvent defaultPosition = AccelerometerEvent(0, 0, 0);
+
+  int cellNumber = 0;
+  Random _random;
+  AppLifecycleState appLifecycleState;
+
+  @override
+  void initState() {
+    final channel = MethodChannel('dev.flutter.example/cell');
+    channel.setMethodCallHandler((MethodCall call) async {
+      if (call.method == 'setCellNumber') {
+        setState(() {
+          cellNumber = call.arguments as int;
+          _random = Random(cellNumber);
+        });
+      }
+    });
+    // Keep track of what the current platform lifecycle state is.
+    WidgetsBinding.instance.addObserver(this);
+    super.initState();
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    setState(() {
+      appLifecycleState = state;
+    });
+  }
+
+  // Show a random bright color.
+  Color randomLightColor() {
+    _random ??= Random(cellNumber);
+
+    return Color.fromARGB(255, _random.nextInt(50) + 205,
+        _random.nextInt(50) + 205, _random.nextInt(50) + 205);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      // The Flutter cells will be noticeably different (due to background color
+      // and the Flutter logo). The banner breaks immersion.
+      debugShowCheckedModeBanner: false,
+      home: Container(
+        color: Colors.white,
+        child: Builder(
+          builder: (BuildContext context) {
+            return Card(
+              // Mimic the platform Material look.
+              margin: EdgeInsets.symmetric(horizontal: 36, vertical: 24),
+              shape: RoundedRectangleBorder(
+                borderRadius: BorderRadius.circular(16),
+              ),
+              elevation: 16,
+              color: randomLightColor(),
+              child: Stack(
+                children: [
+                  Center(
+                    child: Column(
+                      mainAxisAlignment: MainAxisAlignment.center,
+                      children: [
+                        Text(
+                          // Show a number provided by the platform based on
+                          // the cell's index.
+                          cellNumber.toString(),
+                          style: Theme.of(context).textTheme.headline3,
+                        ),
+                      ],
+                    ),
+                  ),
+                  Positioned(
+                    left: 42,
+                    top: 0,
+                    bottom: 0,
+                    child: Opacity(
+                      opacity: 0.2,
+                      child: StreamBuilder(
+                        // Don't continuously rebuild for nothing when the
+                        // cell isn't visible.
+                        stream: appLifecycleState == AppLifecycleState.resumed
+                            ? accelerometerEvents
+                            : Stream.value(defaultPosition),
+                        initialData: defaultPosition,
+                        builder: (BuildContext context,
+                            AsyncSnapshot<AccelerometerEvent> snapshot) {
+                          return Transform(
+                              // Figure out the phone's orientation relative
+                              // to gravity's direction. Ignore the z vector.
+                              transform: Matrix4.rotationX(
+                                  snapshot.data.y / gravity * pi / 2)
+                                ..multiply(Matrix4.rotationY(
+                                    snapshot.data.x / gravity * pi / 2)),
+                              alignment: Alignment.center,
+                              child: FlutterLogo(size: 72));
+                        },
+                      ),
+                    ),
+                  ),
+                ],
+              ),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}