Add a pigeon sample that demonstrates a "realistic" integration scenario with middleware and business logic (#465)

add_to_app/flutter_module_books/lib/main.dart

@@ -0,0 +1,201 @@
+// Copyright 2020 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_module_books/api.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      theme: ThemeData(
+        primaryColor: const Color(0xff6200ee),
+      ),
+      home: BookDetail(),
+    );
+  }
+}
+
+typedef BookReceived = void Function(Book book);
+
+class FlutterBookApiHandler extends FlutterBookApi {
+  FlutterBookApiHandler(this.callback);
+
+  final BookReceived callback;
+
+  @override
+  void displayBookDetails(Book book) {
+    assert(
+      book != null,
+      'Non-null book expected from FlutterBookApi.displayBookDetails call.',
+    );
+    callback(book);
+  }
+}
+
+class BookDetail extends StatefulWidget {
+  const BookDetail({this.hostApi, this.flutterApi});
+
+  // These are the outgoing and incoming APIs that are here for injection for
+  // tests.
+  final HostBookApi hostApi;
+  final FlutterBookApi flutterApi;
+
+  @override
+  _BookDetailState createState() => _BookDetailState();
+}
+
+class _BookDetailState extends State<BookDetail> {
+  Book book;
+
+  HostBookApi hostApi;
+
+  FocusNode textFocusNode = FocusNode();
+  TextEditingController titleTextController = TextEditingController();
+  TextEditingController subtitleTextController = TextEditingController();
+  TextEditingController authorTextController = TextEditingController();
+
+  @override
+  void initState() {
+    super.initState();
+
+    // This `HostBookApi` class instance lets us make outgoing calls to the
+    // platform.
+    hostApi = widget.hostApi ?? HostBookApi();
+
+    // Registering this `FlutterBookApiHandler` class lets us receive incoming
+    // calls from the platform.
+    // TODO(gaaclarke): make the setup method an instance method so it's
+    // injectable https://github.com/flutter/flutter/issues/59119.
+    FlutterBookApi.setup(FlutterBookApiHandler(
+        // The `FlutterBookApi` just has one method. Just give a closure for that
+        // method to the handler class.
+        (Book book) {
+      setState(() {
+        // This book model is what we're going to return to Kotlin eventually.
+        // Keep it bound to the UI.
+        this.book = book;
+        titleTextController.text = book.title;
+        titleTextController.addListener(() {
+          this.book?.title = titleTextController.text;
+        });
+        // Subtitle could be null.
+        // TODO(gaaclarke): https://github.com/flutter/flutter/issues/59118.
+        subtitleTextController.text = book.subtitle ?? '';
+        subtitleTextController.addListener(() {
+          this.book?.subtitle = subtitleTextController.text;
+        });
+        authorTextController.text = book.author;
+        authorTextController.addListener(() {
+          this.book?.author = authorTextController.text;
+        });
+      });
+    }));
+  }
+
+  void clear() {
+    book = null;
+    // Keep focus if going to the home screen but unfocus if leaving
+    // the activity.
+    textFocusNode.unfocus();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Book Details'),
+        leading: IconButton(
+          icon: Icon(Icons.clear),
+          // Pressing clear cancels the edit and leaves the activity without
+          // modification.
+          onPressed: () {
+            hostApi.cancel();
+            clear();
+          },
+        ),
+        actions: [
+          IconButton(
+            icon: Icon(Icons.check),
+            // Pressing save sends the updated book to the platform.
+            onPressed: () {
+              hostApi.finishEditingBook(book);
+              clear();
+            },
+          ),
+        ],
+      ),
+      body: book == null
+          // Draw a spinner until the platform gives us the book to show details
+          // for.
+          ? Center(child: CircularProgressIndicator())
+          : Focus(
+              focusNode: textFocusNode,
+              child: ListView(
+                padding: EdgeInsets.all(24),
+                children: [
+                  TextField(
+                    controller: titleTextController,
+                    decoration: InputDecoration(
+                      border: OutlineInputBorder(),
+                      filled: true,
+                      hintText: "Title",
+                      labelText: "Title",
+                    ),
+                  ),
+                  SizedBox(height: 24),
+                  TextField(
+                    controller: subtitleTextController,
+                    maxLines: 2,
+                    decoration: InputDecoration(
+                      border: OutlineInputBorder(),
+                      filled: true,
+                      hintText: "Subtitle",
+                      labelText: "Subtitle",
+                    ),
+                  ),
+                  SizedBox(height: 24),
+                  TextField(
+                    controller: authorTextController,
+                    decoration: InputDecoration(
+                      border: OutlineInputBorder(),
+                      filled: true,
+                      hintText: "Author",
+                      labelText: "Author",
+                    ),
+                  ),
+                  SizedBox(height: 32),
+                  Divider(),
+                  Center(
+                    child: Padding(
+                      padding: const EdgeInsets.all(8.0),
+                      child: Text(
+                          '${book.pageCount} pages  ~  published ${book.publishDate}'),
+                    ),
+                  ),
+                  Divider(),
+                  SizedBox(height: 32),
+                  Center(
+                    child: Text(
+                      'BOOK DESCRIPTION',
+                      style: TextStyle(
+                        fontSize: 15,
+                        fontWeight: FontWeight.bold,
+                        decoration: TextDecoration.underline,
+                      ),
+                    ),
+                  ),
+                  SizedBox(height: 12),
+                  Text(
+                    book.description,
+                    style: TextStyle(color: Colors.grey.shade600, height: 1.24),
+                  ),
+                ],
+              ),
+            ),
+    );
+  }
+}