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

add_to_app/flutter_module_books/lib/api.dart

@@ -0,0 +1,100 @@
+// Autogenerated from Pigeon (v0.1.0), do not edit directly.
+// See also: https://pub.dev/packages/pigeon
+// ignore_for_file: public_member_api_docs, non_constant_identifier_names, avoid_as, unused_import
+import 'dart:async';
+import 'package:flutter/services.dart';
+
+class Book {
+  String title;
+  String subtitle;
+  String author;
+  String description;
+  String publishDate;
+  int pageCount;
+  // ignore: unused_element
+  Map<dynamic, dynamic> _toMap() {
+    final Map<dynamic, dynamic> pigeonMap = <dynamic, dynamic>{};
+    pigeonMap['title'] = title;
+    pigeonMap['subtitle'] = subtitle;
+    pigeonMap['author'] = author;
+    pigeonMap['description'] = description;
+    pigeonMap['publishDate'] = publishDate;
+    pigeonMap['pageCount'] = pageCount;
+    return pigeonMap;
+  }
+
+  // ignore: unused_element
+  static Book _fromMap(Map<dynamic, dynamic> pigeonMap) {
+    final Book result = Book();
+    result.title = pigeonMap['title'];
+    result.subtitle = pigeonMap['subtitle'];
+    result.author = pigeonMap['author'];
+    result.description = pigeonMap['description'];
+    result.publishDate = pigeonMap['publishDate'];
+    result.pageCount = pigeonMap['pageCount'];
+    return result;
+  }
+}
+
+abstract class FlutterBookApi {
+  void displayBookDetails(Book arg);
+  static void setup(FlutterBookApi api) {
+    {
+      const BasicMessageChannel<dynamic> channel = BasicMessageChannel<dynamic>(
+          'dev.flutter.pigeon.FlutterBookApi.displayBookDetails',
+          StandardMessageCodec());
+      channel.setMessageHandler((dynamic message) async {
+        final Map<dynamic, dynamic> mapMessage =
+            message as Map<dynamic, dynamic>;
+        final Book input = Book._fromMap(mapMessage);
+        api.displayBookDetails(input);
+      });
+    }
+  }
+}
+
+class HostBookApi {
+  Future<void> cancel() async {
+    const BasicMessageChannel<dynamic> channel = BasicMessageChannel<dynamic>(
+        'dev.flutter.pigeon.HostBookApi.cancel', StandardMessageCodec());
+
+    final Map<dynamic, dynamic> replyMap = await channel.send(null);
+    if (replyMap == null) {
+      throw PlatformException(
+          code: 'channel-error',
+          message: 'Unable to establish connection on channel.',
+          details: null);
+    } else if (replyMap['error'] != null) {
+      final Map<dynamic, dynamic> error = replyMap['error'];
+      throw PlatformException(
+          code: error['code'],
+          message: error['message'],
+          details: error['details']);
+    } else {
+      // noop
+    }
+  }
+
+  Future<void> finishEditingBook(Book arg) async {
+    final Map<dynamic, dynamic> requestMap = arg._toMap();
+    const BasicMessageChannel<dynamic> channel = BasicMessageChannel<dynamic>(
+        'dev.flutter.pigeon.HostBookApi.finishEditingBook',
+        StandardMessageCodec());
+
+    final Map<dynamic, dynamic> replyMap = await channel.send(requestMap);
+    if (replyMap == null) {
+      throw PlatformException(
+          code: 'channel-error',
+          message: 'Unable to establish connection on channel.',
+          details: null);
+    } else if (replyMap['error'] != null) {
+      final Map<dynamic, dynamic> error = replyMap['error'];
+      throw PlatformException(
+          code: error['code'],
+          message: error['message'],
+          details: error['details']);
+    } else {
+      // noop
+    }
+  }
+}