Provide APIs for exchanging messages with the host application

packages/flutter/lib/services.dart

@@ -12,6 +12,7 @@
 library services;
 
 export 'src/services/activity.dart';
+export 'src/services/app_messages.dart';
 export 'src/services/assertions.dart';
 export 'src/services/asset_bundle.dart';
 export 'src/services/binding.dart';

packages/flutter/lib/src/services/app_messages.dart

+// Copyright 2016 The Chromium Authors. 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:async';
+
+import 'package:flutter/shell.dart';
+import 'package:mojo/core.dart' as core;
+import 'package:sky_services/flutter/platform/app_messages.mojom.dart';
+
+// APIs for exchanging messages with the host application.
+
+ApplicationMessagesProxy _initHostAppMessagesProxy() {
+  ApplicationMessagesProxy proxy = new ApplicationMessagesProxy.unbound();
+  shell.connectToViewAssociatedService(proxy);
+  return proxy;
+}
+
+final ApplicationMessagesProxy _hostAppMessagesProxy = _initHostAppMessagesProxy();
+
+typedef Future<String> HostMessageCallback(String message);
+typedef Object _SendStringResponseFactory(String response);
+
+class _ApplicationMessagesImpl extends ApplicationMessages {
+  final Map<String, HostMessageCallback> handlers = <String, HostMessageCallback>{};
+
+  _ApplicationMessagesImpl() {
+    shell.provideService(ApplicationMessages.serviceName,
+      (core.MojoMessagePipeEndpoint endpoint) {
+        ApplicationMessagesStub stub = new ApplicationMessagesStub.fromEndpoint(endpoint);
+        stub.impl = this;
+      }
+    );
+  }
+
+  @override
+  dynamic sendString(String messageName, String message, [_SendStringResponseFactory responseFactory]) {
+    HostMessageCallback callback = handlers[messageName];
+    if (callback == null)
+      return responseFactory(null);
+
+    return callback(message).then((String s) => responseFactory(s));
+  }
+}
+
+final _ApplicationMessagesImpl _appMessages = new _ApplicationMessagesImpl();
+
+class HostMessages {
+  /// Send a message to the host application.
+  static Future<String> sendToHost(String messageName, String message) async {
+    return (await _hostAppMessagesProxy.ptr.sendString(messageName, message)).reply;
+  }
+
+  /// Register a callback for messages received from the host application.
+  /// The callback function must return a String, Future<String>, or null.
+  static void addMessageHandler(String messageName, HostMessageCallback callback) {
+    _appMessages.handlers[messageName] = callback;
+  }
+}