Polish HostMessages (#4515)

Cleans up a few style nits in hello_services and adds support for automatic
JSON encoding and decoding to the HostMessages interface.

examples/hello_services/ios/HelloServices/AppDelegate.h

@@ -6,7 +6,6 @@
 
 @interface AppDelegate : UIResponder <UIApplicationDelegate>
 
-@property (strong, nonatomic) UIWindow *window;
+@property(strong, nonatomic) UIWindow *window;
 
 @end
-

examples/hello_services/ios/HelloServices/AppDelegate.m

@@ -14,13 +14,13 @@
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
     FlutterDartProject* project = [[FlutterDartProject alloc] initFromDefaultSourceForConfiguration];
     self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
-    FlutterViewController* controller = [[FlutterViewController alloc] initWithProject:project
-                                                                               nibName:nil
-                                                                                bundle:nil];
+    FlutterViewController* flutterController = [[FlutterViewController alloc] initWithProject:project
+                                                                                      nibName:nil
+                                                                                       bundle:nil];
     _locationProvider = [[LocationProvider alloc] init];
-    [controller addMessageListener: _locationProvider];
+    [flutterController addMessageListener:_locationProvider];
 
-    self.window.rootViewController = controller;
+    self.window.rootViewController = flutterController;
     [self.window makeKeyAndVisible];
     return YES;
 }

examples/hello_services/lib/main.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:math';
 
 import 'package:flutter/material.dart';
@@ -11,15 +10,13 @@ import 'package:flutter/services.dart';
 
 final Random random = new Random();
 
-Future<String> handleGetRandom(String json) async {
-  Map<String, dynamic> message = JSON.decode(json);
-  double min = message['min'].toDouble();
-  double max = message['max'].toDouble();
+Future<dynamic> handleGetRandom(Map<String, dynamic> message) async {
+  final double min = message['min'].toDouble();
+  final double max = message['max'].toDouble();
 
-  double value = (random.nextDouble() * (max - min)) + min;
-
-  Map<String, double> reply = <String, double>{'value': value};
-  return JSON.encode(reply);
+  return <String, double>{
+    'value': (random.nextDouble() * (max - min)) + min
+  };
 }
 
 class HelloServices extends StatefulWidget {
@@ -50,14 +47,14 @@ class _HelloServicesState extends State<HelloServices> {
     );
   }
 
-  void _getLocation() {
-    Map<String, String> message = <String, String>{'provider': 'network'};
-    HostMessages.sendToHost('getLocation', JSON.encode(message))
-        .then(_onReceivedLocation);
-  }
-
-  void _onReceivedLocation(String json) {
-    Map<String, num> reply = JSON.decode(json);
+  Future<Null> _getLocation() async {
+    final Map<String, String> message = <String, String>{'provider': 'network'};
+    final Map<String, dynamic> reply = await HostMessages.sendJSON('getLocation', message);
+    // If the widget was removed from the tree while the message was in flight,
+    // we want to discard the reply rather than calling setState to update our
+    // non-existant appearance.
+    if (!mounted)
+      return;
     setState(() {
       _latitude = reply['latitude'].toDouble();
       _longitude = reply['longitude'].toDouble();
@@ -68,5 +65,5 @@ class _HelloServicesState extends State<HelloServices> {
 void main() {
   runApp(new HelloServices());
 
-  HostMessages.addMessageHandler('getRandom', handleGetRandom);
+  HostMessages.addJSONMessageHandler('getRandom', handleGetRandom);
 }

packages/flutter/lib/services.dart

@@ -15,11 +15,11 @@
 library services;
 
 export 'src/services/activity.dart';
-export 'src/services/app_messages.dart';
 export 'src/services/asset_bundle.dart';
 export 'src/services/binding.dart';
 export 'src/services/clipboard.dart';
 export 'src/services/haptic_feedback.dart';
+export 'src/services/host_messages.dart';
 export 'src/services/image_cache.dart';
 export 'src/services/image_decoder.dart';
 export 'src/services/image_resource.dart';

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

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 
 import 'package:mojo/core.dart' as core;
 import 'package:sky_services/flutter/platform/app_messages.mojom.dart' as mojom;
@@ -48,13 +49,28 @@ final _ApplicationMessagesImpl _appMessages = new _ApplicationMessagesImpl();
 /// Flutter framework to exchange application-specific messages.
 class HostMessages {
   /// Send a message to the host application.
-  static Future<String> sendToHost(String messageName, String message) async {
+  static Future<String> sendToHost(String messageName, [String message = '']) async {
     return (await _hostAppMessagesProxy.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.
+  /// Sends a JSON-encoded message to the host application and JSON-decodes the response.
+  static Future<dynamic> sendJSON(String messageName, [dynamic json]) async {
+    return JSON.decode((await _hostAppMessagesProxy.sendString(messageName, JSON.encode(json))).reply);
+  }
+
+  /// Register a callback for receiving messages from the host application.
   static void addMessageHandler(String messageName, HostMessageCallback callback) {
     _appMessages.handlers[messageName] = callback;
   }
+
+  /// Register a callback for receiving JSON messages from the host application.
+  ///
+  /// Messages received from the host application are decoded as JSON before
+  /// being passed to `callback`. The result of the callback is encoded as JSON
+  /// before being returned to the host application.
+  static void addJSONMessageHandler(String messageName, Future<dynamic> callback(dynamic json)) {
+    _appMessages.handlers[messageName] = (String message) async {
+      return JSON.encode(await callback(JSON.decode(message)));
+    };
+  }
 }