1. 前言

在文章Flutter框架分析（八）-Platform Channel中，我们分析了BasicMessageChannel的原理和结构，并详细讲解了与其相关的一些核心类，例如MessageHandler和MessageCodec等，本文主要讲解使用BasicMessageChannel的示例。

2. 使用流程

BasicMessageChannel可用于Flutter给native发消息，也可用于native给Flutter发消息，所以接下来将分别分析这两种使用流程。

2.1 Flutter给native发消息

流程如下：

1）native端创建某channel name的BasicMessageChannel。

2）native端使用setMessageHandler函数，设置该BasicMessageChannel的MessageHandler。

3）Flutter端创建该channel name的BasicMessageChannel。

4）Flutter端使用该BasicMessageChannel通过send函数向native端发送消息。

5）native端刚刚注册的MessageHandler收到发送的消息，在onMessage中处理消息，通过reply函数进行回复。

6）Flutter端处理该回复。

Flutter端关键代码如下：

class _MessageChannelState extends State<MessageChannelWidget> {
  static const _channel = BasicMessageChannel('flutter2/testmessagechannel', JSONMessageCodec());
  int i = 0;

  void _sendMessage() async {
    final String reply = await _channel.send('Hello World i: $i');
    print('MessageChannelTest in dart $reply');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("message channel test"),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$i',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed:_sendMessage,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

native端关键代码如下：

class MessageChannelActivity: FlutterActivity() {

    companion object {
        fun startActivity(activity: Activity) {
            val intent = Intent(activity, MessageChannelActivity::class.java)
            activity.startActivity(intent)
        }
    }

    override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {

        Log.d("MessageChannelTest", "MessageChannelActivity configureFlutterEngine")

//        GeneratedPluginRegistrant.registerWith(flutterEngine)

        val channel = BasicMessageChannel(
                flutterEngine.dartExecutor.binaryMessenger,
                "flutter2/testmessagechannel",
            JSONMessageCodec.INSTANCE)

        // Receive messages from Dart
        channel.setMessageHandler { message, reply ->
            Log.d("MessageChannelTest", "in android Received message = $message")
            reply.reply("Reply from Android")
        }
    }
}

2.2 native给Flutter发消息

流程如下：

1） Flutter端创建该channel name的BasicMessageChannel。

2） Flutter端使用setMessageHandler函数，设置该BasicMessageChannel的Handler函数。

3） native端创建某channel name的BasicMessageChannel。

4） native端使用该BasicMessageChannel通过send函数向Flutter端发送消息。

5） Flutter端刚刚注册的Handler收到发送的消息，并处理消息，然后通过reply函数进行回复。

6） native端处理该回复。

Flutter端关键代码如下：

class _MessageChannelState extends State<MessageChannelWidget> {
  static const _channel = BasicMessageChannel('flutter2/testmessagechannel', JSONMessageCodec());
  int i = 0;

  void _sendMessage() async {
    final String reply = await _channel.send('Hello World i: $i');
    print('MessageChannelTest in dart $reply');
  }


  @override
  void initState() {
    // Receive messages from platform
    _channel.setMessageHandler((dynamic message) async {
      print('MessageChannelTest in dart Received message = $message');
      return 'Reply from Dart';
    });
    super.initState();
  }
}

native端关键代码如下：

class MessageChannelActivity: FlutterActivity() {

    companion object {
        fun startActivity(activity: Activity) {
            val intent = Intent(activity, MessageChannelActivity::class.java)
            activity.startActivity(intent)
        }
    }

    override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {

        Log.d("MessageChannelTest", "MessageChannelActivity configureFlutterEngine")

//        GeneratedPluginRegistrant.registerWith(flutterEngine)

        val channel = BasicMessageChannel(
                flutterEngine.dartExecutor.binaryMessenger,
                "flutter2/testmessagechannel",
            JSONMessageCodec.INSTANCE)

        // Send message to Dart
        Handler().postDelayed({
            channel.send("Hello World from Android") { reply ->
                Log.d("MessageChannelTest", "in android $reply")
            }
        }, 500)
    }
}

3. 小结

本文主要介绍了BasicMessageChannel的使用流程，并列举了一个使用BasicMessageChannel的示例。