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');
????setState(() {
??????i++;
????});
??}


??@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")
        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');
????setState(() {
??????i++;
????});
??}

??@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")

   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)
    }
}