flutter在移动端开发领域凭借其一处代码，多端运行的特点，备受关注。但是目前来说，市面上的app功能复杂多变，因此也很难见到纯flutter的项目，其中会夹杂着很多native的插件及相关代码。因此，了解flutter与native端是如何建立数据通信的对于开发而言至关重要。下面将以集成百度语音识别的SDK到flutter项目中为例，介绍如何实现两端通信。

插件代码编写

首先在百度AI开放平台上下载SDK，同时，在本地项目中用android studio打开android目录，新建asr_plugin文件夹存放与插件相关的代码。之后按需提取相关代码到该目录下。具体教程可以参考官网，因为此处主要讲述如何实现通信，设计功能开发类的部分简单跳过。

为了能让我们的包被本地识别，需要设置asr_plugin Module下的build.gradle，

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {localPropertiesFile.withReader('UTF-8') { reader ->localProperties.load(reader)}
}
def flutterRoot = localProperties.getProperty('flutter.sdk')
...
flutter {source '../..'
}

MethodChannel使用

android端

c++层通过调用flutterJNI的handlePlatformMessage方法将消息传递给java层，在java层通过调用handleMessageFormDart

public void handleMessageFromDart(final String channel, byte[] message, final int replyId) {FlutterNativeView.this.assertAttached();BinaryMessageHandler handler = (BinaryMessageHandler)FlutterNativeView.this.mMessageHandlers.get(channel);if (handler != null) {try {ByteBuffer buffer = message == null ? null : ByteBuffer.wrap(message);handler.onMessage(buffer, new BinaryReply() {private final AtomicBoolean done = new AtomicBoolean(false);public void reply(ByteBuffer reply) {if (!FlutterNativeView.this.isAttached()) {Log.d("FlutterNativeView", "handleMessageFromDart replying ot a detached view, channel=" + channel);} else if (this.done.getAndSet(true)) {throw new IllegalStateException("Reply already submitted");} else {if (reply == null) {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);} else {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageResponseCallback(replyId, reply, reply.position());}}}});} catch (Exception var6) {Log.e("FlutterNativeView", "Uncaught exception in binary message listener", var6);FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);}} else {FlutterNativeView.this.mFlutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);}
}

在MainActivity.java中进行设置，注册我们的自定义插件到activity身上。（注： flutter升级之后，很多搜到的代码均是使用的== onCreate 进行注册，目前已不再兼容。需使用 configureFlutterEngine ==进行设置。）

//channel的名称，由于app中可能会有多个channel，这个名称需要在app内是唯一的。private static final String CHANNEL = "flutter_asr_plugin";private MethodChannel mothodChannel;private static String TAG = "MainActivity";@Overridepublic void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {//这里的CHANNEL即是我们在dart端定义的名称，需保持一致mothodChannel = new MethodChannel(flutterEngine.getDartExecutor(), CHANNEL);ShimPluginRegistry shimPluginRegistry = new ShimPluginRegistry(flutterEngine);FlutterWebviewPlugin.registerWith(shimPluginRegistry.registrarFor("com.flutter_webview_plugin.FlutterWebviewPlugin"));mothodChannel.setMethodCallHandler((methodCall, result) -> {//call.method是获取调用的方法名字//call.arguments 是获取参数});}

如下（可参考已导入的第三方插件注册过程）
MainActivity.java

@Overridepublic void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {Log.i(TAG, "start");ShimPluginRegistry shimPluginRegistry = new ShimPluginRegistry(flutterEngine);FlutterWebviewPlugin.registerWith(shimPluginRegistry.registrarFor("com.flutter_webview_plugin.FlutterWebviewPlugin"));AsrPlugin.registerWith(shimPluginRegistry.registrarFor("flutter_asr_plugin"));}

AsrPlugin.java

  //做旧版本兼容public static void registerWith(PluginRegistry.Registrar registrar) {if (registrar.activity() != null) {channel = new MethodChannel(registrar.messenger(), CHANNEL_NAME);final AsrPlugin instance = new AsrPlugin(registrar.activity());registrar.addActivityResultListener(instance);channel.setMethodCallHandler(instance);}}

flutter端

首先，在dart端创建一个MethodChannel对象，定义其名称为flutter_asr_plugin，这个名字需要与android端保持一致，方便之后调用。之后，我们为其定义一些与native通信的相关接口，在dart端通过调用这些接口来触发android端SDK的相对应的功能。

class AsrManager {//MethodChannel flutter与native通信的通道static const MethodChannel _channel = const MethodChannel('flutter_asr_plugin');/// 定义一些与native通信的接口/// 开始录音 --- start回调中可以拿到语音识别返回的数据static Future<String> start({Map params}) async {return await _channel.invokeMethod('start', params ?? {});}/// 停止录音static Future<String> stop() async {return await _channel.invokeMethod('stop');}/// 取消录音static Future<String> cancel() async {return await _channel.invokeMethod('cancel');}
}

这里使用了invokeMethod这个方法，具体实现如下：

//method 为方法名；arguments 为相关的传入参数
Future<T> _invokeMethod<T>(String method, { bool missingOk, dynamic arguments }) async {assert(method != null);//使用binaryMessenger.send这个方法传递给android端final ByteData result = await binaryMessenger.send(name,//使用codec对根据方法名和参数构建的MethodCall对象进行编码得到的对象codec.encodeMethodCall(MethodCall(method, arguments)),);if (result == null) {if (missingOk) {return null;}//抛出插件不存在的错误throw MissingPluginException('No implementation found for method $method on channel $name');}return codec.decodeEnvelope(result) as T;}

最终Dart本地接口方法==_sendPlatformMessage使用的是ui.window.sendPlatformMessage==来启用C++层进行数据传递。

Future<ByteData> _sendPlatformMessage(String channel, ByteData message) {final Completer<ByteData> completer = Completer<ByteData>();// ui.window is accessed directly instead of using ServicesBinding.instance.window// because this method might be invoked before any binding is initialized.// This issue was reported in #27541. It is not ideal to statically access// ui.window because the Window may be dependency injected elsewhere with// a different instance. However, static access at this location seems to be// the least bad option.ui.window.sendPlatformMessage(channel, message, (ByteData reply) {try {completer.complete(reply);} catch (exception, stack) {FlutterError.reportError(FlutterErrorDetails(exception: exception,stack: stack,library: 'services library',context: ErrorDescription('during a platform message response callback'),));}});return completer.future;}

踩坑

• W/FlutterJNI(18011): Tried to send a platform message to Flutter, but FlutterJNI was detached from native C++. Could not send. Channel: xyz.luan/audioplayers. Response ID: 0
  在flutter启用真机调试时发生，与native通信失败。
• Error connecting to the service protocol: failed to connect to http://127.0.0.1:61203
  连接模拟器出错，重启模拟器或调整网络在同一个局域网内
• cannot resolve symbol MethodChannel
  相关插件未注册成功，build.gradle建议修改
• error: cannot find symbol new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL)
• Plugin key com.example.plugin.asr.AsrPlugin is already in use
  register注册插件时，多次注册
• PlatformException(No recognition result match
  语音识别未返回内容

至此，也就完成了flutter与native基于MethodChannel上的通信。