Flutter与android之间的通讯

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Platform Channel简介

Flutter引入Platform Channel机制来支持不同平台的API调用。在Flutter中,提供了三种Platform Channel用来支持和平台之间数据的传递:

  • BasicMessageChannel:支持字符串和半结构化的数据传递,可以通过BasicMessageChannel来获取Native项目的图标等资源
  • MethodChannel:支持传递方法调用,Flutter主动调用Native的方法,并获取相应的返回值。既可以从Flutter发平台发起方法调用,也可以从平台代码向Flutter发起调用
  • EventChannel:支持数据流通信,传递事件。收到消息后无法回复此次消息,通常用于Native向Dart的通信

使用方法

BasicMessageChannel

Android端:

BasicMessageChannel mBasicMessageChannel = new BasicMessageChannel(getFlutterView(), "basic_channel", StringCodec.INSTANCE);
mBasicMessageChannel.setMessageHandler(new BasicMessageChannel.MessageHandler() {
    //接受消息
    @Override
    public void onMessage(Object o, BasicMessageChannel.Reply reply) {
        Log.e("basic_channel", "接收到来自flutter的消息:"+o.toString());
        reply.reply("回馈消息");
    }
});
//发送消息
mBasicMessageChannel.send("向flutter发送消息");
//发送消息并接受flutter的回馈
mBasicMessageChannel.send("向flutter发送消息", new BasicMessageChannel.Reply() {
            @Override
            public void reply(Object o) {
                
            }
});

Flutter端:

const basicMessageChannel = const BasicMessageChannel('basic_channel', StringCodec());
//接受并回复消息
basicMessageChannel.setMessageHandler(
      (String message) => Future<String>(() {
            setState(() {
              this.message = message;
            });
            return "回复native消息";
      }),
);
//发送消息
basicMessageChannel.send("来自flutter的message");
//flutter并没有发送并接受回复消息的`send(T message, BasicMessageChannel.Reply<T> callback)`方法

MethodChannel

Android端:

MethodChannel mMethodChannel = new MethodChannel(getFlutterView(), "method_channel");
mMethodChannel.setMethodCallHandler(new MethodChannel.MethodCallHandler() {
    //响应flutter端的调用
    @Override
    public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
        if (methodCall.method.equals("noticeNative")) {
            todo()
            result.success("接受成功");
        }
    }
});
//原生调用flutter
mMethodChannel.invokeMethod("noticeFlutter", "argument", new MethodChannel.Result() {
            @Override
            public void success(Object o) {
                //回调成功
            }
            @Override
            public void error(String s,String s1, Object o) {
                //回调失败
            }
            @Override
            public void notImplemented() {

            }
});

Flutter端:

const methodChannel = const MethodChannel('method_channel');
Future<Null> getMessageFromNative() async {
    //flutter调原生方法
    try {
      //回调成功
      final String result = await methodChannel.invokeMethod('noticeNative');
      setState(() {
        method = result;
      });
    } on PlatformException catch (e) {
      //回调失败
    }
  }
methodChannel.setMethodCallHandler(
      (MethodCall methodCall) => Future<String>(() {
            //响应原生的调用
          if(methodCall.method == "noticeFlutter"){
            setState(() {
              
            });
          }
      }),
); 

EventChannel

Android端:

EventChannel eventChannel = new EventChannel(getFlutterView(),"event_channel");
eventChannel.setStreamHandler(new EventChannel.StreamHandler() {
    @Override
    public void onListen(Object o, EventChannel.EventSink eventSink) {
        eventSink.success("成功");
        //eventSink.error("失败","失败","失败");
    }
    @Override
    public void onCancel(Object o) {
        //取消监听时调用
    }
});

Flutter端:

const eventChannel = const EventChannel('event_channel');
eventChannel.receiveBroadcastStream().listen(_onEvent,onError:_onError);
void _onEvent(Object event) {
    //返回的内容
}
void _onError(Object error) {
    //返回的回调
}

其中:Object args是传递的参数,EventChannel.EventSink eventSink是Native回调Dart时的会回调函数,eventSink提供success、error与endOfStream三个回调方法分别对应事件的不同状态

源码初探

Platform Channel基本结构

首先了解一下这三种Channel的代码:

BasicMessageChannel
class BasicMessageChannel<T> {
  const BasicMessageChannel(this.name, this.codec);
  final String name;
  final MessageCodec<T> codec;
  Future<T> send(T message) async {
    return codec.decodeMessage(await BinaryMessages.send(name, codec.encodeMessage(message)));
  }
  void setMessageHandler(Future<T> handler(T message)) {
    if (handler == null) {
      BinaryMessages.setMessageHandler(name, null);
    } else {
      BinaryMessages.setMessageHandler(name, (ByteData message) async {
        return codec.encodeMessage(await handler(codec.decodeMessage(message)));
      });
    }
  }
  void setMockMessageHandler(Future<T> handler(T message)) {
    if (handler == null) {
      BinaryMessages.setMockMessageHandler(name, null);
    } else {
      BinaryMessages.setMockMessageHandler(name, (ByteData message) async {
        return codec.encodeMessage(await handler(codec.decodeMessage(message)));
      });
    }
  }
}
MethodChannel
class MethodChannel {
  const MethodChannel(this.name, [this.codec = const StandardMethodCodec()]);
  final String name;
  final MethodCodec codec;
  void setMethodCallHandler(Future<dynamic> handler(MethodCall call)) {
    BinaryMessages.setMessageHandler(
      name,
      handler == null ? null : (ByteData message) => _handleAsMethodCall(message, handler),
    );
  }
  void setMockMethodCallHandler(Future<dynamic> handler(MethodCall call)) {
    BinaryMessages.setMockMessageHandler(
      name,
      handler == null ? null : (ByteData message) => _handleAsMethodCall(message, handler),
    );
  }
  Future<ByteData> _handleAsMethodCall(ByteData message, Future<dynamic> handler(MethodCall call)) async {
    final MethodCall call = codec.decodeMethodCall(message);
    try {
      return codec.encodeSuccessEnvelope(await handler(call));
    } on PlatformException catch (e) {
      returun ...
    } on MissingPluginException {
      return null;
    } catch (e) {
      return ...
    }
  }
  Future<T> invokeMethod<T>(String method, [dynamic arguments]) async {
    assert(method != null);
    final ByteData result = await BinaryMessages.send(
      name,
      codec.encodeMethodCall(MethodCall(method, arguments)),
    );
    if (result == null) {
      throw MissingPluginException('No implementation found for method $method on channel $name');
    }
    final T typedResult = codec.decodeEnvelope(result);
    return typedResult;
  }
}
EventChannel
class EventChannel {
  const EventChannel(this.name, [this.codec = const StandardMethodCodec()]);
  final String name;
  final MethodCodec codec;
  Stream<dynamic> receiveBroadcastStream([dynamic arguments]) {
    final MethodChannel methodChannel = MethodChannel(name, codec);
    StreamController<dynamic> controller;
    controller = StreamController<dynamic>.broadcast(onListen: () async {
      BinaryMessages.setMessageHandler(name, (ByteData reply) async {
        ...
      });
      try {
        await methodChannel.invokeMethod<void>('listen', arguments);
      } catch (exception, stack) {
        ...
      }
    }, onCancel: () async {
      BinaryMessages.setMessageHandler(name, null);
      try {
        await methodChannel.invokeMethod<void>('cancel', arguments);
      } catch (exception, stack) {
        ...
      }
    });
    return controller.stream;
  }
}

这三种Channel都有两个成员变量:

  • name:表示Channel名字,用于区分不同Platform Channel的唯一标志,每个Channel使用唯一的name作为其唯一标志
  • codec: 表示消息的编解码器,Flutter采用了二进制字节流作为数据传输协议:发送方需要把数据编码成二进制数据,接受方再把数据解码成原始数据.而负责编解码操作的就是Codec。 每个Channel中都使用到了BinaryMessages,它起到了信使的作用,负责将信息进行跨平台的搬运,是消息发送和接受的工具。
setMessageHandler

在创建好BasicMessageChannel后,让其接受来自另一平台的消息,BinaryMessenger调用它的setMessageHandler方法为其设置一个消息处理器,配合BinaryMessenger完成消息的处理以及回复;

send

在创建好BasicMessageChannel后,可以调用它的send方法向另一个平台传递数据。

setMethodCallHandler

设置用于在此MethodChannel上接收方法调用的回调

receiveBroadcastStream

设置广播流以接收此EventChannel上的事件

Handler

Flutter使用Handler处理Codec解码后的消息。三种Platform Channel相对应,Flutter中也定义了三种Handler:

  • MessageHandler: 用于处理字符串或者半结构化消息,定义在BasicMessageChannel中.
  • MethodCallHandler: 用于处理方法调用,定义在MethodChannel中.
  • StreamHandler: 用于事件流通信,定义在EventChannel中

使用Platform Channel时,需要为其注册一个对应BinaryMessageHandler为其设置对应的Handler。二进制数据会被BinaryMessageHanler进行处理,首先使用Codec进行解码操作,然后再分发给具体Handler进行处理。

结语

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