Platform Channel簡介

Flutter引入Platform Channel機制來支持不同平臺的API調(diào)用。在Flutter中,提供了三種Platform Channel用來支持和平臺之間數(shù)據(jù)的傳遞:

• BasicMessageChannel：支持字符串和半結(jié)構(gòu)化的數(shù)據(jù)傳遞，可以通過BasicMessageChannel來獲取Native項目的圖標等資源
• MethodChannel：支持傳遞方法調(diào)用,Flutter主動調(diào)用Native的方法，并獲取相應(yīng)的返回值。既可以從Flutter發(fā)平臺發(fā)起方法調(diào)用,也可以從平臺代碼向Flutter發(fā)起調(diào)用
• EventChannel：支持數(shù)據(jù)流通信，傳遞事件。收到消息后無法回復(fù)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", "接收到來自flutter的消息:"+o.toString());
        reply.reply("回饋消息");
    }
});
//發(fā)送消息
mBasicMessageChannel.send("向flutter發(fā)送消息");
//發(fā)送消息并接受flutter的回饋
mBasicMessageChannel.send("向flutter發(fā)送消息", new BasicMessageChannel.Reply() {
            @Override
            public void reply(Object o) {
                
            }
});

Flutter端：

const basicMessageChannel = const BasicMessageChannel('basic_channel', StringCodec());
//接受并回復(fù)消息
basicMessageChannel.setMessageHandler(
      (String message) => Future<String>(() {
            setState(() {
              this.message = message;
            });
            return "回復(fù)native消息";
      }),
);
//發(fā)送消息
basicMessageChannel.send("來自flutter的message");
//flutter并沒有發(fā)送并接受回復(fù)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() {
    //響應(yīng)flutter端的調(diào)用
    @Override
    public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
        if (methodCall.method.equals("noticeNative")) {
            todo()
            result.success("接受成功");
        }
    }
});
//原生調(diào)用flutter
mMethodChannel.invokeMethod("noticeFlutter", "argument", new MethodChannel.Result() {
            @Override
            public void success(Object o) {
                //回調(diào)成功
            }
            @Override
            public void error(String s,String s1, Object o) {
                //回調(diào)失敗
            }
            @Override
            public void notImplemented() {

            }
});

Flutter端：

const methodChannel = const MethodChannel('method_channel');
Future<Null> getMessageFromNative() async {
    //flutter調(diào)原生方法
    try {
      //回調(diào)成功
      final String result = await methodChannel.invokeMethod('noticeNative');
      setState(() {
        method = result;
      });
    } on PlatformException catch (e) {
      //回調(diào)失敗
    }
  }
methodChannel.setMethodCallHandler(
      (MethodCall methodCall) => Future<String>(() {
            //響應(yīng)原生的調(diào)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("失敗","失敗","失敗");
    }
    @Override
    public void onCancel(Object o) {
        //取消監(jiān)聽時調(diào)用
    }
});

Flutter端：

const eventChannel = const EventChannel('event_channel');
eventChannel.receiveBroadcastStream().listen(_onEvent,onError:_onError);
void _onEvent(Object event) {
    //返回的內(nèi)容
}
void _onError(Object error) {
    //返回的回調(diào)
}

其中：Object args是傳遞的參數(shù)，EventChannel.EventSink eventSink是Native回調(diào)Dart時的會回調(diào)函數(shù)，eventSink提供success、error與endOfStream三個回調(diào)方法分別對應(yīng)事件的不同狀態(tài)

源碼初探

Platform Channel基本結(jié)構(gòu)

首先了解一下這三種Channel的代碼：

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;
  }
}

這三種Channel都有兩個成員變量：

• name:表示Channel名字,用于區(qū)分不同Platform Channel的唯一標志，每個Channel使用唯一的name作為其唯一標志
• codec: 表示消息的編解碼器,Flutter采用了二進制字節(jié)流作為數(shù)據(jù)傳輸協(xié)議:發(fā)送方需要把數(shù)據(jù)編碼成二進制數(shù)據(jù),接受方再把數(shù)據(jù)解碼成原始數(shù)據(jù).而負責編解碼操作的就是Codec。
  每個Channel中都使用到了BinaryMessages，它起到了信使的作用，負責將信息進行跨平臺的搬運，是消息發(fā)送和接受的工具。

setMessageHandler

在創(chuàng)建好BasicMessageChannel后，讓其接受來自另一平臺的消息，BinaryMessenger調(diào)用它的setMessageHandler方法為其設(shè)置一個消息處理器,配合BinaryMessenger完成消息的處理以及回復(fù)；

send

在創(chuàng)建好BasicMessageChannel后，可以調(diào)用它的send方法向另一個平臺傳遞數(shù)據(jù)。

setMethodCallHandler

設(shè)置用于在此MethodChannel上接收方法調(diào)用的回調(diào)

receiveBroadcastStream

設(shè)置廣播流以接收此EventChannel上的事件

Handler

Flutter使用Handler處理Codec解碼后的消息。三種Platform Channel相對應(yīng),Flutter中也定義了三種Handler:

• MessageHandler: 用于處理字符串或者半結(jié)構(gòu)化消息,定義在BasicMessageChannel中.
• MethodCallHandler: 用于處理方法調(diào)用,定義在MethodChannel中.
• StreamHandler: 用于事件流通信,定義在EventChannel中

使用Platform Channel時，需要為其注冊一個對應(yīng)BinaryMessageHandler為其設(shè)置對應(yīng)的Handler。二進制數(shù)據(jù)會被BinaryMessageHanler進行處理,首先使用Codec進行解碼操作,然后再分發(fā)給具體Handler進行處理。

結(jié)語

欲更進一步了解Platform Channel設(shè)計與實現(xiàn)，可前往深入Flutter技術(shù)內(nèi)幕:Platform Channel設(shè)計與實現(xiàn)，關(guān)注大神博客