Flutter定義了三種不同類型的Channel，它們分別是
? BasicMessageChannel：用于傳遞字符串和半結(jié)構(gòu)化的信息。
? MethodChannel：用于傳遞方法調(diào)用（method invocation）
? EventChannel: 用于數(shù)據(jù)流（event streams）的通信。
原理講解咸魚團(tuán)隊(duì)的技術(shù)文章講解的很通透了：深入理解Flutter Platform Channel
這篇文章在官方platform_channel的示例基礎(chǔ)上，補(bǔ)全了示例子中沒有實(shí)踐的種方式。

Fultter側(cè)實(shí)現(xiàn)

BasicMessageChannel

1 創(chuàng)建messageChannel

static const BasicMessageChannel messageChannel = 
BasicMessageChannel('samples.flutter.io/message', StandardMessageCodec());

1.1 發(fā)送數(shù)據(jù)到NA flutter -> NA

// result接收異步回調(diào)的數(shù)據(jù)
String parama = await messageChannel.send('flutter 參數(shù)');

1.2 接收NA發(fā)過來的數(shù)據(jù) NA -> flutter

Future<String> messageHandler(parama) async {
    print('messageHandler ' + parama);
    _naToFlutterText = parama;
    return 'messageHandler flutter return';
}
// void setMessageHandler(Future<T> handler(T message)) 
messageChannel.setMessageHandler(messageHandler);

MethodChannel

2 創(chuàng)建MethodChannel

static const MethodChannel methodChannel = MethodChannel('samples.flutter.io/battery');

2.1 flutter調(diào)用NA方法 flutter -> NA

try {
 final int result = await methodChannel.invokeMethod('getBatteryLevel');
  // 處理成功結(jié)果
} on PlatformException {
  // 處理異常結(jié)果
}

2.2 NA調(diào)用flutter方法 NA -> flutter

Future<String> methodHandler (MethodCall call) async {
    print(call.method + '-----method');
    print(call.arguments + '-----argument');
    return  '回調(diào)結(jié)果';
}
// void setMethodCallHandler(Future<dynamic> handler(MethodCall call)) 
methodChannel.setMethodCallHandler(methodHandler);

EventChannel

3 創(chuàng)建EventChannel

static const EventChannel eventChannel = EventChannel('samples.flutter.io/charging');

3.1 接收NA發(fā)送過來的stream NA -> flutter

// Stream<dynamic> receiveBroadcastStream([ dynamic arguments ])
// StreamSubscription<T> listen(void onData(T event), {Function onError, void onDone(), bool cancelOnError});
eventChannel.receiveBroadcastStream().listen(_onEvent, onError: _onError);

void _onEvent(Object event) {
  // 正常數(shù)據(jù)流
}
void _onError(Object error) {
  // 失敗數(shù)據(jù)流
}

對于 eventChannel.receiveBroadcastStream() 添加多次沒有意義，只會有一個(gè)生效，添加第二個(gè)的時(shí)候，會先把前一個(gè)cancel掉。

iOS側(cè)實(shí)現(xiàn)

1 創(chuàng)建channel

// 創(chuàng)建massageChannel
    FlutterBasicMessageChannel *massageChannel = [FlutterBasicMessageChannel messageChannelWithName:@"samples.flutter.io/message" binaryMessenger:controller];
    
    // 創(chuàng)建methodChannel
    FlutterMethodChannel *methodChannel = [FlutterMethodChannel
    methodChannelWithName:@"samples.flutter.io/battery" binaryMessenger:controller];
    
    // 創(chuàng)建eventChannel 代理方法里處理stream流 delegate設(shè)置為self
    FlutterEventChannel *eventChannel = [FlutterEventChannel eventChannelWithName:@"samples.flutter.io/charging" binaryMessenger:controller];
    [eventChannel setStreamHandler:self];

2 接收flutter發(fā)送過來的message和method flutter -> NA

    // 接收flutter發(fā)過來的message
    [self.massageChannel setMessageHandler:^(id  _Nullable message, FlutterReply  _Nonnull callback) {
        NSLog(@"flutter->na message %@ ", message);
        callback([NSString stringWithFormat:@"flutter->na message callback %@ ", message]);
    }];
    
    // 接收flutter調(diào)用的方法
    __weak typeof(self) weakSelf = self;
    [methodChannel setMethodCallHandler:^(FlutterMethodCall* call,
                                          FlutterResult result) {
        
        if ([@"getBatteryLevel" isEqualToString:call.method]) {
            int batteryLevel = [weakSelf getBatteryLevel];
            if (batteryLevel == -1) {
                result([FlutterError errorWithCode:@"UNAVAILABLE"
                                           message:@"Battery info unavailable"
                                           details:nil]);
            } else {
                result(@(batteryLevel));
            }
        } else {
          result(FlutterMethodNotImplemented);
        }
    }];

3 NA發(fā)送message和method到flutter NA -> flutter

    //  na to flutter 發(fā)送普通消息
    [self.massageChannel sendMessage:@"na message 參數(shù)" reply:^(id  _Nullable reply) {
        NSLog(@"na->flutter message callback %@ ", reply);
    }];
    
    //  na to flutter 調(diào)用flutter方法
    [self.methodChannel invokeMethod:@"naCallFlutter" arguments:@"na method 參數(shù)" result:^(id  _Nullable result) {
        NSLog(@"na->flutter method callback %@ ", result);
    }];

在NA側(cè)FlutterEventChannel 接收和發(fā)送stream流通過協(xié)議FlutterStreamHandler進(jìn)行的，可以方便的進(jìn)行NA->flutter方向的數(shù)據(jù)發(fā)送，但是從flutter給NA發(fā)送呢?cái)?shù)據(jù)流，dart側(cè)沒有提供api。通過StreamController應(yīng)該是可行的，它的繼承關(guān)系如下自StreamController<StreamSink<StreamSink<EventSink，查詢dart文檔，在EventSink 中有

/**
   * Adds a data [event] to the sink.
   *
   * Must not be called on a closed sink.
   */
  void add(T event);

不過沒有試驗(yàn)成功從flutter數(shù)據(jù)流到NA側(cè)，在dart側(cè)是可以內(nèi)部發(fā)送數(shù)據(jù)流的。

Android側(cè)實(shí)現(xiàn)

1 創(chuàng)建channel

 // 創(chuàng)建massageChannel
     BasicMessageChannel channel = new BasicMessageChannel(registrar.messenger(), "flutter_message_plugin", StringCodec.INSTANCE );
      channel.setMessageHandler(new BasicMessageChannel.MessageHandler() {
      @java.lang.Override
      public void onMessage(java.lang.Object object, BasicMessageChannel.Reply reply) {  //flutter發(fā)送消息到平臺測
        Log.i("BasicMessageChannel", "接收到Flutter的消息:" + object);
      }
    });
    channel.send("發(fā)送消息到Flutter");
  // 創(chuàng)建methodChannel
        new MethodChannel(getFlutterView(), "samples.flutter.io/battery").setMethodCallHandler(
          // 直接 new MethodChannel，然后設(shè)置一個(gè)Callback來處理Flutter端調(diào)用
                new MethodCallHandler() {
                    @Override
                    public void onMethodCall(MethodCall call, Result result) {
                        // 在這個(gè)回調(diào)里處理從Flutter來的調(diào)用
                        if (call.method.equals("getBatteryLevel")) {
                            int batteryLevel = getBatteryLevel();
                            //result是給Flutter的返回值
                            if (batteryLevel != -1) {
                                result.success(batteryLevel);
                            } else {
                                result.error("UNAVAILABLE", "Battery level not available.", null);
                            }     
                          }
                        } else {
                          result.notImplemented();
                        }
                    }
                });
    
    // 創(chuàng)建eventChannel 代理方法里處理stream流 delegate設(shè)置為self
    new EventChannel(getFlutterView(), "samples.flutter.io/charging").setStreamHandler(
        new StreamHandler() {
          // 接收電池廣播的BroadcastReceiver。
          private BroadcastReceiver chargingStateChangeReceiver;
          @Override
         // 這個(gè)onListen是Flutter端開始監(jiān)聽這個(gè)channel時(shí)的回調(diào)，第二個(gè)參數(shù) EventSink是用來傳數(shù)據(jù)的載體。
          public void onListen(Object arguments, EventSink events) {
            chargingStateChangeReceiver = createChargingStateChangeReceiver(events);
            registerReceiver(
                chargingStateChangeReceiver, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
          }

          @Override
          public void onCancel(Object arguments) {
            // 對面不再接收
            unregisterReceiver(chargingStateChangeReceiver);
            chargingStateChangeReceiver = null;
          }
        }
    );