PlatformChannel功能简介

PlatformChannel分为BasicMessageChannel、MethodChannel以及EventChannel三种。其各自的主要用途如下：

• BasicMessageChannel: 用于传递数据。Flutter与原生项目的资源是不共享的，可以通过BasicMessageChannel来获取Native项目的图标等资源。
• MethodChannel: 传递方法调用。Flutter主动调用Native的方法，并获取相应的返回值。比如获取系统电量，发起Toast等调用系统API，可以通过这个来完成。
• EventChannel: 传递事件。这里是Native将事件通知到Flutter。比如Flutter需要监听网络情况，这时候MethodChannel就无法胜任这个需求了。EventChannel可以将Flutter的一个监听交给Native，Native去做网络广播的监听，当收到广播后借助EventChannel调用Flutter注册的监听，完成对Flutter的事件通知。

其实可以看到，无论传方法还是传事件，其本质上都是数据的传递，不过上层包的一些逻辑不同而已。

具体实现

BasicMessageChannel

初始直接在Native端发送消息, flutter是收不到的, 因为还没有执行接收监听方法, 所以在Native端发送消息, 需要注意一下时机
flutter

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

 static const messageChannel2 = const BasicMessageChannel('samples.flutter.io/message2', StandardMessageCodec());

 Future<String> sendMessage() async {
    String reply = await messageChannel.send('发送给Native端的数据');
    print('reply: $reply');
    return reply;
  }

  void receiveMessage() {
    messageChannel2.setMessageHandler((message) async {
      print('message: $message');
      return '返回Native端的数据';
    });
  }
  
   @override
  void initState() {
    // TODO: implement initState
    super.initState();
    receiveMessage();
    sendMessage();

  }

ios

// 初始化定义
FlutterBasicMessageChannel* messageChannel = [FlutterBasicMessageChannel messageChannelWithName:@"samples.flutter.io/message" binaryMessenger:controller];

// 接收消息监听
[messageChannel setMessageHandler:^(id message, FlutterReply callback) {
    NSLog(message);
    callback(@"返回flutter端的数据");
}];




// 触发事件执行

FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;
FlutterBasicMessageChannel* messageChannel2 = [FlutterBasicMessageChannel messageChannelWithName:@"samples.flutter.io/message2" binaryMessenger:controller];

// 发送消息
[messageChannel2 sendMessage:(@"发送给flutter的数据") reply:^(id reply) {
    NSLog(reply);
}];

android

BasicMessageChannel<Object> messageChannel = new BasicMessageChannel<Object>(getFlutterView(), "samples.flutter.io/message", StandardMessageCodec.INSTANCE);

// 接收消息监听
messageChannel.setMessageHandler(new BasicMessageChannel.MessageHandler<Object>() {
    @Override
    public void onMessage(Object o, BasicMessageChannel.Reply<Object> reply) {
        System.out.println("onMessage: " + o);
        reply.reply("返回给flutter的数据");
    }
});



// 触发事件执行

BasicMessageChannel<Object> messageChannel2 = new BasicMessageChannel<Object>(getFlutterView(), "samples.flutter.io/message2", StandardMessageCodec.INSTANCE);

// 发送消息
messageChannel2.send("发送给flutter的数据", new BasicMessageChannel.Reply<Object>() {
    @Override
    public void reply(Object o) {

        System.out.println("onReply: " + o);
    }
});

MethodChannel

flutter

 static const platform = const MethodChannel('samples.flutter.io/battery');
 
 Future<Null> _getBatteryLevel() async {
    String batteryLevel;
    try {
      final int result = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'Battery level at $result % .';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    }

    setState(() {
      _batteryLevel = batteryLevel;
    });
  }
  
  // 执行_getBatteryLevel方法

ios

FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;
    
FlutterMethodChannel* batteryChannel = [FlutterMethodChannel
                                        methodChannelWithName:@"samples.flutter.io/battery"
                                        binaryMessenger:controller];



[batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
    // TODO
    if ([@"getBatteryLevel" isEqualToString:call.method]) {
        int batteryLevel = [self getBatteryLevel];
        
        if (batteryLevel == -1) {

            result([FlutterError errorWithCode:@"UNAVAILABLE"
                                       message:@"Battery info unavailable"
                                       details:nil]);
        } else {
            result(@(batteryLevel));
        }
    } else {
        result(FlutterMethodNotImplemented);
    }
}];

- (int)getBatteryLevel {
    UIDevice* device = UIDevice.currentDevice;
    device.batteryMonitoringEnabled = YES;
    
    if (device.batteryState == UIDeviceBatteryStateUnknown) {
        return -1;
    } else {
        return (int)(device.batteryLevel * 100);
    }
}

android

new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
    new MethodCallHandler() {
        @Override
        public void onMethodCall(MethodCall call, Result result) {
            // TODO
            if (call.method.equals("getBatteryLevel")) {
                int batteryLevel = getBatteryLevel();

                if (batteryLevel != -1) {
                    result.success(batteryLevel);
                } else {
                    result.error("UNAVAILABLE", "Battery level not available.", null);
                }
            } else {
                result.notImplemented();
            }
        }
    }
);


private int getBatteryLevel() {
    int batteryLevel = -1;
    if (VERSION.SDK_INT >= VERSION_CODES.LOLLIPOP) {
        BatteryManager batteryManager = (BatteryManager) getSystemService(BATTERY_SERVICE);
        batteryLevel = batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY);
    } else {
        Intent intent = new ContextWrapper(getApplicationContext()).
                registerReceiver(null, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
        batteryLevel = (intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) * 100) /
                intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
    }

    return batteryLevel;
}

FlutterEventChannel

flutter

static const EventChannel _eventChannel =
      const EventChannel('samples.flutter.io/test');

 void _onEvent(Object event) {
    print('返回的内容: $event');
  }

  void _onError(Object error) {
    print('返回的错误');
  }
  
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    // 监听开始
    _eventChannel.receiveBroadcastStream().listen(_onEvent, onError: _onError);

  }

ios

FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;
    
FlutterEventChannel* eventChannel = [FlutterEventChannel eventChannelWithName:@"samples.flutter.io/test" binaryMessenger:controller];
[eventChannel setStreamHandler:self];


FlutterEventSink     eventSink;
// // 这个onListen是Flutter端开始监听这个channel时的回调，第二个参数 EventSink是用来传数据的载体。
- (FlutterError* _Nullable)onListenWithArguments:(id _Nullable)arguments
                                       eventSink:(FlutterEventSink)events {
    eventSink = events;
    // arguments flutter给native的参数
    // 回调给flutter， 建议使用实例指向，因为该block可以使用多次
    if (events) {
        events(@"主动发送通知到flutter");
    }

    // 监听电池状态
    [[NSNotificationCenter defaultCenter] addObserver:self
                                           selector:@selector(onBatteryStateDidChange:)
                                               name:UIDeviceBatteryStateDidChangeNotification
                                             object:nil];
    return nil;
}

/// flutter不再接收
- (FlutterError* _Nullable)onCancelWithArguments:(id _Nullable)arguments {
    // arguments flutter给native的参数
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    eventSink = nil;
    return nil;
}

- (void)onBatteryStateDidChange:(NSNotification*)notification {
    if (eventSink == nil) return;
    UIDeviceBatteryState state = [[UIDevice currentDevice] batteryState];
    switch (state) {
            case UIDeviceBatteryStateFull:
            case UIDeviceBatteryStateCharging:
            eventSink(@"charging");
            break;
            case UIDeviceBatteryStateUnplugged:
            eventSink(@"discharging");
            break;
        default:
            eventSink([FlutterError errorWithCode:@"UNAVAILABLE"
                                           message:@"Charging status unavailable"
                                           details:nil]);
            break;
    }
}

android

new EventChannel(getFlutterView(), CHANNEL2).setStreamHandler(
    new EventChannel.StreamHandler() {
        @Override
        public void onListen(Object o, EventChannel.EventSink eventSink) {
            this.eventSink = eventSink;

            handler.sendEmptyMessageDelayed(1, 1000);
        }

        @Override
        public void onCancel(Object o) {

        }

        private EventChannel.EventSink eventSink;
        private int count = 0;
        private Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                eventSink.success((count++) + "主动发送消息给flutter");
//              handler.sendEmptyMessageDelayed(1,1000);
            }
        };

    }
);

源码

github: https://github.com/shimuhui/flutterToNative