Flutter Channel实践

Flutter定义了三种不同类型的Channel，它们分别是
▪ BasicMessageChannel：用于传递字符串和半结构化的信息。
▪ MethodChannel：用于传递方法调用（method invocation）
▪ EventChannel: 用于数据流（event streams）的通信。
原理讲解咸鱼团队的技术文章讲解的很通透了：深入理解Flutter Platform Channel
这篇文章在官方platform_channel的示例基础上，补全了示例子中没有实践的种方式。

Fultter侧实现

BasicMessageChannel

1 创建messageChannel

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

1.1 发送数据到NA flutter -> NA

// result接收异步回调的数据
String parama = await messageChannel.send('flutter 参数');

1.2 接收NA发过来的数据 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 创建MethodChannel

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

2.1 flutter调用NA方法 flutter -> NA

try {
 final int result = await methodChannel.invokeMethod('getBatteryLevel');
  // 处理成功结果
} on PlatformException {
  // 处理异常结果
}

2.2 NA调用flutter方法 NA -> flutter

Future<String> methodHandler (MethodCall call) async {
    print(call.method + '-----method');
    print(call.arguments + '-----argument');
    return  '回调结果';
}
// void setMethodCallHandler(Future<dynamic> handler(MethodCall call)) 
methodChannel.setMethodCallHandler(methodHandler);

EventChannel

3 创建EventChannel

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

3.1 接收NA发送过来的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) {
  // 正常数据流
}
void _onError(Object error) {
  // 失败数据流
}

对于 eventChannel.receiveBroadcastStream() 添加多次没有意义，只会有一个生效，添加第二个的时候，会先把前一个cancel掉。

iOS侧实现

1 创建channel

// 创建massageChannel
    FlutterBasicMessageChannel *massageChannel = [FlutterBasicMessageChannel messageChannelWithName:@"samples.flutter.io/message" binaryMessenger:controller];
    
    // 创建methodChannel
    FlutterMethodChannel *methodChannel = [FlutterMethodChannel
    methodChannelWithName:@"samples.flutter.io/battery" binaryMessenger:controller];
    
    // 创建eventChannel 代理方法里处理stream流 delegate设置为self
    FlutterEventChannel *eventChannel = [FlutterEventChannel eventChannelWithName:@"samples.flutter.io/charging" binaryMessenger:controller];
    [eventChannel setStreamHandler:self];

2 接收flutter发送过来的message和method flutter -> NA

    // 接收flutter发过来的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调用的方法
    __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发送message和method到flutter NA -> flutter

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

在NA侧FlutterEventChannel 接收和发送stream流通过协议FlutterStreamHandler进行的，可以方便的进行NA->flutter方向的数据发送，但是从flutter给NA发送呢数据流，dart侧没有提供api。通过StreamController应该是可行的，它的继承关系如下自StreamController<StreamSink<StreamSink<EventSink，查询dart文档，在EventSink 中有

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

不过没有试验成功从flutter数据流到NA侧，在dart侧是可以内部发送数据流的。

Android侧实现