通信原理

我们分几种场景来介绍Flutter和Native之间的通信。

• Native发送数据给Flutter
• Flutter发送数据给Native
• Flutter发送数据给Native，然后Native回传数据给Flutter

Flutter与Native通信机制

在讲解Flutter与Native之间是如何传递数据之前，我们先了解下Flutter与Native的通信机制，Flutter和Native的通信是通过Channel来完成的。

消息使用Channel(平台通道)在客户端(UI)和主机(平台)之间传递，如下图所示：

• Channel所支持的数据类型对照表

• Flutter定义了三种不同类型的Channel：
  1. BasicMessageChannel：用于传递字符串和半结构化的信息，持续通信，收到消息后可以回复此次消息，如：Native将遍历到的文件信息陆续传递到Flutter，在比如：Flutter将从服务端陆陆续续获取到信息交给Native加工，Native处理完返回等；
  2. MessageChannel：用于传递方法调用，一次性通信：如Flutter调用Native拍照；
  3. EventChannel：用于数据流的通信，持续通信，通常用于Native向Flutter的通信，如：手机电量变化，网络连接变化等；

这三种类型的Channel都是全双工通信，即A<=>B，Flutter可以主动发送消息给Native端，并且Native接收到消息后可以做出回应，同样，Native端可以主动发送消息给Flutter端，Flutter端接收到数据后返回给Native。

通信原理

• 无论是哪一种类型的Channel，它能和Flutter进行通信主要是借助BinaryMessenger来实现的。
• 三种类型的Channel在Flutter侧都有对应的实现。

实践

• flutter主动发送数据给native，native接受到消息后回信给flutter
  1. Android端

class MainActivity : FlutterActivity() {

    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)
        val channel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "HiFlutterBridge")//HiFlutterBridge需要和flutter端的对应

        channel.setMethodCallHandler { call, result ->
            Log.e("HiFlutterBridge", "argus is ${call.arguments}")
            if (call.method == "goToNative") {
                val data = "Hello from native!"
                result.success(data)
            } else {
                result.notImplemented()
            }
        }
    }

2. Flutter端

import 'package:flutter/cupertino.dart';
import 'package:flutter/services.dart';

class HiFlutterBridge {
  static HiFlutterBridge _instance = HiFlutterBridge._();
  final MethodChannel _bridge = const MethodChannel("HiFlutterBridge");//需要和原生端的MethodChannel方法的第二个参数一致
  var _listener = {};

  HiFlutterBridge._() {
    _bridge.setMethodCallHandler((MethodCall call) {
      String method = call.method;
      if (_listener[method] != null) {
        return _listener[method](call);
      }
      return Future(() => null);
    });
  }

  static HiFlutterBridge getInstance() {
    return _instance;
  }

  register(String method, Function(MethodCall) callBack) {
    _listener[method] = callBack;
  }

  unRegister(String method) {
    _listener.remove(method);
  }

  goToNative(Map params) async {
    String result = await _bridge.invokeMethod("goToNative", params);
    debugPrint("HiFlutterBridge result is $result");
  }

  MethodChannel bridge() {
    return _bridge;
  }
}

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
        resizeToAvoidBottomInset: false, // 设置为false以避免在打开软键盘时widget被顶上去
        body: HideKeyboard(
            child: Stack(
          children: <Widget>[
            Align(
              alignment: Alignment.center,
              child: ElevatedButton(
                onPressed: () => {
                  //flutter向native端通信

                  HiFlutterBridge.getInstance()
                      .goToNative({"test": "hello world"})
                },
                // style: ElevatedButton.styleFrom(
                //     minimumSize: const Size(100, 50),
                //     maximumSize: const Size(100, 50),
                //     shape:
                //     RoundedRectangleBorder(borderRadius: BorderRadius.circular(30))),
                child: const Text('login'),
              ),
            )
          ],
        )));
  }

• native主动发送数据给flutter，flutter接受到消息后回信给native
  1. Android端

class MainActivity : FlutterActivity() {

    override fun onResume() {
        super.onResume()
        sendDataToFlutter()
    }

    private fun sendDataToFlutter() {
        val channel = MethodChannel(flutterEngine!!.dartExecutor.binaryMessenger, "test")
        channel.invokeMethod(" sendData ", "Hello Flutter", object : MethodChannel.Result {
            override fun success(o: Any?) {
                Log.d("Native", "Received back: " + o.toString())
            }

            override fun error(s: String, s1: String?, o: Any?) {
                Log.e("Native", "Error: $s1")
            }

            override fun notImplemented() {}
        })
    }
}

2. Flutter端

import 'package:flutter/cupertino.dart';
import 'package:flutter/services.dart';

class HiFlutterBridge {
  static HiFlutterBridge _instance = HiFlutterBridge._();
  final MethodChannel _bridge = const MethodChannel("test");
  var _listener = {};

  HiFlutterBridge._() {
    _bridge.setMethodCallHandler((MethodCall call) async{
      debugPrint("test ===> ${call}");
      return "flutter ===> native";
      String method = call.method;
      if (_listener[method] != null) {
        return _listener[method](call);
      }
      return Future(() => null);
    });
  }

  static HiFlutterBridge getInstance() {
    return _instance;
  }

  register(String method, Function(MethodCall) callBack) {
    _listener[method] = callBack;
  }

  unRegister(String method) {
    _listener.remove(method);
  }

  goToNative(Map params) async {
    String result = await _bridge.invokeMethod("goToNative", params);
    debugPrint("HiFlutterBridge result is $result");
  }

  MethodChannel bridge() {
    return _bridge;
  }
}

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    HiFlutterBridge.getInstance();
  }

• 运行结果

I/flutter (27181): test ===> MethodCall( sendData , Hello Flutter)
D/Native  (27181): Received back: flutter ===> native