class _MyHomePageState extends State<MyHomePage> {
  static const MethodChannel methodChannel = MethodChannel("channel_test"); //
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: RaisedButton(
          child: Text("a"),
          onPressed: () {
                () async {
              String str = await methodChannel.invokeMethod('getRes');
              print(str);
            }();
          },
        ),
      ),
    );
  }
}

package com.rockcheck.flutter_app1;

import android.os.Bundle;

import androidx.annotation.Nullable;

import io.flutter.embedding.android.FlutterActivity;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugins.GeneratedPluginRegistrant;

public class MainActivity extends FlutterActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        GeneratedPluginRegistrant.registerWith(getFlutterEngine());

        new MethodChannel(getFlutterEngine().getDartExecutor().getBinaryMessenger(),"channel_test").setMethodCallHandler(
        (call, result) -> {
            if (call.method.equals("getRes")) {
                result.success("aaa");
            } else {
                result.notImplemented();
            }
        }
);
    }
}

class _MyHomePageState extends State<MyHomePage> {
  static const MethodChannel methodChannel = MethodChannel("channel_test"); //
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: RaisedButton(
          child: Text("a"),
          onPressed: () {
                () async {
              await methodChannel.invokeMethod('set', 'bbb');
              String str = await methodChannel.invokeMethod('get');
              print(str);
            }();
          },
        ),
      ),
    );
  }
}

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    
    let controller:FlutterViewController = window.rootViewController as! FlutterViewController
    let userdefault = FlutterMethodChannel(name: "channel_test", binaryMessenger: controller.binaryMessenger)
    userdefault.setMethodCallHandler { (call, result) in
        //赋值给了ios端
        if "set" == call.method{
            self.setPlatString(result: result, str: call.arguments as! String)
        }
        //回传给了flutter
        else if "get" == call.method{
            self.getPlatString(result: result)
        }
        else{
            result(FlutterMethodNotImplemented)
        }
    }
    
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
    fileprivate func getPlatString(result:FlutterResult){
        let token:String = UserDefaults.standard.value(forKey: "v") as! String
        result(token)
    }
    fileprivate func setPlatString(result:FlutterResult,str:String){
        UserDefaults.standard.set(str, forKey: "v")
        UserDefaults.standard.synchronize()
        result(NSNumber(booleanLiteral: true))
    }
}