Flutter项目中，调用iOS代码步骤：(共三步)

1.flutter代码中，添加MethodChannel

2.flutter代码中，调用iOS方法获取想要的值，关键词：.invokeMethod

3.iOS的AppDelegate里面FlutterMethodChannel、setMethodCallHandler 返回需要的值。

class _MixPageState extends State<MixPage> {
// 第一步添加 MethodChannel
  static const plantform = MethodChannel("test.flutter.io/testAction1");
// 获取iOS代码的返回值
  String iosResultString = "";
@override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("混合开发测试"),
      ),
      body: Center(
        child: Column(
          children: [
              buildButton(),
            Text("获取iOS结果为：${iosResultString}", style: TextStyle(fontSize: 26),)
          ],
        ),
      ),
    );
  }

  Widget buildButton() {
    return MaterialButton(onPressed:
        () {
      setState(() {
        testAction1();
      });
    },
      child: Text("获取调用iOS代码结果", style: TextStyle(fontSize: 30),),
      color: Colors.blue,
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(20),
      ),
      elevation: 0,
    );
  }

// 调用iOS代码
  Future testAction1() async {
    String result = await plantform.invokeMethod("testAction1");
    print("获取iOS结果：====${result}");
    setState(() {
      iosResultString = result;
    });
  }
}

iOS中添加代码：

@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
      
      // flutter调用iOS的代码，在这里写。 ----- BEGIN
      if let controller = window.rootViewController as? FlutterBinaryMessenger {
          let channel = FlutterMethodChannel.init(name: "test.flutter.io/testAction1", binaryMessenger: controller);
          channel.setMethodCallHandler { (call: FlutterMethodCall, result: FlutterResult) -> Void in
              if (call.method == "testAction1") {
                  result("ios假数据"); // 根据需求，调用具体方法返回特定数据.
              }
              result(FlutterMethodNotImplemented);
          }
      }
      // flutter调用iOS的代码，在这里写。 ----- END
      
      GeneratedPluginRegistrant.register(with: self)
      return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}