Flutter和iOS互相调用

flutter调用ios并有返回值：

BXTestChannel.shared.initEventChannel(context);
              String a = await BXTestChannel.shared.getIosString();
              dlog(a);

flutter中的channel类：

import 'dart:io';

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

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:wiki_flu/providers.dart';
import 'package:wiki_flu/wiki_flu.dart';


const String channelName_wikifxsaveLaunage = 'com.wikifx.BXTestChannel';

const String method_BXTestChannel = 'method_BXTestChannel';

class BXTestChannel {

  static final BXTestChannel shared = BXTestChannel._();
  BXTestChannel._();
  factory BXTestChannel() => shared;
  static final _channel = const MethodChannel(channelName_wikifxsaveLaunage);



  ///flutter调ios
  Future<String> getIosString() async {
    var a = await _channel.invokeMethod(method_BXTestChannel, {
      "test":