How to add utf8 decoder to solve the problem of Garbled characters in flutter http request?

I use this topology model to parse Json data. The data become garbled when I receive Chinese characters.

I have tried to add utf8.decode like

List<Client> clientFromJson(String str) => List<Client>.from(json.decode(utf8.decode(str)).map((x) => Client.fromJson(x)));

but the IDE tell me that "The argument type 'String' can't be assigned to the parameter type 'List< int >'".

What should I do to add utf8 decoder in the model?

///topology.dart// To parse this JSON data, do////     final client = clientFromJson(jsonString);//     final topology = topologyFromJson(jsonString);import 'dart:convert';List<Client> clientFromJson(String str) => List<Client>.from(json.decode(str).map((x) => Client.fromJson(x)));String clientToJson(List<Client> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));List<Topology> topologyFromJson(String str) => List<Topology>.from(json.decode(str).map((x) => Topology.fromJson(x)));String topologyToJson(List<Topology> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));class Topology {  String location;  String mode;  String macAddress;  String ipAddress;  String downloadSpeed;  String downloadSpeedUnit;  String uploadSpeed;  String uploadSpeedUnit;  List<Client> client;  Topology({    this.location,    this.mode,    this.macAddress,    this.ipAddress,    this.downloadSpeed,    this.downloadSpeedUnit,    this.uploadSpeed,    this.uploadSpeedUnit,    this.client,  });  factory Topology.fromJson(Map<String, dynamic> json) => Topology(    location: json["location"],    mode: json["mode"],    macAddress: json["macAddress"],    ipAddress: json["ipAddress"],    downloadSpeed: json["downloadSpeed"],    downloadSpeedUnit: json["downloadSpeedUnit"],    uploadSpeed: json["uploadSpeed"],    uploadSpeedUnit: json["uploadSpeedUnit"],    client: List<Client>.from(json["client"].map((x) => Client.fromJson(x))),  );  Map<String, dynamic> toJson() => {"location": location,"mode": mode,"macAddress": macAddress,"ipAddress": ipAddress,"downloadSpeed": downloadSpeed,"downloadSpeedUnit": downloadSpeedUnit,"uploadSpeed": uploadSpeed,"uploadSpeedUnit": uploadSpeedUnit,"client": List<dynamic>.from(client.map((x) => x.toJson())),  };}class Client {  String hostname;  String macAddress;  String ipAddress;  String deviceType;  String connectedNebula;  String connectionType;  String connectTime;  Client({    this.hostname,    this.macAddress,    this.ipAddress,    this.deviceType,    this.connectedNebula,    this.connectionType,    this.connectTime,  });  factory Client.fromJson(Map<String, dynamic> json) => Client(    hostname: json["hostname"],    macAddress: json["macAddress"],    ipAddress: json["ipAddress"],    deviceType: json["deviceType"],    connectedNebula: json["connectedNebula"],    connectionType: json["connectionType"],    connectTime: json["connectTime"],  );  Map<String, dynamic> toJson() => {"hostname": hostname,"macAddress": macAddress,"ipAddress": ipAddress,"deviceType": deviceType,"connectedNebula": connectedNebula,"connectionType": connectionType,"connectTime": connectTime,  };}

//main.dart

import 'package:flutter/material.dart';import 'package:flutter/services.dart';void main() => runApp(MyApp());class MyApp extends StatelessWidget {  // This widget is the root of your application.  @override  Widget build(BuildContext context) {    SystemChrome.setPreferredOrientations([      DeviceOrientation.portraitUp,      DeviceOrientation.portraitDown,    ]);     return MaterialApp(      debugShowCheckedModeBanner: false,      title: 'Topology',      theme: ThemeData(),      home: Home(),    );  }}class Home extends StatefulWidget {  final String title;  const Home({Key key, this.title}) : super(key: key);  @override  _HomeState createState() => _HomeState();}class _HomeState extends State<Home> {  var topologies = const [];  Future loadTopologyList() async {    SharedPreferences prefs = await SharedPreferences.getInstance();    String managementPassword = prefs.getString("management_password");    String username = 'admin';    String basicAuth ='Basic '+ base64Encode(utf8.encode('$username:$managementPassword'));    final String url ="some url";    bool trustSelfSigned = true;    HttpClient httpClient = HttpClient()      ..badCertificateCallback =      ((X509Certificate cert, String host, int port) => trustSelfSigned);    IOClient ioClient = IOClient(httpClient);    final response = await ioClient.get(url,      headers: <String, String>{'authorization': basicAuth},    );    ioClient.close();    String content = response.body;    setState(() {      topologies = topologyFromJson(content);    });  }  void initState() {    loadTopologyList();    super.initState();  }  @override  Widget build(BuildContext context) {    return Scaffold(      backgroundColor: Color(0xFFFAFAFA),      appBar: AppBar(),      body: Column(        crossAxisAlignment: CrossAxisAlignment.start,        children: <Widget>[          Padding(            padding: const EdgeInsets.fromLTRB(17, 0, 0, 0),            child: Text('Topology'),          ),          Expanded(            child: ListView.separated(              itemCount: topologies.length,              separatorBuilder: (context, index) => Divider(),              itemBuilder: (BuildContext context, int index) {                Topology topology = topologies[index];                 return ListTile(                  title: Text(                    topology.location,                  ),                  leading: CircleAvatar(                      child: Image.asset("assets/drawable-mdpi/app_icon_circle.png")),                  onTap: () {                  },                );              },            ),          ),        ],      ),    );  }}