关于 API 调用后有状态小部件上的 dart:flutter 计时问题

flutter timing problems on stateful widget after API call

我遇到了计时问题，我从 api 获取数据，然后从 JSON 创建一个列表。我认为使用结果列表的长度作为我的列表视图中的项目计数。但是，它会在 itemcount 上引发 null 错误，然后完成处理并显示列表视图。我试图找出时间问题出在哪里以及如何处理项目和小部件，以便避免错误。如果有人对我的代码有缺陷有任何想法，我的代码如下。

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class Specialty extends StatefulWidget {
Specialty({Key key, this.title}) : super(key: key);

final String title;

@override
_SpecialtyState createState() => new _SpecialtyState();
}

class _SpecialtyState extends State<Specialty> {

bool _dataReceived = false;
bool _authenticated = false;
SharedPreferences prefs;
List mylist;

@override
void initState() {
super.initState();

_getPrefs();
_getSpecialty();
}

_getPrefs() async {
prefs = await SharedPreferences.getInstance();
_authenticated = prefs.getBool('authenticated');
print('AUTH2: ' + _authenticated.toString());
print('AUTHCODE2: ' + prefs.getString('authcode'));

}

_getSpecialty() async {
var _url = 'http://$baseurl:8080/support/specialty';

var http = createHttpClient();
var response = await http.get(_url);

var specialties = jsonCodec.decode(response.body);

mylist = specialties.toList();
//_dataReceived = true;

setState(() {
_dataReceived = true;
});
}

Future<Null> _onRefresh() {
Completer<Null> completer = new Completer<Null>();
Timer timer = new Timer(new Duration(seconds: 3), () {
completer.complete();
});
return completer.future;
}

@override
Widget build(BuildContext context) {
return new Scaffold(
body: new RefreshIndicator(
child: new ListView.builder(
itemBuilder: _itemBuilder,
itemCount: mylist.length,
),
onRefresh: _onRefresh,

));
}

Widget _itemBuilder(BuildContext context, int index) {
Specialties spec = getSpec(index);
return new SpecialtyWidget(spec: spec,);
}

Specialties getSpec(int index) {
return new Specialties(
mylist[index]['id'], mylist[index]['name'], mylist[index]['details'],
new Photo('lib/images/' + mylist[index]['image'], mylist[index]['name'],
mylist[index]['name']));
//return new Specialties.fromMap(mylist[index]);

}

var jsonCodec = const JsonCodec();

}