未处理的异常： NoSuchMethod错误： 方法 'setString' 在 null 上调用

Question

通过应用程序，我从数据库中进行查询，以获取一些以前存储的数据。我试图将id存储在myPreferences中，但我得到了这样的错误：

E/flutter (29170): [ERROR:flutter/lib/ui/ui_dart_state.cc(166)] Unhandled Exception: NoSuchMethodError: The method 'setString' was called on null.
E/flutter (29170): Receiver: null
E/flutter (29170): Tried calling: setString("id", "6")
E/flutter (29170): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
E/flutter (29170): #1      MyPreferences.commit (package:flutter_apptestqeuriy/MyPreferences.dart:32:30)
E/flutter (29170): #2      AddCommentsState.getLogin.<anonymous closure> (package:flutter_apptestqeuriy/AddComment.dart:82:23)
E/flutter (29170): #3      State.setState (package:flutter/src/widgets/framework.dart:1240:30)
E/flutter (29170): #4      AddCommentsState.getLogin (package:flutter_apptestqeuriy/AddComment.dart:76:5)
E/flutter (29170): <asynchronous suspension>
E/flutter (29170): #5      AddCommentsState.initState.<anonymous closure> (package:flutter_apptestqeuriy/AddComment.dart:57:9)
E/flutter (29170): #6      interval.function (package:flutter_apptestqeuriy/AddComment.dart:21:9)
E/flutter (29170): #7      _rootRun (dart:async/zone.dart:1182:47)
E/flutter (29170): #8      _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter (29170): #9      _CustomZone.runGuarded (dart:async/zone.dart:997:7)
E/flutter (29170): #10     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
E/flutter (29170): #11     _rootRun (dart:async/zone.dart:1190:13)
E/flutter (29170): #12     _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter (29170): #13     _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:1021:23)
E/flutter (29170): #14     Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18:15)
E/flutter (29170): #15     _Timer._runTimers (dart:isolate-patch/timer_impl.dart:397:19)
E/flutter (29170): #16     _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:428:5)
E/flutter (29170): #17     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
E/flutter (29170): 

我通过以下查询存储号码：

Future<String> getLogin() async {
var response = await http.get(
Uri.encodeFull("http://xxxxxxxxx/ApplicationP.php"),);
setState(() {
var convertDataToJson = json.decode(response.body);
data = convertDataToJson['result'];
if (data != null) {
user_name = data[0]['id'];
_myPreferences.id = user_name;
_myPreferences.commit();
}
});
}

import 'package:shared_preferences/shared_preferences.dart';
class MyPreferences {
static const ID = "id";

static final MyPreferences instance = MyPreferences._internal();
static SharedPreferences _sharedPreferences;
String id = "";

MyPreferences._internal() {}
factory MyPreferences() => instance;
Future<SharedPreferences> get preferences async {
if (_sharedPreferences != null) {
return _sharedPreferences;
} else {
_sharedPreferences = await SharedPreferences.getInstance();
id = _sharedPreferences.getString(ID);
return _sharedPreferences;
}
}
Future<bool> commit() async {
await _sharedPreferences.setString(ID, id);
}
Future<MyPreferences> init() async {
_sharedPreferences = await preferences;
return this;
}
static Future<bool> clearPreference() async {
if (_sharedPreferences != null) {
_sharedPreferences.clear();
}
}
}

我在这里跟进了一些类似的标题，但问题不同。

我不知道是否有人知道这个问题的解决办法。

Answer 1

您的_sharedPreferences实例未初始化。

Future<bool> commit() async {
_sharedPreferences ??= await SharedPreferences.getInstance();
await _sharedPreferences.setString(ID, id);
}

Answer 2

static final String TOKEN = "TOKEN";
SharedPreferences sharedPreferences;
setToken(String token) async {
sharedPreferences  = await SharedPreferences.getInstance();
await sharedPreferences.setString(TOKEN, token);
}
getToken() async {
return _preferences.getString(TOKEN);
}