Question

来自面向对象的编程背景，我计划制作一个专用的Settings类来存储有关该应用程序的某些基本数据。

我计划首先使用SharedPreferences和LocalStorage保存应用程序的主题。

但是，SharedPreferences.getInstance（）似乎总是返回null。

我尝试过简单地运行，以调试模式运行，有一个单独的异步方法来加载SharedPreferences并返回一个使用.then（）展开的Future。我似乎无法弄清楚为什么我总是从我编写的AppSettings.getInstance（）方法的SharedPreferences.getInstance（）中得到空值。

import 'package:shared_preferences/shared_preferences.dart';
import 'package:localstorage/localstorage.dart';
import 'package:flutter/material.dart';

class AppSettings {
  // Singleton Instance
  static AppSettings _appSettings;

  // For First Launch Recognition
  bool _initialize;

  // Storage instances for persistent settings storage
  static SharedPreferences _prefs;
  static LocalStorage _dayColors = new LocalStorage('_dayColors');
  static LocalStorage _nightColors = new LocalStorage('_nightColors');

  // App Settings
  bool _nightTheme;

  Color _dayBgColor;
  Color _primaryDayColor;
  Color _secondaryDayColor;
  Color _accentDayColor;

  Color _nightBgColor;
  Color _primaryNightColor;
  Color _secondaryNightColor;
  Color _accentNightColor;

  static AppSettings getInstance() {
    SharedPreferences.getInstance().then((prefs) => _prefs = prefs);
    _appSettings ??= AppSettings._();
    return _appSettings;
  }

  ///
  /// Initialize App Settings
  ///
  AppSettings._() {
    _checkIfFirstLaunch();
    if (_initialize) {
      _loadDefaultSettings();
      _saveSettings();
    } else {
      _loadSettings();
    }
  }

  _checkIfFirstLaunch() {
    try {
      _initialize = _prefs.getBool("_initialize");
    } catch (e) {
      _initialize = true;
    }
  }

  _loadSettings() {
    _nightTheme = _prefs.getBool("_nightTheme");

    _dayColors.ready.then((_) => _loadDayColors());
    _nightColors.ready.then((_) => _loadNightColors());
  }

  _loadDefaultSettings() {
    _nightTheme = false;

    _dayBgColor = Colors.white;
    _primaryDayColor = Colors.blue;
    _secondaryDayColor = Colors.lightBlue;
    _accentDayColor = Colors.blueAccent;

    _nightBgColor = Colors.black54;
    _primaryNightColor = Colors.green;
    _secondaryNightColor = Colors.lightGreen;
    _accentNightColor = Colors.amber;
  }

  _saveSettings() {
    _prefs.setBool("_nightTheme", _nightTheme);

    _dayColors.ready.then((_) => _saveDayColors());
    _nightColors.ready.then((_) => _saveNightColors());
  }
}

SharedPreferences.getInstance（）应该返回SharedPreferences单例实例。它一直返回null。

Answer 1

您的函数异步，并且您的回调（then）在返回getInstance()之后执行。您必须更改函数以使用await并获取SharedPreferences.getInstance()的值，而不要使用SharedPreferences.getInstance().then(...)

查看文档：{{3}}

SharedPreferences.getInstance（）的实现。

static Future<SharedPreferences> getInstance() async {
  if (_instance == null) {
    final Map<String, Object> preferencesMap =
        await _getSharedPreferencesMap();
    _instance = SharedPreferences._(preferencesMap);
  }
  return _instance;
}

Answer 2

以下是基于Augusto答案的代码：

static Future<AppSettings> getInstance() async {
    _prefs = await SharedPreferences.getInstance();
    _appSettings ??= AppSettings._();
    return _appSettings;
}

SharedPreferences.getInstance（）始终返回null

2 个答案: