dependencies:
  flutter:
    sdk: flutter
  shared_preferences: ^2.0.15

flutter pub get

import 'package:shared_preferences/shared_preferences.dart';

Future<void> saveUserName(String name) async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.setString('username', name);
}

Future<String?> getUserName() async {
  final prefs = await SharedPreferences.getInstance();
  return prefs.getString('username');
}

Future<void> removeUserName() async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.remove('username');
}

Future<void> clearAllPreferences() async {
  final prefs = await SharedPreferences.getInstance();
  await prefs.clear();
}

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  ThemeMode _themeMode = ThemeMode.light;

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

  Future<void> _loadThemePreference() async {
    final prefs = await SharedPreferences.getInstance();
    final isDarkMode = prefs.getBool('isDarkMode') ?? false;
    setState(() {
      _themeMode = isDarkMode ? ThemeMode.dark : ThemeMode.light;
    });
  }

  Future<void> _toggleTheme(bool isDark) async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setBool('isDarkMode', isDark);
    setState(() {
      _themeMode = isDark ? ThemeMode.dark : ThemeMode.light;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      themeMode: _themeMode,
      theme: ThemeData.light(),
      darkTheme: ThemeData.dark(),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Shared Preferences Example'),
        ),
        body: Center(
          child: SwitchListTile(
            title: Text('ダークモード'),
            value: _themeMode == ThemeMode.dark,
            onChanged: (bool value) {
              _toggleTheme(value);
            },
          ),
        ),
      ),
    );
  }
}

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:id="@+id/action_home"
        android:title="ホーム"
        android:icon="@drawable/ic_baseline_home_24" />
    <item android:id="@+id/action_setting"
        android:title="設定"
        android:icon="@drawable/ic_baseline_settings_24" />
</menu>

class MainActivity : AppCompatActivity() ,BottomNavigationView.OnNavigationItemSelectedListener{


	override fun onCreate(savedInstanceState: Bundle?) {
		super.onCreate(savedInstanceState)
		setContentView(R.layout.activity_main)

		initializeResource()
	}

	fun initializeResource(){
//		BottomNavigationViewを設定
		val bottomNavigationView = findViewById<BottomNavigationView>(R.id.bottom_navigation_view)
		bottomNavigationView.setOnNavigationItemSelectedListener(this)
		bottomNavigationView.itemIconSize = 70
		bottomNavigationView.scaleX = 1.2f
		bottomNavigationView.scaleY = 1.2f

	}

	override fun onNavigationItemSelected(item: MenuItem): Boolean {
		Log.d(TAG,"Selected item: " + item)

		when(item.itemId){
//			ホームボタンが押された時
			R.id.action_home -> {
				supportFragmentManager.beginTransaction()
						.replace(R.id.container,HomeFragment())
						.setReorderingAllowed(true)
						.commit()
			}
//			設定ボタンが押された時
			R.id.action_setting -> {
				supportFragmentManager.beginTransaction()
						.replace(R.id.container,SettingFragment())
						.setReorderingAllowed(true)
						.commit()
			}

		}

		return true
	}

	companion object{
		const val TAG : String = "MainActivity"
	}
}

class MySharedPrefs (context: Context){
	private val preferences = context.getSharedPreferences("MyPrefs", Context.MODE_PRIVATE)
	private val urlKey = "url"
	private val cacheKey = "cache"
	private val zoomKey = "zoom"

	fun getUrl() : String?{
		return preferences.getString(urlKey,"https://techgrowup.net/")
	}

	fun setUrl(value : String){
		val editor = preferences.edit()
		editor.putString(urlKey,value)
		editor.apply()
	}

	fun getCache() :  Int{
		return preferences.getInt(cacheKey, CACHE_DEFAULT)
	}

	fun setCache(value: Int){
		val editor = preferences.edit()
		editor.putInt(cacheKey,value)
		editor.apply()
	}

	fun getZoom() :  Int{
		return preferences.getInt(zoomKey, ZOOM_DEFAULT)
	}

	fun setZoom(value: Int){
		val editor = preferences.edit()
		editor.putInt(zoomKey,value)
		editor.apply()
	}

	companion object{
		const val CACHE_DEFAULT= 1
		const val NO_CACHE = 2
		const val CACHE_ONLY = 3

		const val ZOOM_DEFAULT = 100
	}
}

class SettingFragment : Fragment() {
    lateinit var  mySharedPrefs : MySharedPrefs

    lateinit var saveButton: Button
    lateinit var urlText : TextView
    lateinit var cacheRadioGroup : RadioGroup
    lateinit var zoomText: TextView


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_setting, container, false)
        initializeResource(view)

        return view
    }

    fun initializeResource(view : View){
//        SharedPreferenceのインスタンス生成
        mySharedPrefs = MySharedPrefs(context!!)

//        保存ボタンの初期設定
        saveButton = view.findViewById<Button>(R.id.save_button)
        saveButton.setOnClickListener {
            saveData(saveButton)
        }

//        埋め込みURLの初期設定
        urlText = view.findViewById<TextView>(R.id.url_plaintext)
        urlText.text = mySharedPrefs.getUrl()

//        キャッシュタイプラジオボタンの初期設定
        cacheRadioGroup = view.findViewById<RadioGroup>(R.id.cache_radiogroup)
        when (mySharedPrefs.getCache()){
            MySharedPrefs.CACHE_DEFAULT -> cacheRadioGroup.check(R.id.cache_default_radiobutton)
            MySharedPrefs.NO_CACHE -> cacheRadioGroup.check(R.id.nocache_radiobutton)
            MySharedPrefs.CACHE_ONLY -> cacheRadioGroup.check(R.id.cacheonly_radiobutton)
        }

//        ズーム倍率の初期設定
        zoomText = view.findViewById<TextView>(R.id.textzoom_plaintext)
        zoomText.text = mySharedPrefs.getZoom().toString()
    }


    fun saveData(view : View){
//        データの取得
        Log.d(TAG,"save datas")
        val url = urlText.text.toString()
        val checkedRadioButton = cacheRadioGroup.checkedRadioButtonId
        var cacheType = MySharedPrefs.CACHE_DEFAULT
        when(checkedRadioButton){
            R.id.cache_default_radiobutton -> cacheType = MySharedPrefs.CACHE_DEFAULT
            R.id.nocache_radiobutton -> cacheType = MySharedPrefs.NO_CACHE
            R.id.cacheonly_radiobutton -> cacheType = MySharedPrefs.CACHE_ONLY
        }
        val zoom = Integer.parseInt(zoomText.text.toString())

//        データの保存
        mySharedPrefs.setUrl(url)
        mySharedPrefs.setCache(cacheType)
        mySharedPrefs.setZoom(zoom)

        Toast.makeText(context,"データが更新されました。",Toast.LENGTH_LONG).show()
    }

    companion object {
       const val TAG : String = "SettingFragment"
    }
}

class HomeFragment : Fragment() {
    lateinit var  mySharedPrefs : MySharedPrefs

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_home, container, false)
        initializeResource(view)

        return view
    }

    fun initializeResource(view : View){
//        SharedPreferenceのインスタンス生成
        mySharedPrefs = MySharedPrefs(context!!)

//        データの取得
        val url = mySharedPrefs.getUrl()
        val cacheType = mySharedPrefs.getCache()
        val zoom = mySharedPrefs.getZoom()

//        WebViewの設定
        val webView = view.findViewById<WebView>(R.id.webview)
        webView.webViewClient = WebViewClient()
        webView.settings.javaScriptEnabled = true

        when(cacheType){
            MySharedPrefs.CACHE_DEFAULT -> webView.settings.cacheMode = WebSettings.LOAD_DEFAULT
            MySharedPrefs.NO_CACHE -> webView.settings.cacheMode = WebSettings.LOAD_NO_CACHE
            MySharedPrefs.CACHE_ONLY -> webView.settings.cacheMode = WebSettings.LOAD_CACHE_ONLY
        }

        webView.settings.textZoom = zoom
        webView.loadUrl(url!!)
    }

    companion object {
        const val TAG : String = "HomeFragment"
    }
}