【Flutter】設定情報の利用

pubspec.yamlにShared Preferencesというパッケージを追加する必要がある

dependencies:
  flutter:
    sdk: flutter
  path_provider: any
  shared_preferences: any

class _MyHomePageState extends State<MyHomePage> {
  final _controller = TextEditingController();
  double _r = 0.0;
  double _g = 0.0;
  double _b = 0.0;

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Padding(
        padding: EdgeInsets.all(20),
        child:Column(
          children:<Widget> [
               Text('PREFERENCES ACCESS',
               style: TextStyle(fontSize: 32,
               fontWeight: ui.FontWeight.w500),
               ),
              Padding(padding: EdgeInsets.all(10.0)),
              TextField(
                controller: _controller,
                style: TextStyle(fontSize: 24),
                minLines: 1,
                maxLines: 5,
              ),
              Padding(padding: EdgeInsets.all(10.0)),
              Slider(
                min: 0.0,
                max: 255.0,
                value: _r,
                divisions: 255,
                onChanged: (double value) {
                  setState(() {
                    _r = value;
                  });
                },
              ),
            Slider(
              min: 0.0,
              max: 255.0,
              value: _g,
              divisions: 255,
              onChanged: (double value) {
                setState(() {
                  _g = value;
                });
              },
            ),
            Slider(
              min: 0.0,
              max: 255.0,
              value: _b,
              divisions: 255,
              onChanged: (double value) {
                setState(() {
                  _b = value;
                });
              },
            ),
              Container(
                padding: EdgeInsets.all(20),
                width: 125,
                height: 125,
                color: Color.fromARGB(255, _r.toInt(), _g.toInt(), _b.toInt()),
              ),
              ],
            ),
          ),
          floatingActionButton: FloatingActionButton(
            child: Icon(Icons.open_in_new),
            onPressed: () {
              savePref();
              showDialog(
                context: context,
                builder: (BuildContext context) =>
                    AlertDialog(
                      title: Text("saved!"),
                      content: Text("save preferences."),
                    )
              );
            }
          )
    );
  }

  void loadPref() async {
    final prefs = await SharedPreferences.getInstance();
    setState((){
      _r = (prefs.getDouble('r') ?? 0.0);
      _g = (prefs.getDouble('g') ?? 0.0);
      _b = (prefs.getDouble('b') ?? 0.0);
      _controller.text = (prefs.getString('input') ?? '');
    });
  }

  void savePref() async {
    final prefs = await SharedPreferences.getInstance();
    prefs.setDouble('r', _r);
    prefs.setDouble('g', _g);
    prefs.setDouble('b', _b);
    prefs.setString('input', _controller.text);
  }
}