import 'dart:async';

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

class Pref {
  static Completer<Preferences> _completer;

  static Future<Preferences> getInstance(path) async {
    if (_completer == null) {
      _completer = Completer<Preferences>();
      try {
        final test = await Preferences.preferences(path: path);
        _completer.complete(test);
      } on Exception catch (e) {
        _completer.completeError(e);
        final Future<Preferences> prefFuture = _completer.future;
        _completer = null;
        return prefFuture;
      }
    }
    return _completer.future;
  }
}

class AppBody extends StatefulWidget {
  @override
  _AppBodyState createState() => _AppBodyState();
}

class _AppBodyState extends State<AppBody> {
  double _sliderValue = 1;
  Preferences preferences;
  TextEditingController tec = new TextEditingController();

  @override
  void initState() {
    loadSavedData();

    super.initState();
  }

  Future<void> loadSavedData() async {
    preferences = await Pref.getInstance('./lib/prefs');
//    preferences = await Preferences.preferences(path: "save");
    print(preferences.containsKey('tmp'));
    print(preferences.getString('tmp'));
    setState(() {
      if (preferences.containsKey('tmp')) tec.text = preferences.getString('tmp');
    });
  }

  @override
  Widget build(BuildContext context) {
    return getBody();
  }

  getBody() {
    return Container(
        child: Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Text((tec.text == null || tec.text == "") ? "Empty value" : tec.text),
        TextFormField(
          controller: tec,
        ),
        RaisedButton(
          child: Text("Save"),
          onPressed: () async {
            if (await preferences.setString('tmp', tec.text)) {
              setState(() {});
              print(preferences.get('tmp'));
            } else {
              setState(() {
                tec.text = "";
              });
            }
          },
        )
      ],
    ));
  }

The above code does not save/load the values.

…fixed in this pull request

If one uses multiple setValue calls in his/her application, each call causes async writeAsString call. This can lead to the file corruption. At least I found that effect on Linux Desktop platform. To solve this problem I propose a delayed write: setValue just create a delayd task for writing. It no other setValue-s are called the writing occures while if new setValue comes the delay shifts a little bit to the future.