Flutter App with provider and persistent storage

Question

I have been trying to figure out how to build an app in flutter with persistant state manager. I can't seem to get it to work. This is my app with a state manager.

I want to store actual classes, and not just an integer, which makes this a bit tricker, but hey, that's my goal.

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

void main() {
  runApp(
    /// Use a provider. Multiprovider works just fine
    MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (_) => State()),
      ],
      child: const MyApp(),
    ),
  );
}

// Define the data type we want to use
// We will use time and value to track data over time
class MyData {
  final DateTime time;
  final int value;
  MyData(this.time, this.value);
}

// Use a state with a change notifier (provider stuff)
class State with ChangeNotifier {
  late List<MyData> _dataset = [];
  List<MyData> get dataset => _dataset;

  State() {
    // The dataset is a list of objects
    _dataset = [];
  }

  void addData(time, value) {
    // Add data to the dataset
    MyData datapoint = MyData(time, value);
    _dataset.add(datapoint);
  }

  void clearData() {
    // Clear the dataset
    _dataset = [];
  }
}

// The actual widget
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Example'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('You have added this many datapoints'),
            const Count(),
            IconButton(
                onPressed: () =>
                    context.read<State>().addData(DateTime.now(), 100),
                icon: const Icon(Icons.add)),
            IconButton(
                onPressed: () => context.read<State>().clearData(),
                icon: const Icon(Icons.remove))
          ],
        ),
      ),
    );
  }
}

// And the parsing of the data to a widget
class Count extends StatelessWidget {
  const Count({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Text(
      /// Calls `context.watch` to make [Count] rebuild when [Counter] changes.
      '${context.watch<State>().dataset.length}',
      key: const Key('counterState'),
      style: Theme.of(context).textTheme.headlineMedium,
    );
  }
}

The question is. How can I add a persistent logic to this?

Answer 1

The persistent data can be added in the initialization of the state. In order to save the data in a Key-value storage, each object needs to be stringified using something like json.encode and json.decode.

Here's an updated code sn-p that will work.

I removed your comment, and added cmets wherever I added code that will add the persistence logic.

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
// Add shared_preferences and convert
import 'package:shared_preferences/shared_preferences.dart';
import 'dart:convert';

void main() {
  runApp(
    MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (_) => State()),
      ],
      child: const MyApp(),
    ),
  );
}

class MyData {
  final DateTime time;
  final int value;
  MyData(this.time, this.value);

  // Add a converter to from JSON
  static MyData fromJSON(Map<String, dynamic> jsonData) {
    return MyData(DateTime.fromMillisecondsSinceEpoch(jsonData["time"]),
        jsonData["value"]);
  }

  // Add a converter to from an encoded JSON string
  static MyData fromJSONString(String jsonDataString) {
    Map<String, dynamic> jsonData = json.decode(jsonDataString);
    return MyData.fromJSON(jsonData);
  }

  // Add a converter to JSON
  dynamic toJSON() {
    return {"time": time.millisecondsSinceEpoch, "value": value};
  }

  // Add a converter to JSON string
  String toJSONString() {
    return json.encode(toJSON());
  }
}

class State with ChangeNotifier {
  late List<MyData> _dataset = [];
  List<MyData> get dataset => _dataset;

  State() {
    _dataset = [];
    // Read the data on the creation of a state
    readData();
  }

  void readData() async {
    // Load the data from the shared preferences
    final prefs = await SharedPreferences.getInstance();
    List<String>? datasetStrings = prefs.getStringList("dataset");
    datasetStrings ??= [];

    // Load the data into the state
    _dataset = datasetStrings
        .map((jsonData) => MyData.fromJSONString(jsonData))
        .toList();

    // Notify the listeners
    notifyListeners();
  }

  void setData() async {
    // Load the shared preferences
    final prefs = await SharedPreferences.getInstance();

    // Load the data into the shared preferences
    List<String> datasetStrings =
        _dataset.map((dataPoint) => dataPoint.toJSONString()).toList();
    await prefs.setStringList("dataset", datasetStrings);
  }

  void addData(time, value) {
    MyData dataPoint = MyData(time, value);
    _dataset.add(dataPoint);
    // Save the data and notify listeners
    setData();
    notifyListeners();
  }

  void clearData() {
    _dataset = [];
    setData();
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Example'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('You have pushed the button this many times:'),
            const Count(),
            IconButton(
                onPressed: () =>
                    context.read<State>().addData(DateTime.now(), 100),
                icon: const Icon(Icons.add)),
            IconButton(
                onPressed: () => context.read<State>().clearData(),
                icon: const Icon(Icons.remove)),
            // Also add a refresh button to test
            // loading of data without losing debugging connection
            IconButton(
                onPressed: () => context.read<State>().readData(),
                icon: const Icon(Icons.refresh))
          ],
        ),
      ),
    );
  }
}

class Count extends StatelessWidget {
  const Count({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Text(
      /// Calls `context.watch` to make [Count] rebuild when [Counter] changes.
      '${context.watch<State>().dataset.length}',
      key: const Key('counterState'),
      style: Theme.of(context).textTheme.headlineMedium,
    );
  }
}