persistent_context 0.0.2

A Flutter widget that provides a simple and synchronous interface to store persistent variables, relying on shared preferences under the hood

PersistentContext

A Flutter widget that provides a simple and synchronous interface to store persistent variables, relying on shared preferences under the hood.

Basic Usage

To start using persistent variables in one of your widgets simply wrap it inside a PersistentContext widget. You can access the instance of PersistentContext and its methods through PersistentContext.of(context) from any descendant widget.

Simply call the method set(key, value) to set a persistent variable named key with value value and get(key) to access the value. Be aware that value can only have the following types, due to shared preferences limitations:

• int
• double
• bool
• String

Every time a value is set, it will be reflected immediately on the UI, and made persistent asynchronously in background.

When your app is first started, however, data will be loaded asynchronously from local storage, so the get method may return null (or the default value, if specified) if called before PersistentContext is initialized.

You can check the ready future to wait for initialization completion and avoid any inconvenience.

The following example illustrates how to use PersistentContext to build a simple counter app that holds the count persistently. Note that no stateful widget is required, as PersistentContext acts also as a simple state manager.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'PersistentContext Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('PersistentContext Demo'),
        ),
        body: Center(
          child: PersistentContext(
            child: CounterWidget(),
          ),
        ),
      ),
    );
  }
}

class CounterWidget extends StatelessWidget {
  void _increment(BuildContext context) {
    final currentCount = PersistentContext.of(context).get('counter') ?? 0;
    PersistentContext.of(context).set(
      'counter',
      currentCount + 1,
    );
  }

  void _reset(BuildContext context) {
    PersistentContext.of(context).set('counter', 0);
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Text(
          'This counter will persist after restarting the app:',
        ),
        Text(
          '${PersistentContext.of(context).get('counter') ?? 0}',
          style: Theme.of(context).textTheme.headline4,
        ),
        ButtonBar(
          alignment: MainAxisAlignment.center,
          children: [
            ElevatedButton.icon(
              onPressed: () => _increment(context),
              icon: Icon(Icons.add),
              label: Text('Increment'),
            ),
            ElevatedButton.icon(
              onPressed: () => _reset(context),
              icon: Icon(Icons.refresh),
              label: Text('Reset'),
            ),
          ],
        ),
      ],
    );
  }
}

Advanced features

PersistentContext constructor features the following optional arguments, that may be useful in advanced use cases:

• defaultValues: a map of keys and values that define default values for persistent records. Note that if a default value has been defined, a variable can only be set to values of the same type.

• prefix: a string prefix that will be added to all keys in shared preferences. It should be used in case of multiple instances of PersistentContext to avoid collisions.

• sharedPreferencesInstance: an instance of SharedPreferences that can be used immediately, if already available. If not provided, the constructor will instantiate a new SharedPreferences object asynchronously, resulting in a small delay.

The following snippet illustrates how to use such arguments in the previous example:

PersistentContext(
    child: CounterWidget(),
    prefix: 'counterContext',
    defaultValues: {
        'counter': 0,
    },
    sharedPreferencesInstance: sharedPrefs,
)

Extending

To keep your code as clean and robust as possible, it is recommended that you don't use PersistentContext directly, but rather extend it with a custom class that calls get and set methods through dedicated getters and setters for each persistent variable, and also defines statically the default values.

Consider for example the following implementation for the counter app example:

class CustomPersistentContext extends PersistentContext {
  // Default values
  static final _counterDefault = 0;

  // Constructor
  CustomPersistentContext({
    Key? key,
    required Widget child,
  }) : super(
          key: key,
          child: child,
          defaultValues: {
            'counter': _counterDefault,
          },
        );

  // Getters and setters
  int get counter => get('counter') ?? _counterDefault;

  set counter(int value) {
    set('counter', value);
  }

  // This is needed for `of` method to work
  static CustomPersistentContext of(BuildContext context) {
    return context
        .dependOnInheritedWidgetOfExactType<CustomPersistentContext>()!;
  }
}

The methods _increment and _reset from CounterWidget can then be simplified as follows:

void _increment(BuildContext context) {
  CustomPersistentContext.of(context).counter++;
}

void _reset(BuildContext context) {
  CustomPersistentContext.of(context).counter = 0;
}