prefnotifiers

This library represents shared_preferences as ValueNotifier objects.

It fits in well with the paradigm of data models. Models make data readily available to widgets.

Reads and writes occur asynchronously in background.

Why use PrefNotifier?

Suppose, we have parameter, that can be read with shared_preferences like that:

final prefs = await SharedPreferences.getInstance();
int myParamValue = await prefs.getInt("MyParameter");

There are two lines of problem:

• This code is asynchronous. We cannot use such code directly when building a widget
• The paramValue does not reflect the parameter changes

Instead, we suggest using the new PrefNotifier class for accessing the parameter:

final myParam = PrefNotifier<int>("MyParameter");

• myParam object can be used as the only representation of "MyParameter" in the whole program
• myParam.value allows indirectly read and write the shared preference value without getting out of sync
• Widget build(_) methods can access value without relying on FutureBuilder
• myParam.addListener makes it possible to track changes of the value

What is PrefNotifier?

PrefNotifier.value provides the best value we have for the moment. The actual read/write operations happen asynchronously in background.

PrefNotifier serves as a model for an individual parameter stored in shared preferences.

Types

Basic operations

But the most great is:

myParam.addListener(() => print('Value changed! New value: ${myParam.value}');

How to use PrefNotifier?

Create

final myParam = PrefNotifier<int>("MyParameter");

final myParam = PrefItem<int>(SharedPrefsStorage(), "MyParameter");

Read

Reading is is not finished yet. But we already can access myParam.value. By default, it returns null. We can use it in synchronous code:

Widget build(BuildContext context) {
    if (myParam.value==null)
        return Text("Not initialized yet");
    else
        return Text("Value is ${myParam.value}");
}

Since PrefNotifier inherits from the ValueListenable, we can automatically rebuild the widget when the new value of myParam will be available:

Widget build(BuildContext context) {
    return ValueListenableBuilder(
        valueListenable: myParam,
        builder: (BuildContext context, int? value, Widget child) {
            if (value==null)
                return Text("Not initialized yet");
            else
                return Text("Value is $value");
        });
}

Write

The code above will also rebuild the widget when value is changed. Let's change the value in a button callback:

onTap: () {
    // myParam.value is 3, shared preferences value is 3

    myParam.value += 1;
    myParam.value += 1;

    // myParam.value is already changed to 5
    //
    // The widget will rebuild momentarily (i.e. on the next frame)
    //
    // Shared preferences still contain value 3. But asynchronous writing
    // already started. It will rewrite value in a few milliseconds
}

Load

For a newly created PrefNotifier the value returns null until the object reads the actual data from the storage. Asynchronous loading starts automatically when the object is created.

But what if we want to get the loaded data before doing anything else?

final myParam = PrefNotifier<int>("TheParameter");
await myParam.initialized;

// we waited while the object was reading the data.
// Now myParam.value returns the value from the storage, not default NULL.
// Even if it is NULL, it is a NULL from the storage :)

Keep in sync

Create a single instance of PrefNotifier for a particular parameter. Only access this parameter with this PrefNotifier instance.

final myParam = PrefNotifier<int>("MyParameter");
myParam.value = 5;

await (await SharedPreferences.getInstance())
    .setInt("MyParameter", 10); // DON'T DO THIS

var otherNotifier = PrefNotifier<int>("MyParameter"); // DON'T DO THIS
otherNotifier = 20;

// now the myParam.value is still 5.
// And the myParam has no idea it is changed

Test

To test a program that uses PrefNotifiers, you can populate initial values the same way, as in the case of using SharedPreferences directly:

TestWidgetsFlutterBinding.ensureInitialized();
SharedPreferences.setMockInitialValues(Map<String, dynamic> values);