async_phase_notifier 0.0.1

A variant of ValueNotifier that helps you better manage phases of asynchronous operations.

A variant of ValueNotifier that has AsyncPhase representing the initial / waiting / complete / error phases of an asynchronous operation.

AsyncPhaseNotifier + AsyncPhase is similar to AsyncNotifier + AsyncValue of Riverpod.

Unlike AsyncNotifier and AsyncValue, which are tied to package:riverpod, AsyncPhaseNotifier and AsyncPhase have no such binding. The notifier can be used as just a handy variant of ValueNotifier with AsyncPhase as its value and convenient methods for manipulating the phases.

Sample apps

• Useless Facts - simple
• pub.dev explorer - advanced

Usage

runAsync()

The runAsync() method of AsyncPhaseNotifier executes an asynchronous function, updates the value of AsyncPhaseNotifier automatically according to the phase of the asynchronous operation, and notifies the listeners of those changes.

1. The value of the notifier is switched to AsyncWaiting when the operation starts.
2. The change is notified to listeners.
3. The value is switched to either AsyncComplete or AsyncError depending on the result.
4. The change is notified to listeners.

final notifier = AsyncPhaseNotifier<int>();
notifier.runAsync((data) => someAsyncOperation());

AsyncPhase

The value of AsyncPhaseNotifier is either AsyncInitial, AsyncWaiting, AsyncComplete or AsyncError. They are subtypes of AsyncPhase.

AsyncPhase provides the when() and whenOrNull() methods, which are useful for choosing an action based on the current phase, like returning an appropriate widget.

child: phase.when(
  initial: (data) => Text('phase: AsyncInitial($data)'), // Optional
  waiting: (data) => Text('phase: AsyncWaiting($data)'),
  complete: (data) => Text('phase: AsyncComplete($data)'),
  error: (data, error, stackTrace) => Text('phase: AsyncError($data, $error)'),
)

async_phase is a separate package, contained in this package. See its document for details not covered here.

Listening for errors

listenError()

You can listen for errors to imperatively trigger some action when the phase is turned into AsyncError by runAsync(), like showing an AlertDialog or a SnackBar, or to just log them.

final notifier = AsyncPhaseNotifier<Auth>();
final removeErrorListener = notifier.listenError((e, s) { ... });

...

// Remove the listener if it is no longer necessary.
removeErrorListener();

AsyncErrorListener

It is also possible to use AsyncErrorListener to listen for errors. The onError callback is called when an asynchronous operation results in failure.

child: AsyncErrorListener(
  notifier: notifier,
  onError: (context, error, stackTrace) {
    ScaffoldMessenger.of(context).showMaterialBanner(...);
  },
  child: ...,
)

A listener is added per each AsyncErrorListener. Please note that if you use this widget at multiple places for a single notifier, the callback functions of all those widgets are called on error.

Examples

The examples here illustrate how to show a particular UI component depending on the phase of an asynchronous operation.

class WeatherNotifier extends AsyncPhaseNotifier<Weather> {
  WeatherNotifier();

  final repository = WeatherRepository();

  void fetch() {
    runAsync((weather) => repository.fetchWeather(Cities.tokyo));
  }
}

Above is an AsyncPhaseNotifier that fetches the weather info of a city and notifies its listeners. We'll see in the examples below how the notifier is used in combination with each of the several ways to rebuild a widget.

With ValueListenableBuilder

final notifier = WeatherNotifier();
notifier.fetch();

@override
Widget build(BuildContext context) {
  return ValueListenableBuilder<AsyncPhase<Weather>>(
    valueListenable: notifier,
    builder: (context, phase, _) {
      // Shows a progress indicator while fetching and
      // either the result or an error when finished.
      return phase.when(
        waiting: (weather) => const CircularProgressIndicator(),
        complete: (weather) => Text('$weather'),
        error: (weather, e, s) => Text('$e'),
      );
    },
  ); 
}

Or you can use AnimatedBuilder in a similar way.

With Provider

final notifier = WeatherNotifier();

...

ValueListenableProvider<AsyncPhase<Weather>>.value(
  value: notifier,
  child: MaterialApp(home: ...),
)

...

notifier.fetch();

@override
Widget build(BuildContext context) {
  final phase = context.watch<AsyncPhase<Weather>>();

  return phase.when(
    waiting: (weather) => const CircularProgressIndicator(),
    complete: (weather) => Text('$weather'),
    error: (weather, e, s) => Text('$e'),
  );
}

With Grab

final notifier = WeatherNotifier();
notifier.fetch();

@override
Widget build(BuildContext context) {
  final phase = context.grab<AsyncPhase<Weather>>(notifier);

  return phase.when(
    waiting: (weather) => const CircularProgressIndicator(),
    complete: (weather) => Text('$weather'),
    error: (weather, e, s) => Text('$e'),
  );
}

TODO

• Add API documents
• Write tests