ProviderObserver

ProviderObserver listens to the changes of a ProviderContainer.

To use it, extend the class ProviderObserver and override the method you like to use.

ProviderObserver has four methods :

  • didAddProvider is called every time a provider was initialized, and the value exposed is value.
  • didDisposeProvider is called every time A provider was disposed
  • didUpdateProvider is called every time my providers when they emit a notification.
  • mayHaveChanged is called when the dependency of a provider changed, but it is not yet sure if the computed value changes.
info

It is possible that mayHaveChanged will be called, without didUpdateProvider being called, such as when a Provider is re-computed but returns a value == to the previous one.

Usage :

A simple use case for ProviderObserver, is to log the changes in providers by overriding the method didUpdateProvider.

// A Counter example implemented with riverpod with Logger
class Logger extends ProviderObserver {
@override
void didUpdateProvider(ProviderBase provider, Object newValue) {
print('''
{
"provider": "${provider.name ?? provider.runtimeType}",
"newValue": "$newValue"
}''');
}
}
void main() {
runApp(
// Adding ProviderScope enables Riverpod for the entire project
// Adding our Logger to the list of observers
ProviderScope(observers: [Logger()], child: MyApp()),
);
}
class MyApp extends StatelessWidget {
const MyApp({Key key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(home: Home());
}
}
final counterProvider = StateProvider((ref) => 0);
class Home extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text('Counter example')),
body: Center(
child: Consumer(builder: (context, watch, _) {
final count = watch(counterProvider).state;
return Text('$count');
}),
),
floatingActionButton: FloatingActionButton(
onPressed: () => context.read(counterProvider).state++,
child: const Icon(Icons.add),
),
);
}
}

Now that every time the value of our provider is updated, the logger will log it :

I/flutter (16783): {
I/flutter (16783): "provider": "counter",
I/flutter (16783): "newValue": "Instance of 'StateController<int>'"
I/flutter (16783): }