AsyncValue
- Flutter course
- Advanced
- Lesson 3/69
- 4 min
Model async state with AsyncValue.
What is AsyncValue?
AsyncValue is a class that models the three possible states of an asynchronous operation (data, loading, error) in a single object. It is constructed as AsyncValue.data(value), AsyncValue.loading() or AsyncValue.error(err, stack). This lets you represent async state with a single type.
final value = AsyncValue<int>.data(42);
value.when(
data: (n) => print('Data: $n'),
loading: () => print('Loading'),
error: (e, s) => print('Error: $e'),
);The when method
The when method takes three callbacks: data, loading and error. It calls whichever callback matches the current state. This is safer than checking states with if-else because it forces you to handle every case and keeps the code readable.
snapshot.when(
data: (user) => Text(user.name),
loading: () => CircularProgressIndicator(),
error: (e, s) => Text('Error: $e'),
);Practice Use AsyncValue.when
Create a value with AsyncValue<int>.data(42). With the when method, print n in data, 'Yükleniyor' in loading, and 'Hata' in error.
Starter code
import 'package:flutter_riverpod/flutter_riverpod.dart';
void main() {
// AsyncValue ile 42 değerini oluştur
// when ile üç durumu işle
}Sample solution
import 'package:flutter_riverpod/flutter_riverpod.dart';
void main() {
final value = AsyncValue<int>.data(42);
value.when(
data: (n) => print(n),
loading: () => print('Yükleniyor'),
error: (e, s) => print('Hata'),
);
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which method of AsyncValue forces handling all three states?
- maybeWhen
- fold
- map
- when Correct answer
Explanation: The when method takes data, loading and error callbacks; you are expected to provide all three.
2 AsyncValue data state
final value = AsyncValue<int>.____(42);final value = AsyncValue<int>.data(42);Explanation: The data constructor produces an AsyncValue holding the successful result.
3 AsyncValue states
| Term | Meaning |
|---|---|
AsyncValue.data |
Data state |
AsyncValue.loading |
Loading state |
AsyncValue.error |
Error state |
when |
Handles all three states |