AsyncStateHolder

State container for async operations with loading, error, and data states.

late final user = asyncState<User>();

load(user, () => repo.getUser(id));  // Sets loading → data or error
user.hasData      // Check state
user.dataOrNull   // Access data

Creating Async State

class UserViewModel extends ViewModel {
  late final user = asyncState<User>();
  late final posts = asyncState<List<Post>>();
}

The Four States

sealed class AsyncState<T> {
  AsyncEmpty()
  AsyncLoading()
  AsyncError(String message, {Object? error})
  AsyncData(T data)
}

Loading Data

The load() helper manages the full lifecycle:

void loadUser(String id) {
  load(user, () => repo.getUser(id));
}

1. Sets AsyncLoading
2. Runs async work
3. Sets AsyncData on success, AsyncError on failure
4. Cancels if ViewModel disposes

Manual State Control

user.setLoading();
user.setData(fetchedUser);
user.setError('Failed to load');
user.setEmpty();

Checking State

user.isLoading   // true if AsyncLoading
user.hasData     // true if AsyncData
user.hasError    // true if AsyncError
user.isEmpty     // true if AsyncEmpty

user.dataOrNull  // T?
user.errorOrNull // String?

Building UI

vm.user.build(
  (state) => switch (state) {
    AsyncEmpty() => Text('No user'),
    AsyncLoading() => CircularProgressIndicator(),
    AsyncError(:final message) => Text('Error: $message'),
    AsyncData(:final data) => Text('Hello, ${data.name}'),
  },
)

See Building UI for displayWhen() and listenAsync().

API Summary

Operation	Code
Create	late final user = asyncState<User>()
Load	load(user, () => repo.getUser(id))
Set loading	user.setLoading()
Set data	user.setData(data)
Set error	user.setError('message')
Check	user.isLoading, user.hasData
Get data	user.dataOrNull
Build UI	user.build((state) => ...)