[AsyncNotifier] How to set AsyncError in custom state?

[phamquoctrongnta]

Assume I have MovieState using freezed:

@freezed
class MovieState with _$MovieState {
  const factory MovieState({
    @Default([]) List<MovieResponse> movies,
  }) = _MovieState;
}

In MovieNotifier

class MovieNotifier extends AsyncNotifier<MovieState> {
@override
  Future<MovieState> build() async {
    state = const AsyncLoading();
    final repository = ref.read(movieRepositoryProvider);
    final response = await repository.fetchMovies();
    List<MovieResponse> result = [];
    response.fold((l) {
      state = AsyncError(l, StackTrace.current);
    }, (r) {
      result = r;
      state = const AsyncData(MovieState());
    });
    return MovieState(
      movies: result,
    );
  }
}

In MovieView:

asyncValue.when(
        data: (data) => ListView.builder(
          itemBuilder: (context, position) {
            return MovieItem(movie: data.movies[position]);
          },
          itemCount: data.movies.length,
        ),
        error: (error, trace) => const SizedBox(),
          child: CircularProgressIndicator(),
        ),
      )

When hasError is true and state = AsyncError('My error', StackTrace.current); triggered, error on asyncValue.when not called, the data always called.

Question:

1. How to trigger state = AsyncError in AsyncNotifier?
2. What I should return for each case success and error? How to use copyWith function like StateNotifier

[noriHanda]

Your build method should be changed like this

Future<MovieState> build() async {
    final repository = ref.watch(movieRepositoryProvider);
    final response = await repository.fetchMovies();
    return MovieState(movies: response);
  }

Loading and error associated with Future calls gets automatically handled inside build method of AsyncNotifier

[phamquoctrongnta]

@utamori
In 28 line: (error) => AsyncError(error, StackTrace.current),
I think it won't fire on when.error because the function always returns unit. Have you tested it?

[noriHanda]

@phamquoctrongnta I still don't get it... You can just see if the state hasError.

[phamquoctrongnta]

@noriHanda
You said:

I don't see the need of Either because you can just throw Exception(e).

I reply:

I used Either because I want to catch the error clearly.
Ex:
if(left){
doError();
}else{
doSomething();
}

I want to return Either with Left instead throw an Exception on repository.fetchMovies().
So can I throw an exception on build method of Notifier? Is it the best practice?
Sorry for my English, hope you understand.

[noriHanda]

@phamquoctrongnta Your English is good. English is not my first language either so we are on the same side :)
What I was trying to tell you was that throwing Exception was a valid way of returning details of error. throw is more suitable than return for exceptions because IDE can jump to where exception has occurred, which means it's more developer friendly. If you just return Exception as you mention, you need to handle manually inside build method of Notifier and it gets harder to track where the Exception originates from.

[phamquoctrongnta]

@noriHanda Thanks for detail answer.
I'm clean.
Thank you, again.

[phamquoctrongnta]

@noriHanda Sorry.
Do you know use copyWith method in AsyncNotifier?
Ex:

state = state.copyWith(
a: newA,
)

[noriHanda]

@phamquoctrongnta There's no copyWith method in AsyncValue. There is, however, copyWithPrevious that is used internally. There isn't many valid use cases for copyWithPrevious, so it shouldn't be your first option.
What you should think about using first is AsyncValue.guard. Any asynchronous operations inside guard clause is handled properly.
https://pub.dev/documentation/riverpod/latest/riverpod/AsyncValue/guard.html

[phamquoctrongnta]

@noriHanda
If I have complex state, what should I do to update new state based on old state?
Ex: I have MyState have 2 flag: bool isChecked = false, bool isLogin = false.
Assume click a any button to change isLogin to true.
Then click a another button to change isChecked to true but keep isLogin state is true?

[Trung15010802]

@noriHanda If I have complex state, what should I do to update new state based on old state? Ex: I have MyState have 2 flag: bool isChecked = false, bool isLogin = false. Assume click a any button to change isLogin to true. Then click a another button to change isChecked to true but keep isLogin state is true?

You can do like me. use state.value

[phamquoctrongnta]

Thanks for response, bro.