Bloc: UI does not updates if Iterable is not converted to List manually

[nguyenxndaidev]

Assume we want to load a list of todos:

abstract class TodosState extends Equatable {
  const TodosState();

  @override
  List<Object> get props => [];
}

class TodosFilteredState extends TodosState {
  final List<Todo> todos;

  TodosFilteredState([this.todos = const []]);

  @override
  List<Object> get props => [todos];
}

class TodosFilteredByCategory extends TodosEvent {
  final int categoryId;

  TodosFilteredByCategory(this.categoryId);

  @override
  List<Object> get props => [categoryId];
}

class TodosBloc extends Bloc<TodosEvent, TodosState> {
  @override
  Stream<TodosState> mapEventToState(TodosEvent event) async* {
    // Test data
    List<Todo> todos = [Todo(id = 0, categoryId = 0), Todo(id = 1, categoryId = 1)];

    if (event is TodosFilteredByCategory) {
      yield TodosFilteredState(todos.where((todo) => todo.categoryId == event.categoryId));
    }
  }
}

If I use BlocBuilder to build UI, the UI does not build with the list of filtered todos.

The problem is at the line yield TodosFilteredState(todos.where((todo) => todo.categoryId == event.categoryId));. The where() method returns Iterable<Todo> with no compile error.

To fix the problem, I must convert it to List<Todo>: yield TodosFilteredState(todos.where((todo) => todo.categoryId == event.categoryId).toList());

Expected behavior
The UI should be rebuilt without converting Iterable<Todo> to List<Todo> manually. This kind of runtime error is difficult to detect because the compiler doesn't give any complain or warning.

[nguyenxndaidev]

The same error happen when the type Map<A, B> is needed but we return a more general type Map<dynamic, dynamic>.

[felangel]

Hi @nguyenxndaidev 👋
Thanks for opening an issue!

You should override onError in your bloc if you want to handle exceptions thrown within the bloc. Let me know if that helps, thanks!

[nguyenxndaidev]

@felangel Thanks for you help!

I create a log delegate and print error/stacktrace from there and I get:

I/flutter ( 7135): type 'WhereIterable<Todo>' is not a subtype of type 'List<Todo>'

It's very useful! Thanks again.

[nguyenxndaidev]

I created an issue to improve dart compiler here https://github.com/dart-lang/sdk/issues/41155

[nguyenxndaidev]

I created an issue to improve dart compiler here dart-lang/sdk#41155

For now we can disable implicit-casts in analysis_options.yaml to get compile time error:

analyzer:
  strong-mode:
    implicit-casts: false