Flutter - Transform flutter bloc event to add debounce

Question

I am trying to make an Infinite list using flutter bloc.

I have taken reference from here: https://github.com/felangel/bloc/blob/926029cae2d7614d38b5a9a8952e36bb59054b02/examples/github_search/common_github_search/lib/src/github_search_bloc/github_search_bloc.dart#L15

According to this post, to add a delay between two events so that api doesn't get spammed, you need to override transformEvents and add debounce to events like this:

@override
  Stream<GithubSearchState> transformEvents(
    Stream<GithubSearchEvent> events,
    Stream<GithubSearchState> Function(GithubSearchEvent event) next,
  ) {
    return (events as Observable<GithubSearchEvent>)
        .debounceTime(
          Duration(milliseconds: 300),
        )
        .switchMap(next);
  }

Problem I am facing is that the Observable has been deprecated by RxDart and I am not sure how to accomplish the above requirement.

Solution 1

I found the solution.

@override
  Stream<Transition< GithubSearchEvent, GithubSearchState >> transformEvents(
      Stream< GithubSearchEvent > events, transitionFn) {
    return events
        .debounceTime(const Duration(milliseconds: 300))
        .switchMap((transitionFn));
  }

Solution 2

Since bloc 8.0.0 the syntax changed and you need to pass it as a transformer to the on function. Apart from the debouncing you should think about concurrency. To combine sequential processing with a debounce, you can provide the following function:

import 'package:rxdart/rxdart.dart';
//...

    on<GithubSearchEvent>(
      (event, emit) {},
      transformer: (events, mapper) {
        return events.debounceTime(const Duration(milliseconds: 300)).asyncExpand(mapper);
      },
    );

You can create a reusable transformer as a top-level function like this:

EventTransformer<Event> debounceSequential<Event>(Duration duration) {
  return (events, mapper) => events.debounceTime(duration).asyncExpand(mapper);
}

and use it like this:

    on<GithubSearchEvent>(
      (event, emit) {},
      transformer: debounceSequential(const Duration(milliseconds: 300)),
    );

Solution 3

I cannt add comment to @user2220771 cause my reputation is too low.

This will not work:

EventTransformer<Event> debounceTransformer<Event>(Duration duration) {
  return (events, mapper) => events.debounceTime(duration);
}

you have to switch map as:

EventTransformer<Event> debounceTransformer<Event>(Duration duration) {
  return (events, mapper) {
    return events.debounceTime(duration).switchMap(mapper);
  };
}

Comments

[Sebastian Roth]

This may skip over events if they are sent too frequent?

[Hamed]

EventTransformer is deprecated, how can we implement this with onEvent?

[user2220771]

@Hamed you can pass the transformer to the on function. There are a few predefined in the bloc_concurrency package but to debounce you have to pass the function yourself.

[minato]

stackoverflow.com/a/70834818/10121512

[minato]

stackoverflow.com/a/70834818/10121512