The BLoC Pattern has been designed by Paolo Soares and Cong Hui, from Google and first presented during the DartConf 2018 (January 23-24, 2018). See the video on YouTube.

BLoC stands for Business Logic Component. Initially was conceived to allow to reuse the same code between Flutter and Angular Dart, but in fact is independently of the platform: web application, mobile application, back-end.

It can be considered an alternative to the Redux port for flutter making use of Dart streams. In our case, we’re going to use Observables from the library RxJS, but any other option like xstream will be valid as well.

In short, the BLoC will:

• contain business logic (ideally in bigger applications we will have multiple BLoCs)
• rely exclusively on the use of Observables for both input (Observer) and output (Observable)
• remain platform independent
• remain environment independent

How BLoC works?

I’m not going to explain extensively how BLoC works (there is other people that did a better job I will do here), but just some basic hints.

The BLoC will hold the business logic and components will have no knowledge about what's happening inside. Components will send events_to the BLoC via _Observers and will be notified by the BLoC via Observables.

Implementing the BLoC

This is basic typescript example of a search BLoC using RxJS:

export class SearchBloc {
  private _results$: Observable<string[]>;
  private _preamble$: Observable<string>;
  private _query$ = new BehaviorSubject<string>('');

  constructor(private api: API) {
    this._results$ = this._query$.pipe(
      switchMap(query => {
        return observableFrom(this.api.search(query));
      })
    );
    this._preamble$ = this.results$.pipe(
      withLatestFrom(this._query$, (_, q) => {
        return q ? `Results for ${q}` : 'All results';
      })
    );
  }

  get results$(): Observable<string[]> {
    return this._results$;
  }

  get preamble$(): Observable<string> {
    return this._preamble$;
  }

  get query(): Observer<string> {
    return this._query$;
  }

  dispose() {
    this._query$.complete();
  }
}

results$ and preamble$ are exposed to be subscribed from a component and they express asynchronous values that change in response to changes in query.

query is exposed as Observer<string> to the outside, to allow the addition of new value from the components. Within SearchBloc , we have
_query$: BehaviorSubject<string> as the stream source, and the constructor declares _results$ and _preamble$ to respond to _query$.

Using it on React

In order to use it on React we need to create a new instance of the BLoC and share it to the child components using a React context.

const searchBloc = new SearchBloc(new API());
const SearchContext = React.createContext(searchBloc);

We have to expose it using the context provider:

const App = () => {
  const searchBloc = useContext(SearchContext);

  useEffect(() => {
    return searchBloc.dispose;
  }, [searchBloc]);

  return (
    <SearchContext.Provider>
      <SearchInput />
      <ResultList />
    </SearchContext.Provider>
  );
};

It’s important to have the useEffect returning the dispose method of the BLoC so it will complete the observer when the component is unmounted.

Then we can publish changes to the BLoC from the SearchInput component:

const SearchInput = () => {
   const searchBloc = useContext(SearchContext);
   const [query, setQuery] = useState("");

   useEffect(() => {
      searchBloc.query.next(query);
   }, [searchBloc, query]);

   return <input
            type="text"
            name="Search"
            value={query}
            onChange={({ target }) => setQuery(target.value)}
          />;
}

We got the BLoC using the hook useContext and then with useEffect every time the query change we publish the new value to the BLoC.

Now it’s time to the ResultList:

const ResultList = () => {
  const searchBloc = useContext(SearchContext);
  const [results, setResults] = useState([]);

  useEffect(() => {
    return searchBloc.results$.subscribe(setResults);
  }, [searchBloc]);

  return (
    <div>
      {results.map(({ id, name }) => (
        <div key={id}>{name}</div>
      ))}
    </div>
  );
};

As before, we use the useContext to get the BLoC, but now in the useEffect we subscribe the changes on the results$ interface to update the local state of the component. It’s important to return the return value of the subscribe, since it will unsubscribe when the component is unmounted.

Final thoughts

The final code is not complex, at least if you have a minimal knowledge about Observables and hooks. I have the feeling that the code is quite readable and helps to keep the business logic outside the components. It’s true that we should take care about to unsubscribe from the observables and dispose the BLoC when the components are unmounted, but these issues could be easily solvable creating some new hooks like useBlocObservable and useBlocObserver. But this I’ll try in the future, soon I hope, in a side project I’m using this pattern.

Originally published at magarcia.io on February 18, 2019.