Kyle Nazario

I recently did a presentation at work about Angular Signals. One section comparing RxJS and Signals noted that RxJS remained better at asynchronous work. Consider, for example, a text field that takes user input and maps it into API search results.

const searchResults$ = queryValue.pipe(
  switchMap(query => this.apiService.search(query))
);

During the presentation, I said the best way to create an asynchronous computed Signal was to use RxJS.

computed() will not accept an async callback. It also does not provide a done() function that can be called in a callback. You must synchronously return a value from your computed() callback.

After some experimentation, I figured out how to do it. You actually have to go back to RxJS and wrap it with toSignal().

Then a member of the audience raised his hand and asked if I knew about the resource api. It was a new feature coming to Angular that had been announced just a day or two before my presentation.

You can read the pull request yourself. It creates a new function, resource(), which is essentially asyncComputed().

To return to our previous example, here’s how you would use Signals to map user input into a list of search results with resource():

export class SearchComponent {
  protected query = model('');
  protected searchResults: Signal<string[]>;

  constructor(apiService: ApiService) {
    this.searchResults = resource({
      request: this.query,
      loader: async () => {
        const query = this.query();
        const response = await apiService.search(query);
        return response.json();
      }
    });
  }
}

This tells Angular, “if this.query ever changes, ping the API using this async method and return the results in this.searchResults, a Signal we can use anywhere in the class or template.”

resource() will arrive with Angular 19. There’s a bunch of other stuff you can do with it too. At the moment, the only downside I can see is that it can’t easily do some RxJS things, like debouncing.

But you know what? resource() is a promising start. I’m excited we got an official way to make async computed Signals.