RxJava: How do I make a fetch-once-and-reuse Observable?

Question

On every app launch, I have a Retrofit Observable that fetches the user's username from a server. I want to use this value for every subsequent subscriber, but it seems like every time I call subscribe(), the value is re-fetched from the network. Since the username isn't likely to change during the app's lifecycle, I'm not recreating the Observable object, only instantiating once.

How I want it to work is:

Create the Observable once

Fetch the username once, save that value in the Observable

On subscribe, use that saved value, or if it's not done fetching, wait for it to be fetched

How should I go about this?

Answer 1

You can use cache() which will retrieve the user name for the very first subscriber and will just replay the value to any subscribers then on (including the first of course).

Answer 2

To elaborate on David's correct answer, here's some code that illustrates the use of cache:

public class Caching {
    public static void main(String[] args) throws IOException {
        Observable<String> observable = doSomethingExpensive().cache();
        observable.subscribe(System.out::println);
        observable.subscribe(System.out::println);
    }

    private static Observable<String> doSomethingExpensive(){
        return Observable.create(subscriber -> {
            System.out.println("Doing something expensive");
            subscriber.onNext("A result");
            subscriber.onCompleted();
        });
    }
}

Note that, even though you get results twice, you only do something expensive once.