How to avoid multiple mapper calls when using flatMapSingle

Question

Suppose I have a BehaviorProcessor which contains some value v.

Now if I want to asynchronously request some data, which would depend on v I would do it like this:

val res = v.flatMapSingle { asyncRequest(it) }

Now let's log all the invocations of this block (mapper)

val res = v.flatMapSingle {
    println("mapper")
    asyncRequest(it)
}

It will print mapper multiple times, which means asyncRequest is being called multiple times, it seems every time some other dependant stream is being subscribed to

I am trying to avoid multiple mapper invocations (thus avoiding multiple asyncRequest calls).

Is there a way to do so with standard rxjava2 utils?

Answer 1

Use cache() operator. It will cache result of flatMapSingle.

BehaviorProcessor<String> v = BehaviorProcessor.create();
Flowable<String> res = v.flatMapSingle(item -> {
    System.out.println("mapper");
    return asyncRequest(item);
    })
        .cache();
v.onNext("test");
res.subscribe(s->System.out.println("subscribe1 received: "+ s));
res.subscribe(s->System.out.println("subscribe2 received: "+ s));
v.onNext("test2");

Produces

mapper
mapper
subscribe1 received: test async
subscribe2 received: test async
subscribe1 received: test2 async
subscribe2 received: test2 async