How to use Observables as a lazy data source - event-handling

Question

I'm wrapping an API that emits events in Observables and currently my datasource code looks something like this, with db.getEventEmitter() returning an EventEmitter.
const Datasource = {
getSomeData() {
return Observable.fromEvent(db.getEventEmitter(), 'value');
}
};
However, to actually use this, I need to both memoize the function and have it return a ReplaySubject, otherwise each subsequent call to getSomeData() would reinitialize the entire sequence and recreate more event emitters or not have any data until the next update, which is undesirable, so my code looks a lot more like this for every function
const someDataCache = null;
const Datasource = {
getSomeData() {
if (someDataCache) { return someDataCache; }
const subject = new ReplaySubject(1);
Observable.fromEvent(db.getEventEmitter(), 'value').subscribe(subject);
someDataCache = subject;
return subject;
}
};
which ends up being quite a lot of boilerplate for just one single function, and becomes more of an issue when there are more parameters.
Is there a better/more elegant design pattern to accomplish this? Basically, I'd like that
Only one event emitter is created.
Callers who call the datasource later get the most recent result.
The event emitters are created when they're needed.
but right now I feel like this pattern is fighting the Observable pattern, resulting a bunch of boilerplate.

Answer 1

As a followup to this question, I ended up commonizing the logic to leverage Observables in this way. publishReplay as cartant mentioned does get me most of the way to what I needed. I've documented what I've learned in this post, with the following tl;dr code:
let first = true
Rx.Observable.create(
observer => {
const callback = data => {
first = false
observer.next(data)
}
const event = first ? 'value' : 'child_changed'
db.ref(path).on(event, callback, error => observer.error(error))
return {event, callback}
},
(handler, {event, callback}) => {
db.ref(path).off(event, callback)
},
)
.map(snapshot => snapshot.val())
.publishReplay(1)
.refCount()