RxJava Combining Observables

Question

I would like to have the result of multiple observables combined into a parent observable. I don't have access to all the observables at once, they will be subscribed throughout the execution of the program.

So far this was my approach:

...
compoundObservable = compoundObservable.mergeWith(firstObservable);
...
compoundObservable = compoundObservable.mergeWith(secondObservable);
...

The approach hasn't worked, as the events sent by the otherObservable aren't registered by the subscribers of the compoundObservable.

How can I combine these observables?

So, firstObservable and secondObservable will be created only after someone has subscribed to (what was then) compoundOversable? — david.mihola
firstObservable and secondObservable have already been created. I'm looking to accumulate the result of each observable in the compoundObservable. As for a Subject I'm open to it - post an answer! — bkach
OK, but when are you subscribing to compoundObservable? The crucial point is that all Observables are immutable: mergeWith returns a new instance that's different from the previous value of compoundObservable, even though you're using the same variable name to keep it. The main point is: You'd have to merge(With) before subscribing - if you merge afterwards you'll just never use the merged Observable. — david.mihola

Lamorak Lamorak · Accepted Answer · 2017-04-12T09:44:16

Using a Subject you can subscribe to observables at any time and all the subscribers of the subject will get all the items. This can be a bit tricky when debugging, though.

PublishSubject<Object> subject = PublishSubject.create();
Observable<Object> observable1 = ...;
observable1.subscribe(subject);

subject.subscribe(...) // will eventually also get emisions from observable2

Observable<Object> observable2 = ...;
observable2.subscribe(subject);

RxJava Combining Observables

2 Answers