Hi guys We have this weird scenario where we have to subscri

Question

Hi guys We have this weird scenario where we have to subscribe to one stream let s call it notification stream then before any item appears trigger another completable which immediately returns empty response but as a side effect starts emitting on the notification stream And we simply want to return this notification stream from our function Is there a nice RX way of doing this

kioba · Answer

I am not exactly sure about the request But I try my best and help me out if something not the same as you would expect Here the side effect will emit events downstream ``` Observable merge sideEffectObservable notification ``` Or if you want the SideEffect to trigger and finish before the Notifications ```notifications startWith sideEffectObservable ```

gregd · Answer

Hi I probably didn t describe it correctly slightly smiling face I ll try again

gregd · Answer

We need something like this

gregd · Answer

1 Subscribe to notifications 2 Send START completable which activates notifications 3 Return notifications stream

gregd · Answer

So in other words we only need the notification stream but we need to send this START comand completable to activate it

kioba · Answer

`startWith` helps you with that `sideEffectObservable` will be executed first triggering the notification and than continue listening to the notification events

arekolek · Answer

I was thinking ```notifications mergeWith trigger ```

arekolek · Answer

for example ```val notifications = PublishRelay create lt Int gt val trigger = Completable fromAction thread for i in 1 10 notifications accept i Thread sleep 1000 val latch = CountDownLatch 1 notifications mergeWith trigger take 5 subscribeBy onNext = println it onComplete = latch countDown latch await ```

gregd · Answer

Yeah we ve experimented with merge but The thing with `startWith` and `merge` is that both streams have to be of the same type which is not the case here Also even if they were we don t want the emission of START being present in the returned stream

arekolek · Answer

`mergeWith` takes a `Completable`

arekolek · Answer

as seen above

gregd · Answer

Right sorry for that confusion I got used to Rx2 0 naming But in this case it s actually an Observable that emits a single item It s a really bad API in one of our libraries confused

arekolek · Answer

So `notifications mergeWith trigger ignoreElements ` then

kioba · Answer

you can silence any stream emmision with a ```flatmap Observable empty lt MyType gt ```

gregd · Answer

Hmm interesting Seems a bit hacky to deliberately silence a stream but ok Thanks guys I appreciate your help

arekolek · Answer

I thought you re not interested in that element and just need to subscribe to that stream In that case I m not sure what else is there other than ignoring the elements What should be done with the elements for it not to seem hacky thinking face