CODEWITHSUNDEEP
rx-swift
Artur Hellmann
Artur Hellmann

Reputation: 325

RXSwift first element of one observable after result of other observable

I am quite new in the reacitve world and now I have a problem:

I have two functions that returns observables:

func connect() -> Observable<Connection>

and 

func execute(_ statement: Statement) -> Observable<Result>

What I now need to do is, that connect has to run before execute and then I have to return the result of execute...

What I thought I will do is to run the execute function inside of the map function of the observable returned by connect, like this:

public func executeFetch(_ query: Statement) -> Observable<Result> {
    return self.connect()
    .map({ (connection) -> Result in
        return execute(query) // here is my problem because execute returns an Observable<Result>.
    })
}

But because execute returns an Observable<Result> I get an Observable<Observable<Result>>

How can I solve this problem?

Upvotes: 1

Views: 2052

Answers (1)

drhr
drhr

Reputation: 2281

Instead of map, try using flatMap. flatMap allows you to chain the previous result into another Observable, whereas map is more of a direct transformation of values (e.g. turning an Observable<A> into an Observable<B>.

There's nothing wrong with performing a bunch of synchronous work inside map, especially if you use a Scheduler to ensure that that work is done on an appropriate thread (e.g. .subscribeOn(ConcurrentDispatchQueueScheduler(globalConcurrentQueueQOS: .Background))), but if you're trying to weave a second Observable into your chain then flatMap would be the idiomatic way to do it.

Upvotes: 1

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