Improve chaining multiple (three) Completables by using RxJava

Question

I want to invoke one Completable after another. That is when first is finished, invoke second and when the second is finished, invoke the third one. I already tried some different stuff like andThen(), but found out it does not "wait" for previous Completable to finish as it runs parallel. So far, this is what I have found and it works as I wanna, but is there any better way to improve this. Is there any operation function or something to get rid of Completable.defer at every stage?

Here is so far working example:

   private fun invokeAllThreeDoSomeLogic() {
        
        disposable.add(Completable.concatArray(
                Completable.defer {
                    firstApi.getData().doOnError { t: Throwable? ->
                        Timber.w(
                            t,
                            "first error"
                        )
                    }
                },
                Completable.defer {
                    Completable.fromObservable(secondApi.getData()
                        .doOnError { t: Throwable? ->
                            Timber.i(
                                t,
                                "Second error"
                            )
                        }
                },
                Completable.defer {
                    thirdApi.refresh().doOnError { t: Throwable? ->
                        Timber.i(
                            t,
                            "Third error"
                        )
                    }
                }
            ).subscribe(
                { },
                { t: Throwable? ->
                    Timber.w(t, "something went wrong")
                })
        )
    }

Abu Yousuf · Answer 1 · 2021-12-22T09:39:35.297

0

Use FlatMap for chained api call. It will be like

firstApi.getData()
   .flatmap(dataFromApi1 -> return secondApi.getData())
   ......
   ......

Check this medium blog for better understanding.

Edit:

For Completable you can use andThen operator .

firstCompletable
    .andThen(secondCompletable)

Please check this answer.

From that answer:

In general, this operator is a "replacement" for a flatMap on Completable:

edited Dec 22 '21 at 09:39

answered Dec 22 '21 at 09:21

Abu Yousuf

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Good idea but I tried it. The problem is I can not invoke flatMap on completable as firstApi.getData returns Completable(). Or am I wrong? – DM developing Dec 22 '21 at 09:24
You don't need `Completable` , you can use `Single` as its an api. Check the blog link – Abu Yousuf Dec 22 '21 at 09:27
You are correct, but it is not my decision what type should be returned. – DM developing Dec 22 '21 at 09:28
Updated my answer for `Completable` , please check – Abu Yousuf Dec 22 '21 at 09:40
Yea, as I said in question text, did try andThen(sec).andThen(third) but it does not go from first-> sec -> third...It runs parallel. It is important to run one after another when finished. – DM developing Dec 22 '21 at 11:26
I tried your solution andThen(Completable.defer {sec}) and it works! – DM developing Dec 22 '21 at 11:37

Improve chaining multiple (three) Completables by using RxJava

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