What is the use case for doOnSuccess vs onSuccess in rxJava

Question

I'm confusing about use case for doOnSuccess in rxJava.
Let's see the code:

Case 1:

networkApi.callSomething()
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())               
    .doOnSuccess(__ -> showLog(SUCCESS))
    .doOnError(__ -> showLog(ERROR))
    .subscribeBy(
             onSuccess = {//Do something}, 
             onError = {//Show log here}
          )

Case 2:

networkApi.callSomething()
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())               
    .subscribeBy(
             onSuccess = {
               //Do something
               showLog(SUCCESS)
             }, 
             onError = {showLog(ERROR)}
          )

As normal, I think case 2 is fine.
I also have referred some source code in github and I saw some people do like case 1.
I try to ask myself what is the use case for doOnSuccess here ?

Is there any use case that we need apply doOnSuccess() operator ?

Answer 1

Singles and Maybes have a success signal and the handler has the onSuccess method called. Often though, you'd want to side-effect the success signal at various points in the flow so there is the doOnSuccess operator.

getUserAsSingle()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.doOnSuccess(user -> ui.showUser(user))
.flatMap(user -> 
     getUserFavoritesAsSingle(user)
     .subscribeOn(Schedulers.io())
)
.observeOn(AndroidSchedulers.mainThread())
.doOnSuccess(userFavs -> ui.showUserFavorites(userFavs))
.flatMap(userFavs -> 
     updateLoginCounter(userFavs.userId)
     .subscribeOn(Schedulers.io())
)
.observeOn(AndroidSchedulers.mainThread())
subscribe(newCounter -> ui.showLoginCount(newCounter),
    error -> ui.showError(error));