How do you guys deal with pagination and database ...

# rx

[ursus]

How do you guys deal with pagination and database observables?
I have refresh() in the domain layer which calls the api, and writes to the database

fun refresh(page: Int) : Completable {
    .. call api
   database.write(...)
}

(edited)
and now I want to display progressbar while this is happening + progressbar at the end if page > 1 (edited)
the trouble with this is that presentation layer has a query observable opened liek this

ViewModel {

   database.fooQuery()
     .subscribe { ... post to adapter }
}

which means that this will emit at the time database.write is called
so If map refresh to events of Started, Success, Error
Success will come after the database is refreshed
which to me is weird, since it should be done in one go, but that means refresh would need to return that data too (edited)
tldr; writing to the database is side-effecting - how to deal with that?

[Matej Drobnič]

I solve this by wrapping data into wrapper class that also signals status of the data:

sealed class Resource<T> {
    object Loading<T>() : Resource<T>
    data class Success<T>(val data : T) : Resource<T>

}

[Matej Drobnič]

So your fooQuery would return Resource<Data>

[Matej Drobnič]

in refresh you update fooQuery to switch to Loading and when data actually refreshes, it switches back to Success

[Matej Drobnič]

and then you use this state to display progress bar instead of refresh state

[Matej Drobnič]

You can also have some sort of state machine behind that combines fooQuery and refresh - after you call database.write, refresh would not complete immediatelly, but would wait for fooQuery to trigger before completing

[ursus]

If I were to wrap the query to the Resource, how would I send the Loading down its pipe? Also, how to prevent it from them automatically emitting Success whenever db changes since its Room / SqlDelight observable?

[ursus]

@Matej Drobnič

[Matej Drobnič]

This is how I do it:

private var loading: Boolean = true
private val dataSubject = BehaviorProcessor.create<Resource<List<T>>>()

your fooQuery() is mapped to the dataSubject. Whenever new item arrives from DB, it checks for the value of loading and wraps it around with appropriate resource.

I'm not aware of any way to actually check whether db changes were the ones you wanted or not or something else changing. But it is very likely that first database change after your write is gonna be update from this write, so I don't think it is unreasonable to just assume some things and set Success there.

[ursus]

well, you never know.. its assumption -.-