RxJava2. Execute a request for every item in a list

Question

I have a list List<FileModel>

FileModel is just a class contains id: Int

id - is ID of photo file I need to fetch and cast to Bitmap

I have a request:

fun getFile(fileId: Int): Single<ResponseBody>

This request returns ResponseBody which we can cast to Bitmap

And

fun generatePhoto(responseBody: ResponseBody): Single<Bitmap?>

What I want is to create a function

fun getPhotos(list: List<FileModel>): Single<List<Bitmap>> {
    // Execute getFile(...) for each item in the list
    // Cast getFile(...) result to Bitmap using generatePhoto(...)
    // Return a list of Bitmaps
}

I've tried something like this but it's completely wrong

fun getPhotos(list: List<FileModel>): Single<List<Bitmap>> {
    return Observable.fromIterable(list)
        .flatMap { getFile(it.id) }
        // How to call generatePhoto(...) here?
}

Pharmacist

I'm not an expert in Rx, could you provide an example? – Pharmacist

Actually, it's probably just supposed to be flatMap. – Amorphous

Answer 1

You can do it like this:

fun getPhotos(list: List<FileModel>): Single<List<Bitmap>> {
    // Get an Observable of the list
    return Observable.fromIterable(list)
        // Get a Single<ResponseBody> for every FileModel
        .flatMapSingle { getFile(it.id) }
        // Get a Single<Bitmap> for every ResponseBody
        .flatMapSingle { file -> generatePhoto(file) }
        // Put everything back on a list
        .toList()
}

This way you can iterate over the list flapMapping for your needs and then putting it back together as a list in the end. The toList() operator is just a convenience that puts together the items emitted previously.

And to call this function just go:

    getPhotos(list)
        .doOnSuccess { resultList ->
          Log.d("Rx", "doOnSuccess.resultList=[$resultList]")
        }
        .subscribe()

By the way, if you have RxKotlin as a dependency you can get an Observable from a List with an extension function, like this: myList.toObservable()

Unsuspected

Answer 2

Observable.fromIterable(list) should emit an event for each item in the list which means that you're return type in getPhotos be Flowable<Bitmap> as Flowable can return more than one value.

I think what you actually want is:

fun getPhotos(list: List<FileModel>): Flowable<Bitmap> {
    return Observable.fromIterable(list)
        .map { getFile(it.id) }
        .map { generatePhoto(it) }
}

if you want to emit a list of Bitmaps you can use single like this

fun getPhotos(list: List<FileModel>): Single<List<Bitmap>> {
    return Single.just(list)
        .map { it.map { getFile(it.id) } }
        .flatMap { 
            Single.just(it.map { generatePhoto(it) }) 
        }
}

Thyself

But if we use Single.just(list) inside map { } we will have List<FileModel> instead of FileModel – Pharmacist

i've updated the answer, you can iterate over each list inside of the map block – Thyself

you can flatMap to evaluate the single but you have to emit another single in that case – Thyself

Answer 3

Have you tried:

fun getPhotos(list: List<FileModel>): Single<List<Bitmap>> {
    return Observable.fromIterable(list)
        .flatMapSingle { getFile(it.id) }
        .flatMapSingle { generatePhoto(it) }
}

?

Amorphous

We can't call flatMapSingle after flatMap – Pharmacist

You're right, maybe it should be flatMapSingle on both. – Amorphous