rxjava-challenge-solution.kt

// rxjava-challenge-solution
// this example use RxJava 1.0
// Province class => data class Province(val id: Int ,val name:String, var cities:List<City>? = null)

class DataManager {

    // solution
    fun getProvincesAndCities(): Observable<List<Province>> {
        return getProvinces()
                .subscribeOn(Schedulers.io())
                .flatMap { provinces -> Observable.from(provinces) }
                .flatMap({ (id) -> getCityByProvinceId(id.toLong()) }, { province, cities ->
                    province.cities = cities
                    province
                })
                .toList()
                .observeOn(AndroidSchedulers.mainThread())

    }

    // fake method
    private fun getProvinces(): Observable<List<Province>> {
        return Observable.just(listOf(Province(1, "tehran"), Province(2, "esfehan")))
    }

    // fake method
    private fun getCityByProvinceId(provinceId: Long): Observable<List<City>> {
        if (provinceId == 1L) {
            return Observable.just(listOf(City(1, "tehran"), City(2, "rey")))
        }
        return Observable.just(listOf(City(3, "esfehan"), City(4, "shahin shahr")))
    }
}

Thanks man, great job explaining it. I would like to point out that for this code to work out of the box there are a few more lines needed here they are:

class City(val id: Int, val name: String) data class Province(val id: Int, val name: String, val cities: List<City>)

line 9 did not work for me so here is the replacement:

Observable.fromIterable(provinces)

Province properties are immutable so we have to use copy instead of assignment (line 11):

val newProvince = province.copy(cities = cities)

in order to comply with the return type of getProvincesAndCities() func we need a .toObservable() operator after .toList(), because toList will return a Single not a Observable

Suggestion
there is a operator for line 9:
.flatMapIterable { provinces -> provinces }

gotcha
if we are actually observing something and there is no onComplete signal in the getProvinces() or getCities() the function getProvincesAndCities() wont actually return anything, that is because the toList() relies on onComplete signal to construct the list. so for example if you are watching your DB and trying to approach this problem this way you should watch out for that toList() operator