asyncZipSingle.kt

fun <T1, T2, R> asyncZip(source1: Single<T1>,
                         source2: Single<T2>,
                         zipper: (T1, T2) -> Single<R>): Single<R> =
        Single.zip(
                source1, source2,
                io.reactivex.functions.BiFunction<T1, T2, Single<R>> { t1, t2 -> zipper(t1, t2) }
        ).flatMap { it }