Variant of scan operator but supports different return type than source observable. Used mainly to calculate Diff for recyclerView.

ScanMap.kt

inline fun <T, R> Observable<List<T>>.scanMap(crossinline func2: (List<T>, List<T>) -> R): Observable<R> {
    return this.startWith(emptyList<T>()) //emit a empty list first, otherwise the .buffer() below won't emit at first (needs 2 emissions to emit)
            .buffer(2, 1) //buffer the previous and current emission
            .filter { it.size >= 2 }
            .map { func2.invoke(it[0], it[1]) }
}