Optional Subject

Optional.kt

import io.reactivex.subjects.BehaviorSubject

class Optional<T>(private val value: T? = null) {

    companion object {
        fun <T> subject() = BehaviorSubject.createDefault(Optional<T>())
    }

    fun isPresent() = value != null

    fun value() = value

}

fun <T> BehaviorSubject<Optional<T>>.currentValue() = value?.value()
fun <T> BehaviorSubject<Optional<T>>.isSet() = value?.isPresent() ?: false
fun <T> BehaviorSubject<Optional<T>>.clearValue() = onNext(Optional())
fun <T> BehaviorSubject<Optional<T>>.setValue(value: T) = onNext(Optional(value))

Swift:

import Foundation
import RxSwift
import RxCocoa
import SkySelfCare

extension BehaviorRelay {
    func cachedValue<T>(forceFetchIf: (_ value: T) -> Bool = { _ in false },
                        ifNullFetchFrom: () -> Single<T>) -> Single<T> where Element == T? {
        if let value = value, !forceFetchIf(value) {
            return Single.just(value)
        } else {
            return ifNullFetchFrom().do(onSuccess: { result in
                self.accept(result)
            })
        }
    }
}