Memoization single transformer

MemoizationSingleTransformer

class MemoizationSingleTransformer<T : Expirable>(val expiration: Long) : SingleTransformer<T,
        T> {

    private val TAG: String? = MemoizationSingleTransformer::class.simpleName;
    var value: T? = null

    override fun apply(upstream: Single<T>): SingleSource<T>? {

        val memory = Single.fromCallable {
            value ?: Expirable().expire()
        }.map {
            Log.d(TAG, "[memory] " + it?.isExpired)
            it
        }
        return Single.concat(memory, upstream.map {
            Log.d(TAG, "[upstream] " + it.isExpired)
            it
        })
                .filter { it?.isExpired != true }
                .firstElement().toSingle().map {
            value = it as T
            value?.setExpiration(expiration)
            it as T
        }
    }

}