RxSwift's Variable.swfit convert to Kotlin

Variable.kt

import io.reactivex.Observable
import io.reactivex.subjects.BehaviorSubject

/**
 * Wrapping Rx BehaviorSubject defaultValue object.
 * RxSwfit Variable.swfit https://github.com/jspahrsummers/RxSwift/blob/master/RxSwift/SpinLock.swift
 */
class Variable<T> private constructor(defaultValue: T?) {
    companion object {
        fun <T> create(): Variable<T> = Variable(null)
        fun <T> createDefault(value: T): Variable<T> = Variable(value)
    }

    // Call onNext is toSerialized()
    val isAsyncEmit: Boolean = true
    // Value holder
    private val subject: BehaviorSubject<T> = defaultValue?.let {
        BehaviorSubject.createDefault(it)
    } ?: BehaviorSubject.create()

    var value: T
        set(value) {
            if (isAsyncEmit) subject.toSerialized().onNext(value)
            else subject.onNext(value)
        }
        get() = if (subject.hasValue()) {
            subject.value
        } else {
            throw IllegalStateException("Variable has no valueOrDefault.")
        }

    fun valueOrNull(): T? = if (subject.hasValue()) {
        subject.value
    } else {
        null
    }

    fun valueOrDefault(defaultItem: T): T = subject.blockingFirst(defaultItem)

    fun asObservable(): Observable<T> = subject.hide()
}