listener_to_observable.kt

// inspired by http://www.andreamaglie.com/rxjava-listener-to-observable/

interface ValueUpdateListener {
    fun onValueChanged(value: String)
}

class ValueUpdater {
    fun registerListener(listener: ValueUpdateListener) {
        // ...
    }

    fun unregisterListener(listener: ValueUpdateListener) {
        // ...
    }
}

fun observableWrapper(): Observable<String> {
    return observable<String> {
        subscriber ->
        val listener = object : ValueUpdateListener {
            override fun onValueChanged(value: String) {
                if (subscriber.isUnsubscribed) {
                    valueUpdater.unregisterListener(this)
                } else {
                    subscriber.onNext(value)
                }
            }
        }

        valueUpdater.registerListener(listener)
    }
}

observableWrapper().subscribe {
    // do smth with value
}