ObserveWifiNaive.kt

fun currentNetwork(context: Context): Flowable<Network> {
    val connectivityManager = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    return Flowable.create<Network>({ emitter ->

        emitter.onNext(Network.NONE)
        val request = NetworkRequest.Builder()
                .addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)
                .build()

        val networkCallback = object: ConnectivityManager.NetworkCallback() {
            override fun onAvailable(network: android.net.Network) {
                emitter.onNext(Network.CELLULAR)
            }

            override fun onLost(network: android.net.Network) {
                emitter.onNext(Network.NONE)
            }
        }

        connectivityManager.requestNetwork(request, networkCallback)
        emitter.setCancellable {
            connectivityManager.unregisterNetworkCallback(networkCallback)
        }
    }, BackpressureStrategy.LATEST)
}