ConnectivityManager.NetworkCallback networkCallback = new ConnectivityManager.NetworkCallback() {
    @Override
    public void onAvailable(Network network) {
        // network available
    }

    @Override
    public void onLost(Network network) {
        // network unavailable
    }
};

ConnectivityManager connectivityManager =
        (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
    connectivityManager.registerDefaultNetworkCallback(networkCallback);
} else {
    NetworkRequest request = new NetworkRequest.Builder()
            .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET).build();
    connectivityManager.registerNetworkCallback(request, networkCallback);
}

val connectivityManager = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
connectivityManager?.let {
    it.registerDefaultNetworkCallback(object : ConnectivityManager.NetworkCallback() {
        override fun onAvailable(network: Network) {
            //take action when network connection is gained
        }
        override fun onLost(network: Network?) {
            //take action when network connection is lost
        }
    })
}