interface SharedPreferencesProvider {
    val prefs: SharedPreferences
}

class OptionalStringPrefDelegate(
        private val key: String,
        private val defaultValue: String? = null) : ReadWriteProperty<SharedPreferencesProvider, String?> {

    override fun getValue(thisRef: SharedPreferencesProvider, property: KProperty<*>): String? =
            thisRef.prefs.getString(key, defaultValue)

    override fun setValue(thisRef: SharedPreferencesProvider, property: KProperty<*>, value: String?) {
        thisRef.prefs.edit { putString(key, value) }
    }
}