@Database(entities = [Message::class], version = 1, exportSchema = false)
abstract class MessageDatabase : RoomDatabase() {

    abstract fun messageDao(): MessageDao

    companion object {

        @Volatile
        private var INSTANCE: MessageDatabase? = null

        fun getDatabase(context: Context): MessageDatabase? {
            if (INSTANCE == null) {
                synchronized(MessageDatabase::class.java) {
                    if (INSTANCE == null) {
                        INSTANCE = Room.databaseBuilder(
                            context.applicationContext,
                            MessageDatabase::class.java, "message_database"
                        )
                            .build()
                    }
                }
            }
            return INSTANCE
        }
    }
}