Android Room Data Persistance (SQLite)

@Entity(tableName = "subscriber_data_table")
data class Subscriber(

	@PrimaryKey(autoGenerate = true)
	@ColumnInfo(name = "subscriber_id")
	val id: Int,
	
	@ColumnInfo(name = "subscriber_name")
	val name: String,
	
	@ColumnInfo(name = "subscriber_email")
	val email: String
	
)

@Dao
interface SubscriberDAO {

	@Insert(onConflict = OnConflictStrategy.REPLACE)
	suspend fun insert(subscriber: Subscriber): Long

	@Insert
	suspend fun insert(subscribers: List<Subscriber>): List<Long>

	@Update
	suspend fun update(subscriber: Subscriber)
	
	@Update
	suspend fun update(subscribers: List<Subscriber>)
	
	@Delete
	suspend fun delete(subscriber: Subscriber)
	
	@Delete
	suspend fun delete(subscribers: List<Subscriber>)

	@Query("DELETE FROM subscriber_data_table")
	suspend fun deleteAll()

	// this isn't a suspend function!
	@Query("SELECT * FROM subscriber_data_table")
	fun getAllSubscribers(): LiveData<List<Subscriber>>

}

@Database(entities = [Subscriber::class], version = 1)
abstract class SubscriberDatabase: RoomDatabase() {

	abstract val subscriberDAO: SubscriberDAO

	companion object {
		@Volatile
		private var INSTANCE: SubscriberDatabase? = null
			fun getInstance(context: Context): SubscriberDatabase {
				synchronized(this) {
					var instance = INSTANCE
					if(instance==null) {
						instance = Room.databaseBuilder(
							context.applicationContext,
							SubscriberDatabase::class.java,
							"subscriber_data_database"
						).build()
					}
					return instance
				}
			}
	}

}

class SubscriberRepository(private val dao: SubscriberDao) {
	
	// this already is LiveData
	val subscribers = dao.getAllSubscribers()
	
	suspend fun insert(subscriber: Subscriber) {
		dao.insert(subscriber)
	}
	
	suspend fun update(subscriber: Subscriber) {
		dao.update(subscriber)
	}
	
	suspend fun delete(subscriber: Subscriber) {
		dao.delete(subscriber)
	}
	
	suspend fun deleteAll(subscriber: Subscriber) {
		dao.deleteAll(subscriber)
	}
}

class SubscriberViewModel(private val repository: SubscriberRepository): ViewModel(), Observable {
	
	val subscribers = repository.subscribers
	
	@Bindable
	val inputName = MutableLiveData<String>()
	
	@Bindable
	val inputEmail = MutableLiveData<String>()
	
	@Bindable
	val saveOrUpdateButtonText = MutableLiveData<String>()
	
	@Bindable
	val clearAllOrDeleteButtonText = MutableLiveData<String>()
	
	init {
		saveOrUpdateButtonText.value = "Save"
		clearAllOrDeleteButtonText.value = "Clear All"
	}
	
	fun saveOrUpdate() {
		val name = inputName.value!!
		val email = inputEmail.value!!
		// since the ID is AutoGenerated we can just set 0 and it will be ignored
		insert(Subscriber(0, name, email))
		inputName.value = null
		inputEmail.value = null
	}
	
	fun clearAllOrDelete() {
		clearAll()
	}
	
	// this will be executed in a background thread
	fun insert(subscriber: Subscriber) = viewModelScope.launch {
		repository.insert(subscriber)
	}
	
	fun update(subscriber: Subscriber) = viewModelScope.launch {
		repository.update(subscriber)
	}
	
	fun delete(subscriber: Subscriber) = viewModelScope.launch {
		repository.delete(subscriber)
	}
	
	fun clearAll() = viewModelScope.launch {
		repository.deleteAll()
	}
	
	override fun removeOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {
		
	}
	
	override fun addOnPropertyChangedCallback(callback: Observable.OnPropertyChangedCallback?) {
	
	}
	
}

class SubscriberViewModelFactory(private val repository: SubscriberRepository): ViewModelProvider.Factory {
	
	// this is boilerplate code for all Factories
	override fun <T: ViewModel?> create(modelClass: Class<T>): T {
		if(modelClass.isAssignableFrom(SubscriberViewModel::class.java)) {
			return SubscriberViewModel(repository) as T
		}
		throw IllegalArgumentException("Unknown View Model class")
	}
	
}

class MainActivity: AppCompatActivity() {
	
	private lateinit var binding: ActivityMainBinding
	private lateinit var subscriberViewModel: SubscriberViewModel
	
	override fun onCreate(savedInstanceState: Bundle?) {
		super.onCreate(savedInstanceState)
		binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
		val dao = SubscriberDatabase.getInstance(application).subscriberDAO
		val repository = SubscriberRepository(dao)
		val factory = SubscriberViewModelFactory(repository)
		subscriberViewModel = ViewModelProvider(this, factory).get(SubscriberViewModel::class.java)
		binding.myViewModel = subscriberViewModel
		// this is for livedata usage with data binding
		binding.lifecycleOwner = this
		displaySubscribersList()
	}
	
	private fun displaySubscribersList() {
		subscriberViewModel.subscribers.observe(this, Observer {
			Log.i("MyTAG", it.toString())
		})
	}
	
}

open class Event<out T>(private val content: T) {
        var hasBeenHandled = false
                private set // allows external read but not write
                
        /**
         * Returns the content and prevents its use again
         */
        fun getContentIfNotHandled(): T? {
                return if(hasBeenHandled) {
                        null
                } else {
                        hasBeenHandled = true
                        content
                }
        }
        
        /**
         * Returns the content, even if it's already been handled
         */
        fun peekContent(): T = content
}

class ViewModel {
        // this is MutableLiveData so we can edit its value, but it's private so we only can access it from this class. 
        private val statusMessage = MutableLiveData<Event<String>>()
        
        val message: LiveData<Event<String>>
                get() = statusMessage
                
        fun insert() {
                ...
                statusMessage.value = Event("subscriber has been inserted")
        }
        
        fun delete() {
                ...
                statusMessage.value = Event("subscriber has been deleted")
        }
}