儲存資料有許多方式，其中Database 最為常見，google 提供儲存本地端資料 library Room ，建立 Database，在SQLite上提供了一個抽象層，能夠流暢地訪問資料庫。

使用方式為：
import Room persistence library

    def room_version = "2.2.5"
    implementation "androidx.room:room-runtime:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    
    // optional - Kotlin Extensions and Coroutines support for Room
    implementation "androidx.room:room-ktx:$room_version"

 

Room 包含 3 個主要元件：

Database ：資料庫持有者與提供資料連結的關聯式資料庫

Entity：表示資料庫的table。

DAO：包含用於訪問資料庫的方法

以下為實作方式：
Database
建立DB entities代表 Todos table

@Database(
    version = 1,
    
    entities = [
        Todos::class
    ],
    exportSchema = false
)
abstract class AppDatabase: RoomDatabase() {
    companion object {
        private const val DATABASE_NAME = "todo_db"

        // For Singleton instantiation
        @Volatile private var instance: AppDatabase? = null

        fun getInstance(context: Context): AppDatabase {
            return instance ?: synchronized(this) {
                instance ?: buildDatabase(context).also { instance = it }
            }
        }

           database  建立
        private fun buildDatabase(context: Context): AppDatabase {
            return Room.databaseBuilder(context, AppDatabase::class.java, DATABASE_NAME).build()
        }
    }

    abstract fun todoItemDao(): ItemsDao
}

Entity

建立table

@Entity(
    tableName = Items.TABLE_NAME
)
data class Items (
    @ColumnInfo(name = COLUMN_TITLE) var title: String,
    @ColumnInfo(name = COLUMN_DONE) var done: Boolean,
    @ColumnInfo(name = COLUMN_CREATED_Date) var createdDate: Date
) {
    companion object {
        const val TABLE_NAME = "items"
        const val COLUMN_ID = "id"
        const val COLUMN_TITLE = "title"
        const val COLUMN_DONE = "done"
        const val COLUMN_CREATED_AT = "created_date"
    }
    //主鍵為一值
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = COLUMN_ID) var id: Int = 0
}

Dao
data Access objc 與DB存取刪除動作物件。
建立泛型之後要使用table 繼承後就可以實作新增刪修

interface GenericDao<T> {

    @Insert
    suspend fun insert(obj: T)

    @Insert
    suspend fun insert(vararg obj: T)

    @Update
    suspend fun update(obj: T)

    @Delete
    suspend fun delete(obj: T)
}

@Dao
interface ItemsDao: GenericDao<Items> {

    @Query("SELECT * FROM ${Items.TABLE_NAME} ORDER BY ${Items.COLUMN_CREATED_AT} DESC")
    fun findAll(): List<TodoItemDao>
}

class ItemaRepository(
    private val database: AppDatabase
) {
    fun insertTodoItem(todoItem: TodoItem) {
        database.itemsDao().insert(todoItem)
    }
}
}

ViewModel

class TodoViewModel(private val repository: ItemaRepository) : ViewModel() {

    val onNewTodo = MutableLiveData<String>()

    val todoLiveData: LiveData<List<Todo>> = MediatorLiveData<List<Todo>>().apply {
        addSource(onNewTodo) { text ->
            val todo = Todo.Item(text, false)
            this.value = this.value!! + listOf(todo)
        }
        value = mutableListOf(Todo.Title("This is a title"))

    fun createNewTodo(title: String) {
        val todoItem = TodoItem(
            title = title,
            done = false,
            createdDate = Date()
        )
        repository.insertTodoItem(todoItem)
    }
}

UI 呼叫 TodoViewModel 做 createNewTodo

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        
        val itemsDb = AppDatabase.getInstance(requireActivity().applicationContext)
        val itemsRepo = TodoItemRepository(todoItemDb)
           
        val todoViewModel = ViewModelProvider(requireActivity(), viewModelFactory).get(TodoViewModel::class.java)
        //新增資料
          todoViewModel.createNewTodo(title)
    }

reference:https://www.notion.so/Storage-9-23-Cateyes-7e968a1aa22a48d49b0c79089a9ed651
reference:https://developer.android.com/training/data-storage
reference:https://developer.android.com/training/data-storage/room/accessing-data
reference:https://developer.android.com/reference/android/arch/lifecycle/ViewModelProviders