Entity

Entity的實例對應的是對象模型數據庫中數據表的一行數據,其數據結構自然就是對應著表結構. 而這種映射結構是用注釋來標明的.

• 表名 --> 類名 @Entity(tableName = "user")
• 列名 --> 成員變量名 @ColumnInfo(name = "first_name")
• 存儲數據類型 --> 成員變量數據類型 var name: String = ""
• 主鍵 @PrimaryKey(autoGenerate = true)
• 索引 @Entity(indices = {@Index(value = {"first_name", "last_name"}, unique = true)})
• 外鍵

@Entity(indices = arrayOf(Index(value = *arrayOf("first_name", "last_name"), unique = true)))
 class User {
    @PrimaryKey
    var id: Int = 0

    @ColumnInfo(name = "first_name")
    var firstName: String? = null

    @ColumnInfo(name = "last_name")
    var lastName: String? = null

    @Ignore
    var picture: Bitmap? = null
}

@Entity(foreignKeys = arrayOf(ForeignKey(entity = User::class, parentColumns = arrayOf("id"), childColumns =  arrayOf("user_id"))))
 class Book {
    @PrimaryKey
    var bookId: Int = 0

    var title: String? = null

    @ColumnInfo(name = "user_id")
    var userId: Int = 0
}

Dao (Data Access Objects)

Dao 是數據管理層,負責對數據庫進行增刪改查的操作
你只需要簡單地用注解標注,就能自動生成相關的代碼了.值得注意的是,這些代碼都需要在子線程中運行,所以用其配合Rxjava是再好不過的了

@Dao
 interface UserDao {
    @Query("SELECT * FROM user")
    fun getAll():Flowable<List<User>>

    @Insert(onConflict = REPLACE)
    fun insert(user: User)

    @Update(onConflict = REPLACE)
    fun update(user: User)

    @Delete
    fun delete(user: User)

}

Database

最后,我們來到了Database,它標注了數據庫的名稱,版本,包含的表.最重要的是,生成Dao的獲取方法.

@android.arch.persistence.room.Database(entities = arrayOf(User::class), version = 1)
abstract class Database : RoomDatabase() {
    abstract fun userDao(): UserDao
}

當然,我們還需要一個單例來讓我們獲取這個數據庫的實例,如下

class DataBaseHelper private constructor() {
    companion object {
        val instance by lazy(LazyThreadSafetyMode.SYNCHRONIZED) {
            DataBaseHelper()
        }

    }

    var database: Database? = null
    fun setup(context: Context) {
        database = Room.databaseBuilder(context.applicationContext, Database::class.java, "database.sqlite")
                .build()

    }

}