# Kotlin 新舊 Room 初始化

###### tags: `Kotlin` `Android` `Room`

### 2018 年寫法：

@Database(
        entities = [LogDetail::class, WiFiEntity::class, BlueToothLeEntity::class],
        version = 2
)
@TypeConverters(DateConverter::class)
abstract class AppDataBase : RoomDatabase() {

    abstract fun logDetailDao(): LogDetailDao

    abstract fun wiFiEntityDao(): WiFiEntityDao

    abstract fun bleEntityDao(): BleEntityDao

    companion object {

        @Volatile private var INSTANCE: AppDataBase? = null

        fun getInstance(context: Context): AppDataBase =
                INSTANCE ?: synchronized(this) {
                    INSTANCE ?: buildDatabase(context).also { INSTANCE = it }
                }

        private fun buildDatabase(context: Context) =
                Room.databaseBuilder(
                                context.applicationContext,
                                AppDataBase::class.java,
                                "AppMemoVer1.db")
                        // java.lang.IllegalStateException: A migration from 1 to 2 was required but not found.
                        // Please provide the necessary Migration path viaRoomDatabase.Builder.addMigration(Migration ...) or
                        // allow for destructive migrations via one of the RoomDatabase.Builder.fallbackToDestructiveMigration* methods.
                        .fallbackToDestructiveMigration()
                        .build()
    }
}

### 2019 CodeLab 學習 Kotlin coroutine 寫法

/**
 * Title represents the title fetched from the network
 */
@Entity
data class Title constructor(val title: String, @PrimaryKey val id: Int = 0)

/***
 * Very small database that will hold one title
 */
@Dao
interface TitleDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertTitle(title: Title)

    @Query("select * from Title where id = 0")
    fun loadTitle(): LiveData<Title>
}

/**
 * TitleDatabase provides a reference to the dao to repositories
 */
@Database(
        entities = [Title::class], 
        version = 1, 
        exportSchema = false
)
abstract class TitleDatabase : RoomDatabase() {
    abstract val titleDao: TitleDao
}

private lateinit var INSTANCE: TitleDatabase

/**
 * Instantiate a database from a context.
 */
fun getDatabase(context: Context): TitleDatabase {
    synchronized(TitleDatabase::class) {
        if (!::INSTANCE.isInitialized) {
            INSTANCE = Room
                .databaseBuilder(
                        context.applicationContext,
                        TitleDatabase::class.java,
                        "titles_db"
                )
                .fallbackToDestructiveMigration()
                .build()
        }
    }
    return INSTANCE
}