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終於要把整個資料庫建立起來了，官方文件說建立資料庫的過程很花資源，所以最好使用單例，然後把 Dao 做成抽像類別。

建立 Database

1. 建立類別後，使用@Database，目前我們只有一個實體，所以entities = [Note::class]

   @Database(entities = [Note::class], version = 1, exportSchema = false)
   abstract class HackNoteDatabase : RoomDatabase() {
   
       abstract val noteDatabaseDao: NoteDatabaseDao        
   }    
   

2. 假如我們又多了一個使用者（user）實體就會像這樣

   @Database(entities = [Note::class, User::class], version = 1, exportSchema = false)
   abstract class HackNoteDatabase : RoomDatabase() {
   
       abstract val noteDatabaseDao: NoteDatabaseDao        
       abstract val userDatabaseDao: UserDatabaseDao        
   }    
   

3. 設定資料庫版本用version = 1，以後如果編譯不過就要把版號增加一

   @Database(entities = [Note::class], version = 999, exportSchema = false)
   abstract class HackNoteDatabase : RoomDatabase() {
   
       abstract val noteDatabaseDao: NoteDatabaseDao        
   }    
   

4. 建立單例，照抄就好，沒什麼好講的

   @Database(entities = [Note::class], version = 1, exportSchema = false)
   abstract class HackNoteDatabase : RoomDatabase() {
   
       abstract val noteDatabaseDao: NoteDatabaseDao
   
       companion object {
   
           @Volatile
           private var INSTANCE: HackNoteDatabase? = null
   
           fun getInstance(context: Context): HackNoteDatabase {
               synchronized(this) {
                   var instance = INSTANCE
   
                   if (instance == null) {
                       instance = Room.databaseBuilder(
                           context.applicationContext,
                           HackNoteDatabase::class.java,
                           "note_database"
                       )
                           .fallbackToDestructiveMigration()
                           .build()
   
                           INSTANCE = instance
                   }
                   return instance
               }
           }
       }
   }
   

5. 這樣就建立完成