了解Room框架的使用

Android Jetpack架構組件-Room基本使用
Android Jetpack架構組件-Room數(shù)據(jù)庫查詢藝術
Android Jetpack架構組件-Room升級

在Android中使用任何一種數(shù)據(jù)庫框架，少不了應用的迭代和數(shù)據(jù)庫的升級，那么Room的該如何正確的升級？

一、Room數(shù)據(jù)庫升級

• 第一步：增加version數(shù)據(jù)，及版本號增加

@Database(entities = [Cheese::class, User::class], version = 2, exportSchema = true)
abstract class CheeseDb : RoomDatabase() {
}

• 第二步：創(chuàng)建Room特有的Migration

      val MIGRATION_1_2: Migration = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE user ADD COLUMN birthday INTEGER NOT NULL DEFAULT 23 ")
            }
        }

Migration構造函數(shù)中，第一個參數(shù)代碼需要哪個版本才升級，第二個參數(shù)，是升級到哪個版本，一般情況和version保持一致

• 第三步：將Migration添加到配置中，如注釋1所示

instance = Room.databaseBuilder(context, CheeseDb::class.java, "onexzgj")
                    .allowMainThreadQueries()
                    .addMigrations(MIGRATION_1_2)  //注釋1
                    .build()

到這里，一次完整的數(shù)據(jù)庫升級即可完成

二、Room升級常見錯誤

• 1.數(shù)據(jù)庫的vesion未變 ，添加了Migration的crash

Room cannot verify the data integrity. Looks like you've changed schema but forgot to update the version number. 
You can simply fix this by increasing the version number.

• 2.數(shù)據(jù)庫的version增加，未提供Migration的crash

Caused by: java.lang.IllegalStateException: A migration from 1 to 1 was required but not found.
Please provide the necessary Migration path via RoomDatabase.Builder.addMigration(Migration ...) or 
allow for destructive migrations via one of the RoomDatabase.Builder.fallbackToDestructiveMigration* methods.

• 3.第二種情況的錯誤解決
  3.1配置中添加了fallbackToDestructiveMigration()方法，如下所示

    instance = Room.databaseBuilder(context, CheeseDb::class.java, "onexzgj")
                    .allowMainThreadQueries()
                    .fallbackToDestructiveMigration()  //注釋1
                    .build()

即在如上代碼注釋1處添加.fallbackToDestructiveMigration() ，這種方式雖然不會crash，但是數(shù)據(jù)庫的數(shù)據(jù)會清空
3.2配置中添加Migration，數(shù)據(jù)庫升級成功，且不會清空數(shù)據(jù)，即按照第一部分正確升級姿勢

• 4、增加字段未設置默認值

.IllegalStateException: Migration didn't properly handle tasks(googleroom.android.com.google_room.data.Task).
     Expected:

如果在定義Migration的時候，添加的是INTEGER類型的字段，則需要設置默認值，如果不設置默認值，即會遇到如上crash

錯誤Migration示例代碼

     val MIGRATION_1_2: Migration = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE user ADD COLUMN birthday INTEGER")
            }
        }

正確Migration的實例代碼

     val MIGRATION_1_2: Migration = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE user ADD COLUMN birthday INTEGER NOT NULL DEFAULT 23 ")
            }
        }