要将一个变量保存为密封类对象的实体,可以按照以下步骤进行操作:
Status
的密封类,其中包含Success
和Error
两个子类。sealed class Status
data class Success(val message: String) : Status()
data class Error(val error: String) : Status()
@TypeConverter
注解来指定如何将密封类对象转换为数据库中的存储类型,并反之。@Entity
data class MyEntity(
...
@TypeConverters(StatusConverter::class)
val status: Status
...
)
TypeConverter
类,用于在密封类对象和存储类型之间进行转换。在这个类中,使用@JvmStatic
注解来标记转换方法,以便在类型转换过程中被调用。class StatusConverter {
@TypeConverter
fun fromStatus(status: Status): String {
return when (status) {
is Success -> "Success:${status.message}"
is Error -> "Error:${status.error}"
}
}
@TypeConverter
fun toStatus(value: String): Status {
val parts = value.split(":")
return when (parts[0]) {
"Success" -> Success(parts[1])
"Error" -> Error(parts[1])
else -> throw IllegalArgumentException("Invalid status value: $value")
}
}
}
RoomDatabase
的子类中,添加一个抽象方法来获取包含密封类对象的实体类的Dao对象。@Database(entities = [MyEntity::class], version = 1)
@TypeConverters(StatusConverter::class)
abstract class MyDatabase : RoomDatabase() {
abstract fun myDao(): MyDao
}
@Dao
interface MyDao {
@Insert
suspend fun insert(entity: MyEntity)
@Query("SELECT * FROM MyEntity")
suspend fun getAllEntities(): List
}
现在,你可以使用MyDatabase
和MyDao
类来插入和检索包含密封类对象的实体。例如:
val database = Room.databaseBuilder(context, MyDatabase::class.java, "my-database").build()
val dao = database.myDao()
val successEntity = MyEntity(status = Success("Data loaded successfully"))
val errorEntity = MyEntity(status = Error("Error occurred"))
dao.insert(successEntity)
dao.insert(errorEntity)
val entities = dao.getAllEntities()
for (entity in entities) {
val status = entity.status
when (status) {
is Success -> println("Success message: ${status.message}")
is Error -> println("Error message: ${status.error}")
}
}
这样,你就可以使用Android Room保存一个变量为密封类对象的实体了。