要使用Android Room存储对象层次结构并查询超类,可以按照以下步骤进行解决:
@Entity
open class Animal(
@PrimaryKey val id: Int,
val name: String
)
@Entity
data class Cat(
val color: String
) : Animal(id, name)
@Dao
interface AnimalDao {
@Insert
fun insertAnimal(animal: Animal)
@Query("SELECT * FROM Animal")
fun getAllAnimals(): List
}
@Dao
interface CatDao {
@Insert
fun insertCat(cat: Cat)
@Query("SELECT * FROM Animal WHERE id = :id")
fun getAnimalById(id: Int): Animal
@Query("SELECT * FROM Animal WHERE id = :id")
fun getCatById(id: Int): Cat
}
@Database(entities = [Animal::class, Cat::class], version = 1)
abstract class AnimalDatabase : RoomDatabase() {
abstract fun animalDao(): AnimalDao
abstract fun catDao(): CatDao
companion object {
@Volatile
private var INSTANCE: AnimalDatabase? = null
fun getDatabase(context: Context): AnimalDatabase {
return INSTANCE ?: synchronized(this) {
val instance = Room.databaseBuilder(
context.applicationContext,
AnimalDatabase::class.java,
"animal_database"
).build()
INSTANCE = instance
instance
}
}
}
}
class MainActivity : AppCompatActivity() {
private lateinit var animalDao: AnimalDao
private lateinit var catDao: CatDao
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val animalDatabase = AnimalDatabase.getDatabase(this)
animalDao = animalDatabase.animalDao()
catDao = animalDatabase.catDao()
val animal = Animal(1, "Animal")
val cat = Cat("Black")
animalDao.insertAnimal(animal)
catDao.insertCat(cat)
val allAnimals = animalDao.getAllAnimals()
for (animal in allAnimals) {
if (animal is Cat) {
val cat = catDao.getCatById(animal.id)
// Do something with the cat
} else {
val animal = animalDao.getAnimalById(animal.id)
// Do something with the animal
}
}
}
}
通过以上步骤,您就可以使用Android Room存储对象层次结构并查询超类了。在查询时,您可以使用子类的DAO接口来获取子类的特定属性。