要将多个SQL查询的数据合并到一个ViewModel中,可以使用Android Room库的LiveData和ViewModel来实现。
首先,需要创建一个包含所有查询结果的数据类。假设有两个查询:查询A返回一个包含用户名和年龄的实体类User,查询B返回一个包含用户地址的实体类Address。我们可以创建一个包含这些属性的合并数据类UserWithAddress:
data class UserWithAddress(
val name: String,
val age: Int,
val address: String
)
接下来,创建一个包含所有查询方法的Dao接口:
@Dao
interface UserDao {
@Query("SELECT * FROM user")
fun getAllUsers(): LiveData>
@Query("SELECT * FROM address")
fun getAllAddresses(): LiveData>
}
然后,创建一个Repository类来处理数据的获取和转换:
class UserRepository(private val userDao: UserDao) {
fun getUsersWithAddresses(): LiveData> {
val usersLiveData = userDao.getAllUsers()
val addressesLiveData = userDao.getAllAddresses()
return MediatorLiveData>().apply {
var users: List? = null
var addresses: List? = null
fun update() {
val localUsers = users
val localAddresses = addresses
if (localUsers != null && localAddresses != null) {
val usersWithAddresses = mutableListOf()
for (user in localUsers) {
val address = localAddresses.find { it.userId == user.id }
address?.let {
usersWithAddresses.add(UserWithAddress(user.name, user.age, it.address))
}
}
value = usersWithAddresses
}
}
addSource(usersLiveData) { usersData ->
users = usersData
update()
}
addSource(addressesLiveData) { addressesData ->
addresses = addressesData
update()
}
}
}
}
最后,在ViewModel中使用Repository获取数据并将其暴露给UI层:
class UserViewModel(userRepository: UserRepository) : ViewModel() {
val usersWithAddresses: LiveData> = userRepository.getUsersWithAddresses()
}
现在,可以在UI层中观察usersWithAddresses属性来获取合并后的查询结果:
class MainActivity : AppCompatActivity() {
private lateinit var userViewModel: UserViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val userRepository = UserRepository(Room.databaseBuilder(applicationContext, AppDatabase::class.java, "database-name").build().userDao())
userViewModel = ViewModelProvider(this, UserViewModelFactory(userRepository)).get(UserViewModel::class.java)
userViewModel.usersWithAddresses.observe(this, Observer { usersWithAddresses ->
// 处理合并后的数据
})
}
}
这样,就可以在一个ViewModel中合并多个SQL查询的数据了。