我应该在带有 LiveData 的 Room 中使用@DatabaseView 吗？

Question

如果运行通过主线程查询，它就可以工作。如果我尝试使用 Livedata POJO 获取 DatabaseView，它会抛出错误

java.lang.IllegalArgumentException: Cannot add the same observer with different lifecycles
    at androidx.lifecycle.LiveData.observe(LiveData.java:197)
    at MyFragment.onCreateView(MyFragment.kt:45)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2439)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460)
    at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
    at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:802)
    at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
    at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
    at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
    at androidx.fragment.app.FragmentManagerImpl.run(FragmentManager.java:733)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7058)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)

数据库视图class

@DatabaseView("SELECT ss_pack.id as id, ss_pack.title as title, ss_pack.primary_value as primaryValue FROM ss_pack JOIN favourite WHERE favourite.ss_pack_id = ss_pack.id")
data class AllFavourites(
    val id: Int,
    val title: String,
    val primaryValue: String
)

我的道

interface FavouriteDao {
@Query("SELECT * from AllFavourites")
fun getFavorites(): LiveData<List<AllFavourites>>

@Insert
fun insertFav(favourite: Favourite)
}

我的存储库

class MyRepository(val favoriteDao: FavoriteDao, val categoryDao: CategoryDao) {

val favourites: LiveData<List<AllFavourites>> = favoriteDao.getFavorites()
val categories: LiveData<List<Category>> = categoryDao.loadAllCategories()
}

视图模型

class MyViewModel(application: Application) : AndroidViewModel(application){
private val myRepository: MyRepository
val favourites: LiveData<List<AllFavourites>>

init {
    val favoriteDao = SDatabase.getDatabase(application, viewModelScope).favoriteDao()
    myRepository = myRepository(favoriteDao, categoryDao)
    favourites = passwordRepository.favourites
}
}

当我在我的片段中调用这个 observe 时抛出错误

viewModel.favourites.observe(this, androidx.lifecycle.Observer { favorites ->
        favorites?.let {
            print("FAVORITE SIZE ${favorites.size}")
        }
    })

告诉我如何将 DatabaseView 与 LiveData 或 kotlin 协程与此 MVVM 模式结合使用。

我需要通过后台线程而不是 UI 线程从我的片段中打印 FAVORITE SIZE。

Answer 1

onCreateView() 可以为同一个 Fragment 实例调用多次（因为当 Fragment 被放入返回堆栈时视图被销毁，但 Fragment 实例本身被保留）。

在 onCreateView() 中使用 observe() 时，您应该始终使用 viewLifecycleOwner 而不是 this，因为这样可以确保之前的观察者（绑定到上次 onCreateView() 被调用）被正确清理：

viewModel.favourites.observe(viewLifecycleOwner, androidx.lifecycle.Observer { favorites ->
    favorites?.let {
        print("FAVORITE SIZE ${favorites.size}")
    }
})

我应该在带有 LiveData 的 Room 中使用@DatabaseView 吗？

Shall I use @DatabaseView in Room with LiveData?

android

mvvm

kotlin

database-view

android-livedata