绑定变量为空.. 如何让 UI 中的观察者等到查询的 Livedata 从数据库中提取出来？

Question

我正在尝试从数据库中获取数据，然后将其以片段形式绑定到 XML。所以我有存储库将数据从 DB 获取到 ViewModel，UI 片段正在观察结果，然后将数据绑定到 XML。但问题是该应用程序正在粉碎说数据为空即使我在观察者中使空数据无效。

我已经尝试在后台线程上执行查询，它似乎在返回数据（照片）时正常工作。

我认为问题在于查询需要时间，片段中的观察者没有等到查询完成。所以查询没问题，我完全按照 Google 示例进行操作，但无法找出问题所在。提前致谢。

_照片库

    class PhotoRepository @Inject constructor(
private val photoDao: PhotoDoa
) {

fun loadPhotoById(photoId: Int): LiveData<Photo> {
//        var photo: Photo? = null 
//        this is working and i am getting the photo object
//        appExecutors.diskIO().execute { 
                photo = photoDao.getObjectPhotoById(photoId)
           } 

    return photoDao.getPhotoById(photoId)
 }

}

_PhotoViewModel

class PhotoViewModel @Inject constructor(private val photoRepository: 
             PhotoRepository) :
ViewModel() {

private var _photoId = MutableLiveData<Int>()
val photoId: LiveData<Int>
    get() = _photoId


val photo: LiveData<Photo> = Transformations
    .switchMap(_photoId) { id ->
        photoRepository.loadPhotoById(id)
    }


fun setId(photoId: Int) {
 //        if (_photoId.value == photoId){
 //            return
 //        }
    _photoId.value = photoId

  }

 }

_照片片段

class PhotoFragment : Fragment(), Injectable {

@Inject
lateinit var viewModelFactory: ViewModelProvider.Factory

var binding by autoCleared<FragmentPhotoBinding>()

lateinit var photoViewModel: PhotoViewModel

 var photo = Photo()


override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {

     binding = DataBindingUtil.inflate<FragmentPhotoBinding>(
        inflater,
        R.layout.fragment_photo,
        container,
        false
    )

    return binding.root

}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    val params = PhotoFragmentArgs.fromBundle(arguments!!)

    photoViewModel = ViewModelProviders.of(
        this,
        viewModelFactory).get(PhotoViewModel::class.java)

    photoViewModel.setId(params.photoId)
   // photoViewModel.photo.removeObservers(viewLifecycleOwner)
    photoViewModel.photo.observe(viewLifecycleOwner, Observer {

        if (it != null) {
            binding.photo = it
        }

    })
  }
}

_ Doa 类中的 Query

@Query(" SELECT * FROM Photo WHERE id = :id")
 abstract fun getPhotoById ( id: Int): LiveData<Photo>

_ fragment_photo.xml

<layout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools">

<data>

    <import type="com.mustafa.pixabayapp.models.Photo"/>
    <variable
        name="photo"
        type="Photo"/>

    <import type="com.mustafa.pixabayapp.utils.StringUtils" />
</data>

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <ImageView
        android:id="@+id/photo_fragment_image_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:imageUrl="@{photo.webFormatURL}"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="@color/colorTransparentDark"
        android:orientation="vertical"
        android:padding="16dp">

        <TextView
            android:id="@+id/photo_fragment_tags"
            style="@style/PixabayImageTextUser"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{StringUtils.getTags(photo.tags)}"
            tools:text="TEST - TEST - TEST"/>

        <RelativeLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <TextView
                android:id="@+id/photo_fragment_user_name"
                style="@style/PixabayImageTextUser"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_alignParentStart="true"
                android:text="@{StringUtils.byUser(photo.userName)}"
                tools:text="By: Mustafa"/>

            <TextView
                android:id="@+id/photo_fragment_comments"
                style="@style/PixabayImageTextUser"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_alignParentEnd="true"
                android:layout_marginEnd="4dp"
                android:drawableStart="@drawable/ic_comment"

  android:text="@{StringUtils.getCommentsAsString(photo.commentsCount)}"
                tools:text="2222"/>

            <TextView
                android:id="@+id/photo_fragment_favorites"
                style="@style/PixabayImageTextUser"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginEnd="4dp"
                android:layout_toStartOf="@id/photo_fragment_comments"
                android:drawableStart="@drawable/ic_favorite"


 android:text="@{StringUtils.getFavoritesAsString(photo.favoritesCount)}"
                tools:text="2222"/>

            <TextView
                android:id="@+id/photo_fragment_likes"
                style="@style/PixabayImageTextUser"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginEnd="4dp"
                android:layout_toStartOf="@id/photo_fragment_favorites"
                android:drawableStart="@drawable/ic_like"

   android:text="@{StringUtils.getLikesAsString(photo.likesCount)}"
                tools:text="2222"/>
        </RelativeLayout>
    </LinearLayout>
</RelativeLayout>
</layout>

_错误信息：

 java.lang.IllegalArgumentException:
 Parameter specified as non-null is null: 
 method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter 
 userName at com.mustafa.pixabayapp.utils.StringUtils.byUser(Unknown 
 Source:2)at com.mustafa.pixabayapp.databinding.FragmentPhotoBindingImpl.
 executeBindings(FragmentPhotoBindingImpl.java:138)

Answer 1

是的，您的“需要时间”的假设是正确的。布局想要在绑定后立即绘制一些东西，但此时photo尚未加载。StringUtils.byUser()您可以在布局中处理 null 值或添加 null 检查，如下所示：数据绑定：如果属性不为 null，则设置属性