如何从无法访问 lifeCycleOwner 的存储库中获取 LiveData 的价值以进行观察？

Question

我在应用程序中使用了MVVM和ROOM和databindig。根据 应用程序架构指南，我想使用房间兑现数据。在RecyclerView项目的xml布局中，我使用CategoryViewModel变量。我从Room数据库中获取LiveData类型的类别列表。我想LiveData<list<CategoryItem>>类型更改为MutableLiveData<ArrayList<CategoryViewModel>>类型。因为最后我的适配器使用ArrayList<CategoryViewModel>数据类型。如何获得LiveData的价值？当我调用getValue()方法时，返回 null。 这是CategoryItem模型：

@Entity(tableName = "category_table")
public class CategoryItem implements Serializable {
@PrimaryKey
private int id;
private String title;
private String imagePath;
@TypeConverters({SubCategoryConverter.class})
private ArrayList<String> subCategory;
@TypeConverters({DateConverter.class})
private Date lastRefresh;
public CategoryItem(int id, String title, String imagePath, ArrayList<String> subCategory, Date lastRefresh) {
this.id = id;
this.title = title;
this.imagePath = imagePath;
this.subCategory = subCategory;
this.lastRefresh=lastRefresh;
}
public CategoryItem(int id, String title, String imagePath) {
this.id = id;
this.title = title;
this.imagePath = imagePath;
}
public CategoryItem() {
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getImagePath() {
return imagePath;
}
public void setImagePath(String imagePath) {
this.imagePath = imagePath;
}
public ArrayList<String> getSubCategory() {
return subCategory;
}
public void setSubCategory(ArrayList<String> subCategory) {
this.subCategory = subCategory;
}
public Date getLastRefresh() {
return lastRefresh;
}
public void setLastRefresh(Date lastRefresh) {
this.lastRefresh = lastRefresh;
}
}

这是CategoryViewModel类：

public class CategoryViewModel extends AndroidViewModel {
private String title;
private String imagePath;
private MutableLiveData<ArrayList<CategoryViewModel>> allCategories=new MutableLiveData<>();
private CategoryRepository repository;
public CategoryViewModel(@NonNull Application application) {
super(application);
repository=new CategoryRepository(application, Executors.newSingleThreadExecutor());
}
public void init(CategoryItem categoryItem){
this.title=categoryItem.getTitle();
this.imagePath=categoryItem.getImagePath();
}
public MutableLiveData<ArrayList<CategoryViewModel>> getAllCategories(){
allCategories=repository.getCategory();
return allCategories;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getImagePath() {
return imagePath;
}
}

这是CategoryRepository类：

public class CategoryRepository {
private static final String TAG="CategoryRepository";
private static int FRESH_TIMEOUT_IN_MINUTES = 1;
private final Executor executor;
private APIInterface apiInterface;
public MutableLiveData<ArrayList<CategoryViewModel>> arrayListMutableLiveData=new MutableLiveData<>();

private CategoryDao categoryDao;
private Application application;
public CategoryRepository(Application application,Executor executor) {
this.executor = executor;
this.application = application;
apiInterface= APIClient.getClient().create(APIInterface.class);
LearnDatabase database= LearnDatabase.getInstance(application);
categoryDao=database.categoryDao();
}
public MutableLiveData<ArrayList<CategoryViewModel>>  getCategory(){
refreshCategory();
List<CategoryItem> items;
categoryDao.loadCategoryItem();
items=categoryDao.loadCategoryItem().getValue(); // return null
CategoryItem category;
ArrayList<CategoryViewModel> arrayList=new ArrayList<>();
for(int i=0;i<items.size();i++){
category=items.get(i);
CategoryViewModel categoryViewModel=new CategoryViewModel(application);
categoryViewModel.init(category);
arrayList.add(categoryViewModel);
}

arrayListMutableLiveData.setValue(arrayList);
return arrayListMutableLiveData;
}
private void refreshCategory(){
executor.execute(() -> {
String lastRefresh=getMaxRefreshTime(new Date()).toString();
boolean sliderExists =(!(categoryDao.hasCategory(lastRefresh)).isEmpty());
Log.e(TAG,"sliderExist: "+sliderExists);
Log.e(TAG,"lastrefresh: "+lastRefresh);
Log.e(TAG,"hasSlider: "+categoryDao.hasCategory(lastRefresh).toString());
// If user have to be updated
if (!sliderExists) {
Log.e(TAG,"in if");
apiInterface.getCategory().enqueue(new Callback<List<CategoryItem>>() {
@Override
public void onResponse(Call<List<CategoryItem>> call, Response<List<CategoryItem>> response) {
executor.execute(() -> {
List<CategoryItem> categories=response.body();
for (int i=0;i<categories.size();i++){
categories.get(i).setLastRefresh(new Date());
categoryDao.saveCategory(categories.get(i));
}
});
}
@Override
public void onFailure(Call<List<CategoryItem>> call, Throwable t) {
Log.e(TAG,"onFailure "+t.toString());
}
});
}
});
}
private Date getMaxRefreshTime(Date currentDate){
Calendar cal = Calendar.getInstance();
cal.setTime(currentDate);
cal.add(Calendar.MINUTE, -FRESH_TIMEOUT_IN_MINUTES);
return cal.getTime();
}
}

这是recyclerView项目的xml布局：

<?xml version="1.0" encoding="utf-8"?>
<layout>
<data class="CategoryDataBinding">
<variable
name="category"
type="com.struct.red.alltolearn.viewmodel.CategoryViewModel"/>
</data>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="200dp"
android:layout_height="150dp"
app:cardCornerRadius="15dp">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/imgItemCategory"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="centerCrop"
app:imageUrl="@{category.imagePath}" />
<TextView
android:id="@+id/txtTitleItemCategory"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="@{category.title}"
android:textColor="#FFFFFF"
android:textSize="20sp"
android:textStyle="bold" />
</RelativeLayout>
</android.support.v7.widget.CardView>
</layout>

这是CategoryDao类：

@Dao

公共接口类别道 {

@Query("SELECT * FROM course_table")
LiveData<List<CategoryItem>> loadCategoryItem();

@Insert(onConflict = OnConflictStrategy.REPLACE)
void saveCategory(CategoryItem category);
@Query("SELECT * FROM category_table WHERE lastRefresh > Date(:lastRefreshMax)")
List<CategoryItem> hasCategory(String lastRefreshMax);

}

最后，我在片段中观察到MutableLiveData：

private void setupCategoryRecycler() {
categoryViewModel = ViewModelProviders.of(this).get(CategoryViewModel.class);
categoryViewModel.getAllCategories().observe(this, new Observer<ArrayList<CategoryViewModel>>() {
@Override
public void onChanged(@Nullable ArrayList<CategoryViewModel> categoryViewModels) {
Log.e(TAG, "categoryitem: " + categoryViewModels.toString());
categoryAdapter = new CategoryAdapter(getContext(), categoryViewModels);
LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getContext(), LinearLayoutManager.HORIZONTAL, true);
linearLayoutManager.setReverseLayout(true);
CategoryRecy.setLayoutManager(linearLayoutManager);
CategoryRecy.setAdapter(categoryAdapter);
}
});
}

Answer 1

你的问题就在这里，对吧？

public MutableLiveData<ArrayList<CategoryViewModel>>  getCategory(){
...
items=categoryDao.loadCategoryItem().getValue(); // returns null
...
}

这是因为您的 categoryDao.loadCategoryItem() 方法返回 LiveData 对象。这意味着方法调用将在后台线程中执行。因此，当您调用getValue()方法时，该值在该时刻仍为空。

为了逃避这一点，你可以做两件坏事。

1.更早地调用 loadCategoryItem()，稍后调用 getValue() 时有值;

您的存储库类

public class CategoryRepository {
Livedata<List<CategoryItem>> items; // moved here
...
public void init () { 
items=categoryDao.loadCategoryItem(); 
}
public MutableLiveData<ArrayList<CategoryViewModel>>  getCategory(){
ArrayList<CategoryViewModel> arrayList=new ArrayList<>();
List<CategoryItem> currentList = items.getValue(); 
for(int i=0;i<currentList.size();i++){
...  
}
arrayListMutableLiveData.setValue(arrayList);
return arrayListMutableLiveData;
}
}

您的视图模型类

public class CategoryViewModel extends AndroidViewModel {
public void init(CategoryItem categoryItem){
repository.init();                          // added
this.title=categoryItem.getTitle();
this.imagePath=categoryItem.getImagePath();
}

这可以工作，但我们有 2 个问题。首先，仍然不能保证值不会为空。第二个问题是您无法观察项目的变化。即使你返回arrayListMutableLiveData对象，即livedata，你也要手动设置一次它的值，除非你再次调用getCategory()，否则它的值不会改变。

2.第二个技巧是同步加载类别项目

public interface CategoryDao {
@Query("SELECT * FROM category_table") LiveData<List<CategoryItem>>loadCategoryItem();
@Query("SELECT * FROM category_table") List<CategoryItem> loadCategoryItemsSync();

在这种情况下，您的 getAllCategories() 和 getCategory() 方法也应该同步工作。

像这样的东西

public void  getCategory(Listener listener){
executor.execute(() -> {
ArrayList<CategoryViewModel> arrayList=new ArrayList<>();
List<CategoryItem> currentList = items.getValue(); 
for(int i=0;i<currentList.size();i++){
...  
}
arrayListMutableLiveData.setValue(arrayList);
listener.onItemsLoaded(arrayListMutableLiveData);
}
}

在这种情况下，我们还有第二个问题 ->您无法观察项目更改。

我写这篇文章是为了更好地澄清问题。

真正的问题是您尝试使用CategoryViewModel进行数据绑定。

请改用类别项

我建议从视图模型中删除这两行

private String title;
private String imagePath;

尝试在不将数据从列表解析到 ArrayList 的情况下解决问题。

public LiveData<List<CategoryItem>> getAllCategories(){
if (items == null) {
items = categoryDao.loadCategoryItem()
}
return items;
}

然后尝试使用类别项作为数据对象

<data class="CategoryDataBinding">
<variable
name="category"
type="com.struct.red.alltolearn.///.CategoryItem "/>
</data>

并尝试更改适配器以使其可能

categoryViewModel = ViewModelProviders.of(this).get(CategoryViewModel.class);
categoryViewModel.getAllCategories().observe(this, new Observer<List<CategoryItem >>() {
@Override
public void onChanged(@Nullable List<CategoryItem > categoryItems) {
categoryAdapter = new CategoryAdapter(getContext(), categoryItems);
...

Answer 2

也许你可以使用变形？

//this is returned to the observer in setupCategoryRecycler()
return Transformations.switchMap(repository.getCategory()) { result -> 
//do any other stuff you need here           
allCategories.setValue(result)
}

转换可用于将一个实时数据转换为另一个实时数据。检查：https://developer.android.com/topic/libraries/architecture/livedata#transform_livedata

Answer 3

您正在尝试从错误的表加载数据course_table

@Query("SELECT * FROM course_table") LiveData>loadCategoryItem();

应该是category_table

Answer 4

除非你调用观察，否则你的items=categoryDao.loadCategoryItem().getValue()将没有任何值。