android - 如何从无法访问lifeCycleOwner进行观察的资源库中获取LiveData的值(value)？

我在我的应用程序中使用了MVVM和ROOM和databindig。根据Guide to app architecture，我想使用空间兑现数据。在xml项目的RecyclerView布局中，我使用CategoryViewModel变量。我从Room类型的LiveData数据库中获取类别列表。我想将LiveData<list<CategoryItem>>类型更改为MutableLiveData<ArrayList<CategoryViewModel>>类型。因为最后我的适配器使用了ArrayList<CategoryViewModel>数据类型。如何获取LiveData的值？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xml项的recyclerView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(public)接口(interface)CategoryDao {

@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);
        }
    });
}

最佳答案

您的问题在这里，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，该值此时仍为null。
为了避免这种情况，您可以做两件事。
1. 提前调用loadCategoryItem()，稍后再调用getValue()时获得值；
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ViewModel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，但是我们有两个问题。首先，仍然不能保证值不会为空。第二个问题是您无法观察到项目更改。即使您正在返回arrayListMutableLiveData对象(即livedata)，也要手动设置一次它的值，除非再次调用getCategory()，否则它的值将不会更改。
2. 第二招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

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->您无法观察到项目更改。
我写这是为了更好地阐明问题。 *
真正的问题是您尝试使用CategoryViewModel进行数据绑定(bind)。
请改用CategoryItem
我建议从viewModel中删除这两行

private String title;
private String imagePath;

尝试解决您的问题而不将数据从List解析到ArrayList。

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

然后尝试使用CategoryItem作为数据对象

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

并尝试更改您的适配器以使执行此操作成为可能

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);
...