android - RecyclerView.Adapter在带有viewPager的Fragment中返回空对象引用

我正在尝试创建具有mvvm体系结构的视图分页器的页面。

所以我想做的是通过使用可观察的方法在第一个选项卡的片段中显示回收站视图。

但是当我尝试设置内容是片段中的适配器时，我得到了

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.xxxx.view.adapter.FoodListAdapter.setFoodList(java.util.List)' on a null object reference

我的FragmentStatePagerAdapter适配器

@Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                Log.d("Food Pager", "get Item");
                FoodListFragment foodListFragment = new FoodListFragment();
                return foodListFragment;
            case 1:
                RestaurantListFragment restaurantListFragment2 = new RestaurantListFragment();
                return restaurantListFragment2;
            case 2:
                RestaurantListFragment restaurantListFragment3 = new RestaurantListFragment();
                return restaurantListFragment3;
            default:
                return null;
        }
    }

我的活动

private void observeViewModel(final CategoryViewModel categoryViewModel) {
        // Observe Category Data
        categoryViewModel.getCategory().observe(this, new Observer<Category>() {
            @Override
            public void onChanged(@Nullable Category category) {
                // Update UI
                if (category != null) {
                    if (category.foods != null) {
                        startFoodListFragment(category.foods);
                    }
                } else {
                    Log.e(TAG, "Category Data is Null");
                }
            }
        });
    }

    private void startFoodListFragment(List<CategoryQuery.Food> foodList) {
        Log.d("startFoodListFragment", "yolo");
        FoodListFragment foodListFragment = new FoodListFragment();
        foodListFragment.setFoodList(foodList);
    }

所以在我的片段里面

@Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        Log.d("Food LIst", "On create VIew");
        View rootView = inflater.inflate(R.layout.fragment_food_list, container, false);

        mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recyclerview_food_list);
        // mLoadingIndicator = (ProgressBar) rootView.findViewById(R.id.food_list_loading_indicator);

        Integer gridColumns = 2;

        Float width = getScreenSize().get("width");

        if (width > 600) {
            gridColumns = 3;
        }

        // Create Pinterest Style RecyclerView
        StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(gridColumns, StaggeredGridLayoutManager.VERTICAL);
        mRecyclerView.setLayoutManager(layoutManager);

        // Cool. it's Very easy ,but margin on my LinearLayout didn’t seem to work. So here’s a quick fix.
        SpacesItemDecoration decoration = new SpacesItemDecoration(8);
        mRecyclerView.addItemDecoration(decoration);

        mFoodListAdapter = new FoodListAdapter(foodListAdapterOnClickHandler);
        mRecyclerView.setAdapter(mFoodListAdapter);

        return rootView;
    }

@Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        if (mFoodList != null) {
            mFoodListAdapter.setFoodList(mFoodList);
        }
    }

    public void setFoodList(List<CategoryQuery.Food> foodList) {
        Log.d(TAG, "setFoodlist");
        if (foodList != null) {
            mFoodList = foodList;
            mFoodListAdapter.setFoodList(mFoodList);
            // mFoodListAdapter.notifyDataSetChanged();
        }
    }

这是我的适配器的一部分

public void setFoodList(final List<? extends CategoryQuery.Food> foodList) {
        if (this.foodList == null) {
            this.foodList = foodList;
            notifyItemRangeInserted(0, foodList.size());
        }
    }

这是我得到的相关日志

D/Food Pager: get Item
D/Food LIst: On create VIew
D/startFoodListFragment: yolo
D/Food List Fragment: setFoodlist
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main

然后错误继续

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.xxx.view.adapter.FoodListAdapter.setFoodList(java.util.List)' on a null object reference

所以有人知道即使我在FoodListAdapter中初始化后调用onCreateView时为什么我还是空的？

谢谢你的帮助！

最佳答案

日志告诉我们什么？


  D / Food Pager：获取物品


调用FragmentStatePagerAdapter.getItem(int position)以显示FoodListFragment。这通常意味着Fragment将被添加到屏幕上，因此下一步应调用其onCreateView()。实际上是这样的：


  D / Food List：创建时


另一方面，我们有一个Observer来监听数据更改。每次触发时，startFoodListFragment()将被调用：


  D / startFoodListFragment：yolo


用这种方法，将创建一个新的FoodListFragment实例。您尝试通过调用setFoodList()将新数据传递给它


  D /食物清单片段：setFoodlist


但是由于此新实例将不会附加到屏幕，因此尚未调用其onCreateView()。因此，其适配器尚未实例化。这就是应用崩溃的原因：


  D / AndroidRuntime：关闭虚拟机
  E / AndroidRuntime：致命异常：main


你能做什么？

您应该将数据传递到FoodListFragment的正确实例中。或者只是在Observer中注册FoodListFragment。 Fragment就像Activity一样是生命周期所有者，所以我认为这种方法最适合您。