android - RecyclerView.Adapter 使用 viewPager 在 Fragment 中返回空对象引用

Question

我正在尝试使用 mvvm 架构创建具有查看寻呼机的页面。

所以我尝试做的是使用可观察方法在第一个选项卡的 fragment 内显示 Recycler View 。

但是当我尝试设置内容是我的 fragment 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 Activity

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 fragment 中

@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Log

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

然后报错

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

所以有人知道为什么我在 onCreateView 中初始化之后调用 FoodListAdapter 时得到 null 吗？

感谢您的帮助!

Answer 1

日志告诉我们什么？

D/Food Pager: get Item

FragmentStatePagerAdapter.getItem(int position) 被调用以显示 FoodListFragment。这通常意味着 Fragment 将被添加到屏幕上，因此接下来应该调用它的 onCreateView()。实际情况是这样的:

D/Food LIst: On create VIew

另一方面，我们有一个Observer 来监听数据变化。每次触发时，都会调用 startFoodListFragment():

D/startFoodListFragment: yolo

在此方法中，FoodListFragment 的新实例将被创建。您尝试通过调用 setFoodList()

将新数据传递给它

D/Food List Fragment: setFoodlist

但是由于这个新实例不会被附加到屏幕上，所以它的 onCreateView() 还没有被调用。所以它的适配器还没有被实例化。这就是应用程序崩溃的原因:

D/AndroidRuntime: Shutting down VM E/AndroidRuntime: FATAL EXCEPTION: main

你能做什么？

您应该将数据传递到 FoodListFragment 的正确实例中。或者简单地在 FoodListFragment 中注册 Observer。 Fragment 就像 Activity 一样是生命周期所有者，因此我认为这种方法最适合您。