我正在尝试创建具有mvvm架构的视图寻呼机的页面。
所以我试图通过使用observable方法在第一个选项卡的片段内显示Recycler视图。
但是,当我尝试设置内容是我的片段内的适配器我得到
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
所以任何人都知道为什么我得到null,甚至在我在FoodListAdapter
内部初始化之后调用onCreateView
?
谢谢你的帮助!
日志告诉我们什么?
D / Food Pager:获取物品
FragmentStatePagerAdapter.getItem(int position)
被称为显示FoodListFragment
。这通常意味着Fragment
将被添加到屏幕上,因此它的onCreateView()
应该被称为下一个。实际情况如此:
D / Food LIst:创建VIew
另一方面,我们有一个Observer
来监听数据变化。每当它发射时,startFoodListFragment()
将被称为:
D / startFoodListFragment:yolo
在此方法中,将创建FoodListFragment
的新实例。您尝试通过调用setFoodList()
将新数据传递给它
D / Food List Fragment:setFoodlist
但由于这个新实例不会附加到屏幕上,因此尚未调用其onCreateView()
。所以它的适配器尚未实例化。这就是应用程序崩溃的原因:
D / AndroidRuntime:关闭VM E / AndroidRuntime:FATAL EXCEPTION:main
你能做什么?
您应该将数据传递到正确的FoodListFragment
实例。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你。
在你的活动中你有这个方法startFoodListFragment()
,它创建一个片段并在其上调用setFoodList()
方法,在内部将数据设置为适配器。
在onCreateView()
生命周期方法上创建适配器实例时,在创建对象时不会初始化适配器。所以它在那个时候是空的,这就是它扔NullPointerException
的原因。确保当您访问适配器时,它必须在执行onCreateView()
方法之后才能再次获得NPE
!