问题描述
我试图在我的项目中使用MVVM这么多天。但无法实现。
我的代码：
FeedRepository.java
public class FeedsRepository {
private static FeedsRepository instance;
private List<Feed> FeedsToAppend = new ArrayList<>();
public static FeedsRepository getInstance() {
if (instance == null) {
instance = new FeedsRepository();
}
return instance;
}
public mutablelivedata<List<Feed>> getFeeds() {
mutablelivedata<List<Feed>> mFeeds = new mutablelivedata<>();
Query query;
query = FeedsRef.whereEqualTo("userID",currentUID).whereEqualTo("not_interested",false)
.orderBy("created_at",Query.Direction.DESCENDING).limit(10);
query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
@Override
public void onComplete(@NonNull Task<QuerySnapshot> task) {
if (task.isSuccessful()) {
QuerySnapshot snapshot = task.getResult();
if (snapshot.isEmpty()) {
} else {
List<DocumentSnapshot> documents = snapshot.getDocuments();
for (DocumentSnapshot doc: documents) {
final Feed Feed = doc.toObject(Feed.class);
Feed.setFeedID(doc.getId());
Feed.setFeedAvailable(true);
FeedsToAppend.add(Feed);
System.out.println("Feed: " + Feed.getFeedID());
}
System.out.println("Total Feeds to append: " + FeedsToAppend.size());
mFeeds.setValue(FeedsToAppend);
}
}
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
}
});
return mFeeds;
}
Homeviewmodel.java
public class Homeviewmodel extends viewmodel {
private mutablelivedata<List<Feed>> mFeeds;
private FeedsRepository repo;
public void init() {
if (mFeeds != null) {
return;
}
repo = FeedsRepository.getInstance();
mFeeds = repo.getFeeds();
}
public LiveData<List<Feed>> getFeeds() {
return mFeeds;
}
}
HomeFragment.java
public View onCreateView(@NonNull LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.home_fragment,container,false);
FeedsRecycleView = view.findViewById(R.id.Feeds_recycler_view);
homeviewmodel = viewmodelProviders.of(this).get(Homeviewmodel.class);
homeviewmodel.init();
homeviewmodel.getFeeds().observe(getViewLifecycleOwner(),new Observer<List<Feed>>() {
@Override
public void onChanged(List<Feed> Feeds) {
System.out.println("Feed adapter Feeds: "+Feeds.size());
mFeeds = Feeds;
FeedAdapter.notifyDataSetChanged();
}
});
List<Feed> testFeeds = homeviewmodel.getFeeds().getValue();
System.out.println("Test Feeds are : " + testFeeds.size());
FeedAdapter = new FeedAdapter(getContext(),homeviewmodel.getFeeds().getValue());
FeedsRecycleView.setAdapter(FeedAdapter);
FeedsRecycleView.setHasFixedSize(true);
linearlayoutmanager linearlayoutmanager = new linearlayoutmanager(getContext());
linearlayoutmanager.setStackFromEnd(true); //newer posts will be shown on top
linearlayoutmanager.setReverseLayout(true);
FeedsRecycleView.setLayoutManager(linearlayoutmanager);
return view;
}
我收到错误消息
在此行
List<Feed> testFeeds = homeviewmodel.getFeeds().getValue();
和
FeedAdapter = new FeedAdapter(getContext(),homeviewmodel.getFeeds().getValue());
请任何人告诉我我在哪里犯错。
解决方法
您遇到以下错误：
“试图在空对象引用上调用接口方法'int java.util.List.size（）'”
最有可能在此特定代码行：
System.out.println("Test feeds are : " + testFeeds.size());
这是因为您的testFeeds对象是null，这意味着：
homeViewModel.getFeeds().getValue();
返回null。这也意味着getValue()方法返回一个null的LiveData对象，的确如此，就像您的HomeViewModel类中那样，getFeeds()方法返回一个从不存在的LiveData对象。初始化。要解决此问题，请更改以下代码行：
private MutableLiveData<List<Feed>> mFeeds
到
private MutableLiveData<List<Feed>> mFeeds = new MutableLiveData<>();
我尝试了这个，但是仍然是相同的错误
public MutableLiveData<List<Feed>> getFeeds() {
MutableLiveData<List<Feed>> mFeeds = new MutableLiveData<>();
return mFeeds;
}
好的。我知道了。
我必须在FeedsRepository中添加此行，
public MutableLiveData<ArrayList<Feed>> getFeeds() {
MutableLiveData<ArrayList<Feed>> mFeeds = new MutableLiveData<>();
mFeeds.setValue(feedsToAppend);
......
并且从后端获取记录后，再次
mFeeds.setValue(feedsToAppend);
由于后端进程是异步的，因此需要一段时间才能提取 它将返回空对象。