如何使用viewmodel类在片段中从数据库中获取数据为什么用户等于null

Question

我试图从数据库中获取用户，但我从viewmodel中获取null，有人知道为什么吗？？

java.lang.NullPointerException:尝试在以下位置对空对象引用调用虚拟方法“int java.lang.string.HashCode（）”:omer.lukatz.rooms.ui.games.gamesfragment.onCreateView（gamesfragment.java:81)，androidx.fragment.app.fragment.PerformCreateView（fragment.java:2698)，androidx.fragment.app.fragmentStatemanager.CreateView（fragment.java:320)， 在android.app.activitythread.main（activitythread.java:6938)在java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.zygoteinit.main（zygoteinit.java:1374)在java.lang.reflect.methodandargscaller.run（zygote.java:327)在com.android.internal.os.zygoteinit.java:1374)

公共类GamesFragment扩展片段{

ListView lvGames;
ArrayList<Game> arrayList;
ArrayAdapter<Game> arrayAdapter;
String console;
private GamesViewModel gamesViewModel;
SharedPreferences gameSharedPreferences;

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

    View root = inflater.inflate(R.layout.fragment_games, container, false);
    lvGames = root.findViewById(R.id.lvGames);
    arrayList = new ArrayList<>();
    arrayAdapter = new ArrayAdapter<>(getActivity().getApplicationContext(), android.R.layout.simple_list_item_1,arrayList);
    lvGames.setAdapter(arrayAdapter);

    gameSharedPreferences = getActivity().getSharedPreferences("game",0);

    SharedPreferences usernameSharedPreferences = getActivity().getSharedPreferences("users",0);
    final String username = usernameSharedPreferences.getString("username","");

    gamesViewModel =
            ViewModelProviders.of(this).get(GamesViewModel.class);
    gamesViewModel.getUser().observe(getViewLifecycleOwner(), new Observer<User>() {
        @Override
        public void onChanged(User user) {
            console = user.getConsole();
        }
    });

公共类GamesViewModel扩展了AndroidViewModel{

private MutableLiveData<User> user;

public GamesViewModel(Application application) {
    super(application);
    SharedPreferences usernameSharedPreferences = getApplication().getSharedPreferences("users",0);
    String username = usernameSharedPreferences.getString("username","");
    user = new MutableLiveData<>();
    User.getUser(username, new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            user.setValue(snapshot.getValue(User.class));

        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {

        }
    });
}

public LiveData<User> getUser() {
    return this.user;
}

}

public class User implements Serializable {
private String fullName;
private String userName;
private String email;
private String password;
private String console;
private String url;

public User()
{

}

public User(String fullName, String userName, String email, String password,String console, String url) {
    this.fullName = fullName;
    this.userName = userName;
    this.email = email;
    this.password = password;
    this.console = console;
    this.url = url;
}

public static void addUser(User user, OnCompleteListener listener)
{
    FirebaseDatabase.getInstance().getReference("Users").child(user.getUserName()).setValue(user).addOnCompleteListener(listener);
}

public static void getUser(String username, ValueEventListener listener)
{
    FirebaseDatabase.getInstance().getReference("Users").child(username).addListenerForSingleValueEvent(listener);
}

public String getConsole() {
    return console;
}

public void setConsole(String console) {
    this.console = console;
}

public String getUrl() {
    return url;
}

public void setUrl(String url) {
    this.url = url;
}

public String getFullName() {
    return fullName;
}

public void setFullName(String fullName) {
    this.fullName = fullName;
}

public String getUserName() {
    return userName;
}

public void setUserName(String userName) {
    this.userName = userName;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

}

Answer 1

首先，将ViewModel创建和observe移动到onViewCreate()，而不是onCreateView()。

其次，user.getUser()方法没有返回任何内容（它是void)。 将它的返回类型更改为它应该返回的任何内容，并将函数更改为返回您需要的内容。