使用共享ViewModel将FireStore数据提取为两个片段的有效方法

Question

上下文：我有一个User类，看起来像这样：

public class User {
    private String firstname;
    private String lastname;
    private GeoPoint location;
    
    /* Constructor and getters */
    ...
}

还有两个片段s，在其中我将使用RecyclerView和GoogleMap在第一个片段中显示用户名列表，后者显示第二个片段中用户的设备位置标记。我在这两个片段之间使用共享的ViewModel，以提供所需的数据。ViewModel如下所示：

@HiltViewModel
public class UsersViewModel extends ViewModel {

    private List<User> users;
    private UserRepository repository;

    @Inject
    public UsersViewModel(UserRepository repository) {
        this.repository = repository;
    }

    public void fetchUsers(OnFetchDataCallback callback) {
        repository.getUsers(callback); 
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

    public List<User> getUsers() {
        return users;
    }
}

在我的资源库课上，

public class UserRepository {

    private final FirebaseFirestore db;

    @Inject
    public UserRepository(FirebaseFirestore db) {
        this.db = db;
    }

    public List<User> getUsers(OnFetchDataCallback callback) {
        db.collection("users")
            .addSnapshotListener(new EventListener<QuerySnapshot>() {
                @Override
                public void onEvent(@Nullable QuerySnapshot val,
                                    @FirebaseFirestoreException e) {
                    List<User> users = new ArrayList<>();
    
                    for (QueryDocumentSnapshot document : val) {
                        users.add(document.toObject(User.class);
                    }
                    callback.onFetchData(users);
                }
            });
    }
}

我有一个名为getUsers（）的方法，可以从firestore数据库中实际获取用户。我还创建了一个如下所示的接口回调

public interface OnFetchDataCallback {
    void onFetchData(List<User> users);
}

这样以后我就可以在我的两个片段中访问获取的用户。在我的第一个片段中，

@AndroidEntryPoint
public class DisplayUsersFragment extends Fragment {
    private FragmentDisplayUsersBinding binding;
    private UsersViewModel viewModel;
    private UsersAdapter adapter;

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        viewModel = new ViewModelProvider(this).get(UsersViewModel.class);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        binding = FragmentDisplayUsersBinding.inflate(inflater, container, false);
        binding.usersRecyclerView.setLayoutManager(
            new LinearLayoutManager(getContext()));

        viewModel.fetchUsers(new OnFetchDataCallback() {
            @Override
            public void onFetchData(List<User> users) {
                viewModel.setUsers(users);
                adapter = new UsersAdapter(viewModel.getUsers());
                binding.usersRecyclerView.setAdapter(adapter);
            }
        });
        return binding.getRoot();
    }
}

我调用了fetchUsers（），访问了回调中users的列表，并将其设置为myviewModel的users，然后为myRecyclerView构建了一个适配器，并通过调用viewModel>为其提供了所需的数据。getUsers（）并将其作为适配器的构造函数参数传递。现在，我的问题是，在我的第二个片段中，

@AndroidEntryPoint
public class UsersMapFragment extends Fragment implements OnMapReadyCallback {

    private FragmentUsersMapBinding binding;
    private UsersViewModel viewModel;

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        viewModel = new ViewModelProvider(this).get(UsersViewModel.class);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        binding = FragmentUsersMapBinding.inflate(inflater, container, false);
        return binding.getRoot();
    }

    @Override
    public void onViewCreated(@NonNull View view,
                              @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        SupportMapFragment mapFragment = SupportMapFragment.newInstance();
        getChildFragmentManager().beginTransaction()
            .add(R.id.users_map_fragment_container, mapFragment)
            .commit()
        mapFragment.getMapAsync(this);
    }

    @Override
    public void onMapReady(@NonNull GoogleMap googleMap) {
         viewModel.fetchUsers(new OnFetchDataCallback() {
            @Override
            public void onFetchData(List<User> users) {
                viewModel.setUsers(users);
                for (User user : viewModel.getUsers()) {
                    googleMap.addMarker(new MarkerOptions()
                                .position(new LatLng(
                                    user.getLocation().getLatitude(),
                                    user.getLocation().getLongitude())
                                )
                                .title(user.getFirstname() + " " 
                                       user.getLastname()
                                )
                            );
                }
            }
        });
    }
}

我再次调用了fetchUser（）内部onMap就绪（），以便我可以访问其回调内部的用户，并使用它们在映射中创建标记。本质上，我尝试调用fetchUser（）两次，但我认为有一种更有效的方法可以做到这一点，但我不知道如何处理。请帮帮我，对不起，我的英语不好。




Answer 1

您可以在ViewModel中使用实时数据：请参阅下面给出的代码：

@HiltViewModel
public class UsersViewModel extends ViewModel {

    private List<User> users;
    MutableLiveData<List<User>> usersLiveData = new MutableLiveData();
    private UserRepository repository;

    @Inject
    public UsersViewModel(UserRepository repository) {
        this.repository = repository;
    }

    public void fetchUsers() {
        repository.getUsers(new OnFetchDataCallback() {
            @Override
            public void onFetchData(List<User> users) {
            usersLiveData.setValue(users);
            // you can observe this data after calling fetch users from your activity
            setUsers(users);
}); 
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

    public List<User> getUsers() {
        return users;
    }
}

在活动中：

mViewModel.fetchUsers();

在这两个片段中：

viewModel.usersLiveData.observe(this,{ list->
   // user your list here in both fragments
   // rather than fetch users on both fragments
});