Question

我有一个students_table，并且存储了不同级别的学生。我想按一个级别显示学生，而隐藏其他级别。我选择学生显示如下：

    if (id == R.id.beginners) {
        stLvl = 0;
    }else if (id == R.id.intermediate) {
        stLvl = 1;
    }else if (id == R.id.advanced) {
        stLvl = 2;
    }else if (id == R.id.high_level) {
        stLvl = 3;
    }

    showStud();

这里是showStud（）;

public void showStud() {

    RecyclerView recyclerView = findViewById(R.id.recycler_view);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setHasFixedSize(true);

    final StudentAdapter adapter = new StudentAdapter();
    recyclerView.setAdapter(adapter);

    setStLvl(stLvl);

    if (stLvl == 0) {

        studentViewModel = ViewModelProviders.of(this).get(StudentViewModel.class);

        studentViewModel.getAllStudents().observe(this, new Observer<List<Student>>() {
            @Override
            public void onChanged(@Nullable List<Student> students) {
                // update RecyclerView
                adapter.submitList(students);
            }
        });

    }else {

        studentViewModel = ViewModelProviders.of(this).get(StudentViewModel.class);

        studentViewModel.getStudentsByLevel().observe(this, new Observer<List<Student>>() {
            @Override
            public void onChanged(@Nullable List<Student> students) {
                // update RecyclerView
                adapter.submitList(students);
            }
        });

    }
}

第一次运行代码时，无论stLvl的值如何，它都可以完美运行，但是当我更改代码时，它的值没有显示我想要的东西，或者根本没有显示。我认为问题出在这一行：

studentViewModel = ViewModelProviders.of(this).get(StudentViewModel.class);

第一次运行，它工作正常，转到StudentViewModel.class来执行应做的工作，但是第二次只是跳转到下一行代码，而无需转到StudentViewModel.class。我究竟做错了什么？预先谢谢你！

Answer 1

是的，您是对的，实际上我是android编程的初学者。这是StudentViewModel：

private StudentDao studentDao;
private LiveData<List<Student>> allStudents;

private LiveData<List<Student>> studentsByLevel;

public int stLevel;

public void setStLvl() {
    MainActivity mainActivity = new MainActivity();
    stLevel = mainActivity.getStLvl();
}

public StudentRepository(Application application) {
    AppDatabase database = AppDatabase.getInstance(application);
    studentDao = database.studentDao();


    setStLvl();
    studentsByLevel = studentDao.getStudentsByLevel(stLevel);

    allStudents = studentDao.getAllStudents();
}
.
.
.
public LiveData<List<Student>> getAllStudents() {
    return allStudents;
}
public LiveData<List<Student>> getStudentsByLevel(int stLevel) {
    return studentsByLevel;
}

StudentRepository：

@Query("SELECT * FROM student_table WHERE level = :level")
LiveData<List<Student>> getStudentsByLevel(int level);

在StudentDao中，我有：

                io.emit('match-found', {
                    lobbyID: lobby._id,
                    enemyID: enemyLobby._id,
                    matchID: newMatch._id
                });

我希望我提供了足够的数据。

Answer 2

首先，阅读本Guide to app architecture将有助于您大致了解这些体系结构组件应如何协同工作。经验法则是

每个组件仅取决于其下一级的组件。

这也意味着每个组件都不应依赖于其上方的组件。例如，存储库不应既不依赖于ViewModel，也不依赖于Activity。您的代码可以通过以下方式重构：

StudentRepository：

private StudentDao studentDao;

// public int stLevel; 

// public void setStLvl() {  // Do not read view components. Do not store their states.
//     MainActivity mainActivity = new MainActivity();
//     stLevel = mainActivity.getStLvl();
// }

public StudentRepository(Application application) {
    AppDatabase database = AppDatabase.getInstance(application);
    studentDao = database.studentDao();

    // setStLvl();
}
.
.
.
public LiveData<List<Student>> getAllStudents() {
    return studentDao.getAllStudents();
}

public LiveData<List<Student>> getStudentsByLevel(int stLevel) {
    return studentDao.getStudentsByLevel(stLevel);
}

在上面的示例中，存储库看起来并没有做很多事情，这是正常的，因为它下面只有一层，即Room。实际上，您可以拥有其他数据源，包括网络客户端和缓存。存储库的工作是抽象所有数据源逻辑。

ViewModel：

private MutableLiveData<Integer> studentLevel;  // This will store the student level
private LiveData<List<Student>> studentsByLevel; // This will store the list of students

public StudentViewModel(@NonNull Application application) {
    super(application);
    repository = new StudentRepository(application);

    studentLevel = new MutableLiveData<>();

    // Place your logic inside the ViewModel
    // Change in studentLevel will be reflected to studentsByLevel
    studentsByLevel = Transformations.switchMap(studentLevel, lvl -> {
        if (studentLevel == 0) {
            return repository.getAllStudents();
        } else {
            repository.getStudentsByLevel(stLevel);
        }
    });

    studentLevel.setValue(0) // Set initial student level.
}

.
.
.
public void setStudentLevel(int level) { // Change studentLevel anytime.
    return studentLevel.setValue(level); 
}

public LiveData<List<Student>> getStudentList() {
    return studentsByLevel;
}

我不是LiveData的粉丝，但是我会这样做。将所有逻辑保留在ViewModel中，并使视图层尽可能简单。

最后，活动：

private StudentViewModel studentViewModel

protected void onCreate(Bundle savedInstanceState) {

    ...

    RecyclerView recyclerView = findViewById(R.id.recycler_view);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setHasFixedSize(true);

    final StudentAdapter adapter = new StudentAdapter();
    recyclerView.setAdapter(adapter);

    studentViewModel = ViewModelProviders.of(this).get(StudentViewModel.class);
    studentViewModel.observe(this, students -> {
        adapter.submitList(students);
    });

    // studentViewModel.setValue(1) // call this function anywhere you like.
}

上面的代码将显示所有学生，因为我们在viewmodel中将默认值设置为0。致电studentViewModel.setValue(/*any integer*/)，将列表切换到任意级别。

房间数据库和viewmodel如何工作？

2 个答案: