Java Android LiveData调用Room查询依赖其他LiveData

Question

更新:::

我已经更新了问题以包括演示其他还需要的 LiveData: 所以我们有 userLD，我们需要它的值来获取 goalWeeklyLD，我们需要 goalWeeklyLD 值来获取剩余的 4 个 LiveData 值，因为它们来自在查询中使用 goalWeekly.dateproperties 的 Room 查询

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我遇到了一个问题，我有一个 fragment 必须填充使用依赖于另一个 LiveData 值的查询的 LiveData。当我的实时数据依赖于其他结果时，我如何才能使它正常工作？

如果不使用 Transitions.map()， View 模型会抛出错误，因为其他实时数据的值仍然为空。使用 View 模型中的 Transitions.map() 时， Activity 观察器会抛出错误，因为 LiveData 仍然为空。

我可能会通过使用一个非常大的嵌套查询在一个自定义 DTO 中返回我需要的所有内容来绕过这个问题。但我更想了解这里发生了什么以及如何正确处理这种情况。

希望一些代码能说明这一点

Activity :

public class SomeFragment extends Fragment {

public static SomeFragment newInstance() {
    return new SomeFragment();
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    someViewModel = ViewModelProviders.of(this).get(SomeViewModel.class);


    //getting user details from previous activity
    Intent intent = getActivity().getIntent();
    if (intent != null){
        if (intent.hasExtra(USER_ID)){
            user = new User(intent.getStringExtra(USERNAME));
            user.setId(intent.getLongExtra(USER_ID,0));
            someViewModel.setUserLD(user);
        }
    }


    someViewModel.getUserLD().observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User userVal) {
            user = userVal;
        }
    });
    someViewModel.getGoalWeeklyLD().observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User userVal) {
            user = userVal;
        }
    });
    //the below Observer calls throw an error because LiveData is null. makes sense.
    //but how can i say "don't try and observe these until the transition.map has ran (because then it wont be null after if my understanding is right)" or something to that effect
    someViewModel.getFirstLD(user.getId()).observe(this, new Observer<XObject>() {
        @Override
        public void onChanged(@Nullable Grades avgSportGradeVal) {
            //Update UI
        }
    });
    someViewModel.getSecondLD(user.getId()).observe(this, new Observer<XObject>() {
        @Override
        public void onChanged(@Nullable Grades avgBoulderGradeVal) {
            // Update UI
        }
    });
    someViewModel.getThriLD(user.getId()).observe(this, new Observer<XObject>() {
        @Override
        public void onChanged(@Nullable Grades avgBoulderGradeVal) {
            // Update UI
        }
    });
    someViewModel.getFourthLD(user.getId()).observe(this, new Observer<XObject>() {
        @Override
        public void onChanged(@Nullable Grades avgBoulderGradeVal) {
            // Update UI
        }
    });

}}

View 模型:

public class SomeViewModel extends AndroidViewModel {
DaoRepository daoRepository;

MutableLiveData<User> userLD;
LiveData<XObject> firstLD;
LiveData<XObject> secondLD;
LiveData<XObject> thirdLD;
LiveData<XObject> fourthLD;



public MutableLiveData<User> getUserLD() {
    return userLD;
}
public void setUserLD(User user){
    userLD.setValue(user);
}

public LiveData<XObject> getFirstLD(long userId) {
    return goalWeeklyLD;
}
public LiveData<XObject> getSecondLD(long userId) {
    return goalWeeklyLD;
}
public LiveData<XObject> getThirdLD(long userId) {
    return goalWeeklyLD;
}
public LiveData<XObject> getForthLD(long userId) {
    return goalWeeklyLD;
}


public SomeViewModel(@NonNull Application application) {
    super(application);
    daoRepository = new DaoRepository(application);
    userLD = new MutableLiveData<>();


    //so the first LiveData waits for the user to be populated before getting its LiveData becasue we need the userId for our Room query to run
    firstLD = Transformations.map(userLD, user -> daoRepository.getMostRecentGoalWeekly(user.getId()).getValue());

    //the remaining live data uses values from the first...
    setupOtherTransformMaps(userLD.getValue())
}

public void setupOtherTransformMaps(long userId) {
    //the secondLD, thirdLD and fourthLD all depends on values from the first (in runs a query that uses its dateExpired)
    secondLD = Transformations.map(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()).getValue());
    thirdLD = Transformations.map(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()).getValue());
    fourthLD = Transformations.map(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()).getValue());
}}

Answer 1

谢天谢地，Google 很聪明，它创建了一个组件，让您可以将可变数量的 LiveData 组合到一个 LiveData 中，并且仅在您选择这样做时才发出事件!

这称为 MediatorLiveData。

但在您的情况下，您只需要将 1 个 LiveData ( userLD ) 导入另一个 1 个 LiveData，每次都会发出 userLd有了新的值(value)。

因此您可以使用预定义的 MediatorLiveData 来执行此操作，特别是 Transformations.switchMap .

firstLD = Transformations.switchMap(userLD, user -> daoRepository.getMostRecentGoalWeekly(user.getId()));

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编辑:是的，您似乎需要将这些 LiveData 彼此分开公开，但它们都取决于要执行的第一个查询。

所以你需要替换Transformations.map { ...getValue()与 Transformations.switchMap一切顺利。

public SomeViewModel(@NonNull Application application) {
    super(application);
    CustomApplication app = (CustomApplication) application;
    daoRepository = app.daoRepository();
    userLD = new MutableLiveData<>();

    firstLD = Transformations.switchMap(userLD, user -> daoRepository.getMostRecentGoalWeekly(user.getId()));

    secondLD = Transformations.switchMap(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()));
    thirdLD = Transformations.switchMap(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()));
    fourthLD = Transformations.switchMap(firstLD, first ->
            daoRepository.getAvgGradeRouteInPeriod(userId, first.getDateCreated(),first.getDateExpires()));

}