我一直在用头撞墙，我不明白为什么会这样。我正在使用适用于 Android 的新架构组件，但在使用对象列表更新 LiveData 时遇到问题。 我有两个旋转器。当我更改第一个选项时，第二个必须更改其内容。但是最后一部分没有发生。 谁能帮帮我？

State.java

@Entity(tableName = "states")
public class State{

@PrimaryKey(autoGenerate = false)
private int id;

private String name;

@ColumnInfo(name = "countryId")
private String CountryId;

@Ignore
private Object geoCenter, geoLimit;

public State(){

}

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getCountryId() {
    return CountryId;
}

public void setCountryId(String countryId) {
    CountryId = countryId;
}
}

StateDAO

@Dao
public interface StateDao {

@Query("SELECT * FROM states")
LiveData<List<State>> getAllStates();

@Query("SELECT * FROM states WHERE countryId = :countryID")
LiveData<List<State>> getStatesFromCountry(String countryID);

@Query("SELECT COUNT(*) FROM states")
int getNrStates();

@Query("SELECT COUNT(*) FROM states WHERE countryId = :countryID")
int getNrStatesByCountry(String countryID);

@Insert(onConflict = IGNORE)
void insertAll(List<State> states);

@Delete
void delete(State state);
}

StateRepository

@Singleton
public class StatesRepository {

private final WebServices services;
private final StateDao stateDao;
private final Executor executor;

@Inject
public StatesRepository(Executor executor, StateDao stateDao, WebServices services) {
    this.services = services;
    this.stateDao = stateDao;
    this.executor = executor;
}


public LiveData<List<State>> getStates(String token){
    refreshStates(token);

    return stateDao.getAllStates();
}

public LiveData<List<State>> getStatesFromCountry(String countryID){

    return stateDao.getStatesFromCountry(countryID);
}

private void refreshStates(final String token){

    executor.execute(() -> {

        Log.d("oooooo", stateDao.getNrStates() + "");
        if(stateDao.getNrStates() == 0){

            try {
                Response<List<State>> response = services.getStates("Bearer "+token).execute();

                stateDao.insertAll(response.body());

            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}
}

StateViewModel

public class StatesViewModel extends ViewModel {

private LiveData<List<State>> states;
private StatesRepository repo;

@Inject
public StatesViewModel(StatesRepository repository){

    this.repo = repository;
}

public void init(String token){

    states = repo.getStates(token);
}

public void getStatesFromCountry(String countryID){

    states = repo.getStatesFromCountry(countryID);

}

public LiveData<List<State>> getStates(){

    return this.states;
}

}

fragment

public class EditAddressFragment extends LifecycleFragment implements View.OnClickListener, Injectable{


private Spinner country, city, state, zip_code;
private String token;
private List<Country> countries;
private List<City> cities;
private List<State> states;
@Inject ViewModelFactory viewModelFactory;


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.addresses_edit_layout, container, false);

    city = view.findViewById(R.id.city);
    state = view.findViewById(R.id.state);
    country = view.findViewById(R.id.country);
    ...

    countries = new ArrayList<>();
    cities = new ArrayList<>();
    states = new ArrayList<>();

    return view;
}


@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);


    CountrySpinnerAdapter adapter = new CountrySpinnerAdapter(getActivity(), android.R.layout.simple_spinner_item, countries);
    country.setAdapter(adapter);

    CitySpinnerAdapter cityAdapter = new CitySpinnerAdapter(getActivity(), android.R.layout.simple_spinner_item, cities);
    city.setAdapter(cityAdapter);
    StateSpinnerAdapter stateAdapter = new StateSpinnerAdapter(getActivity(), android.R.layout.simple_spinner_item, states);
    state.setAdapter(stateAdapter);


    CountriesViewModel countriesViewModel = ViewModelProviders.of(this, viewModelFactory).get(CountriesViewModel.class);
    countriesViewModel.init(token);
    countriesViewModel.getCountries().observe(this, adapter::setValues);

    CityViewModel cityViewModel = ViewModelProviders.of(this, viewModelFactory).get(CityViewModel.class);
    cityViewModel.init(token);
    cityViewModel.getCities().observe(this, cityAdapter::setValues);

    StatesViewModel statesViewModel = ViewModelProviders.of(this, viewModelFactory).get(StatesViewModel.class);
    statesViewModel.init(token);
    statesViewModel.getStates().observe(this, states -> { 
      Log.d("called", states.toString()); 
      stateAdapter.setValues(states); } );


    country.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {

            Country c = (Country) adapterView.getItemAtPosition(i);

            Log.d("cd", c.getId());

            //states = new ArrayList<State>();

            statesViewModel.getStatesFromCountry(c.getId());

        }

        @Override
        public void onNothingSelected(AdapterView<?> adapterView) {

        }
    });

....

适配器

public void setValues(List<State> states)
{ 
this.states = states; 
Log.d("s", states.isEmpty()+" "+states.toString()); 
notifyDataSetChanged(); 
}

最佳答案

好吧，我已经找到了解决这个问题的方法，并了解了这个 LiveData 的工作原理。

感谢@MartinMarconcini 在调试方面的所有帮助；)

显然，观察者与您首先设置的对象相关联。您不能替换对象(按归属)，否则它将不起作用。 此外，如果您的变量的值会发生变化，那么您应该使用 MutableLiveData

所以必要的改变是:

1.从 LiveData 更改为 MutableLiveData 并在需要更新时将该 MutableLiveData 传递到存储库

public class StatesViewModel extends ViewModel {

private MutableLiveData<List<State>> states; ;;CHANGED
private StatesRepository repo;

@Inject
public StatesViewModel(StatesRepository repository){
    this.repo = repository;
}


public void init(String token){

    states = repo.getStates(token);
}

public void getStatesFromCountry(String countryID){

    repo.getStatesFromCountry(this.states, countryID); ;;CHANGED
}

public LiveData<List<State>> getStates(){

    return this.states;
}
}

<强>2。在存储库中，使用 setValue 更新 MutableLiveData

@Singleton
public class StatesRepository {

private final WebServices services;
private final StateDao stateDao;
private final Executor executor;

@Inject
public StatesRepository(Executor executor, StateDao stateDao, WebServices services) {
    this.services = services;
    this.stateDao = stateDao;
    this.executor = executor;
}


public MutableLiveData<List<State>> getStates(String token){
    refreshStates(token);

    final MutableLiveData<List<State>> data = new MutableLiveData<>();

    data.setValue(stateDao.getAllStates());

    return data;

}

;; CHANGED
public void getStatesFromCountry(MutableLiveData states, final String countryID){

    states.setValue(stateDao.getStatesFromCountry(countryID));

}

private void refreshStates(final String token){

    executor.execute(() -> {

        if(stateDao.getNrStates() == 0){

            try {
                Response<List<State>> response = services.getStates("Bearer "+token).execute();

                stateDao.insertAll(response.body());

            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}
}

3.将 DAO 更改为返回 List 而不是 LiveData>

@Dao
public interface StateDao {

@Query("SELECT * FROM states")
List<State> getAllStates();

@Query("SELECT * FROM states WHERE ctrId = :countryID")
List<State> getStatesFromCountry(String countryID);

@Query("SELECT COUNT(*) FROM states")
int getNrStates();

@Query("SELECT COUNT(*) FROM states WHERE ctrId = :countryID")
int getNrStatesByCountry(String countryID);

@Insert(onConflict = IGNORE)
void insertAll(List<State> states);

@Delete
void delete(State state);
}

4.终于允许在主线程中执行查询

AppModule.java

@Singleton @Provides
AppDatabase provideDb(Application app) {
    return Room.databaseBuilder(app, AppDatabase.class,"unitail.db")
            .allowMainThreadQueries()
            .fallbackToDestructiveMigration()
            .build();
}

关于android - LiveData 在第一次调用后没有更新它的值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/45889604/