第一次调用后,LiveData不会更新其值

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,他的所有帮助都是调试;)

显然,观察者与您首次设置的对象相关联.您无法替换对象(通过归因),否则将无法使用.此外,如果您的变量的值将要更改,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MutableLiveData传递给存储库

public class StatesViewModel extends ViewModel {

private MutableLiveData> 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> getStates(){

    return this.states;
}
}

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> getStates(String token){
    refreshStates(token);

    final MutableLiveData> 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> 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 getAllStates();

@Query("SELECT * FROM states WHERE ctrId = :countryID")
List 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 states);

@Delete
void delete(State state);
}

4.最后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();
}