第一次调用后，LiveData不会更新其值

问题描述 投票：14回答：4

我一直在撞墙，我无法理解为什么会这样。我正在使用Android的新建筑组件，我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。但最后一部分没有发生。谁能帮我？

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();
            }
        }
    });
}
}

性模型

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;
}

}

分段

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(); 
}
android viewmodel dagger-2 android-room dagger
4个回答
9
投票

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<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;
}
}

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.最后允许在主线程中执行查询

app module.Java

@Singleton @Provides
AppDatabase provideDb(Application app) {
    return Room.databaseBuilder(app, AppDatabase.class,"unitail.db")
            .allowMainThreadQueries()
            .fallbackToDestructiveMigration()
            .build();
}
2
投票

所有操作中的Dao必须相同。您使用不同的Dao实例进行插入和观察

2
投票

设置它并开始观察后，不应更新livingata引用。而不是使用存储库更新实时数据，您应该使用MediatorLiveData。

在您的情况下，请执行以下更改：

private MediatorLiveData<List<State>> states;  // change
.....
.....
states.addSource(repo.getStatesFromCountry(countryID), newData -> states.setValue(newData)); //change
1
投票

写一个答案，以便更好地讨论。

所以我（在Kotlin，sry）一个模型是一个笔记列表（它只是一个沙盒应用程序来播放所有这些）这里是我的架构：我没有Repo，但我有Activity - > ViewModel - >道。

所以Dao暴露了LiveData<MutableList<Note>>

@Query("SELECT * FROM notes")
fun loadAll(): LiveData<MutableList<Note>>

我的ViewModel ...通过以下方式公开它：

val notesList = database.notesDao().loadAll()

和我的活动（onCreate）做...

    viewModel.notesList.observe(this,
            Observer<MutableList<Note>> { notes ->
                if (notes != null) {
                    progressBar?.hide()
                    adapter.setNotesList(notes)
                }
            })

这有效。适配器是一个RecyclerView适配器，除了：

 fun setNotesList(newList: MutableList<Note>) {
        if (notes.isEmpty()) {
            notes = newList
            notifyItemRangeInserted(0, newList.size)
        } else {
            val result = DiffUtil.calculateDiff(object : DiffUtil.Callback() {
                override fun getOldListSize(): Int {
                    return notes.size
                }

                override fun getNewListSize(): Int {
                    return newList.size
                }

                override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
                    return notes[oldItemPosition].id == newList[newItemPosition].id
                }

                override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
                    val (id, title, _, priority) = newList[newItemPosition]
                    val (id1, title1, _, priority1) = notes[oldItemPosition]
                    return id == id1
                            && priority == priority1
                            && title == title1
                }
            })
            notes = newList
            result.dispatchUpdatesTo(this)
        }
    }

如果应用程序的任何其他部分修改了该注释列表，则适配器会自动更新。我希望这能为你提供一个尝试简单（r？）方法的游乐场。

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