获取 LiveData 的初始值总是返回 Null
当应用程序启动时，我正在尝试从本地房间数据库加载loggedInUser 。如果之前保存的用户保存的身份验证令牌仍然有效，我想跳过提示用户登录！
因此，从DAO中，我想返回一个包含先前登录的用户的LiveData对象，然后观察它以进行后续更改。我面临的挑战是，如果我将结果包装在LiveData中，获取当前登录用户的方法总是返回 null，但如果返回为POJO ，它将返回预期的用户。
如何强制LiveData同步运行以初始化值，然后收听后续更改？我真的想结合这两种行为，因为身份验证可能会因后台同步任务或用户注销而失效（这些操作将替换或更新保存的令牌，我希望在实时数据）。
这是我到目前为止所尝试的：
AuthorizationDAO.java
public interface AuthorizationDAO {
@Query("SELECT * FROM Authorization LIMIT 1") //Assume only one Authentication token will exist at any given time
LiveData<Authorization> getLoggedInUser(); //I want to keep this behaviour
@Insert(onConflict = REPLACE)
long insertAuth(Authorization authorization);
@Update
void logoutCurrentUser(Authorization authorization);
}
AuthorizationRepository.java
public class AuthorizationRepository {
private AuthorizationDAO mAuthorizationDAO;
private MutableLiveData<Authorization> mAuthorization = new MutableLiveData<>();
public AuthorizationRepository(Application application){
AppDatabase db = AppDatabase.getDatabase(application);
this.mAuthorizationDAO = db.mAuthorizationDAO();
}
public LiveData<Authorization> getLoggedInUser(){
mAuthorization.postValue(mAuthorizationDAO.getLoggedInUser().getValue()); //this is always null at startup
return this.mAuthorization;
}
AuthorizationViewModel.java
public class AuthorizationViewModel extends AndroidViewModel {
private AuthorizationRepository mAuthorizationRepository;
private LiveData<Resource<Authorization>> mAuthorization;
private LiveData<Authorization> loggedInUserAuth;
public AuthorizationViewModel(@NonNull Application application) {
super(application);
this.mAuthorizationRepository = new AuthorizationRepository(application);
});
如您所见，我正在调用mAuthorizationViewModel.init()以便我可以从本地数据库加载或初始化loggedInUserAuth，然后在下一行使用mAuthorizationViewModel.getLoggedInUserAuth().observe()观察相同的LiveData实例！但是为loggedInUserAuth返回的值始终为 null！
请帮忙，谢谢！
互换的青春浏览 290回答 3
3回答
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手掌心
我终于在@Krishna 的大力帮助下解决了这个问题，以下是要点：DAO 方法应该返回LiveData在Repository 类中，创建一个LiveData 私有成员变量而不是MutableLiveData（这是因为我们将通过更新/插入来改变数据库记录）。成员变量将保存对DAO 方法返回的LiveData对象的引用在Repository 的构造函数中，将LiveData对象初始化为DAO 方法返回的结果。这样，每次activity启动时，都会加载当前保存的记录在Repository 类中，创建一个getter，它将LiveData对象公开给ViewModel在ViewModel 类中，创建一个将LiveData对象公开给View Controller（活动或片段）的方法在Activity 或 Fragment中，只需侦听或订阅ViewModel提供的Accessor Method公开的LiveData上的更改DAO还可以公开更新LiveData的方法，允许Repository通过ViewModel启用Activity 或 Fragment向LiveData发送更新，同时保持所有侦听器的响应！这是此场景的工作代码：AuthorizationDAO.javapublic interface AuthorizationDAO { @Query("SELECT * FROM Authorization LIMIT 1") //Assume only one Authentication token will exist at any given time LiveData<Authorization> getLoggedInUser(); //I want to keep this behaviour @Insert(onConflict = REPLACE) long insertAuth(Authorization authorization); @Update void logoutCurrentUser(Authorization authorization); //this will be used to toggle login status by Activity or Fragment}AuthorizationRepository.javapublic class AuthorizationRepository { private AuthorizationDAO mAuthorizationDAO; private AuthorizationWebAPI mAuthorizationWebAPI; private LiveData<Authorization> mAuthorization; //reference to returned LiveData public AuthorizationRepository(Application application){ AppDatabase db = AppDatabase.getDatabase(application); this.mAuthorizationDAO = db.mAuthorizationDAO(); this.mAuthorization = mAuthorizationDAO.getLoggedInUser(); //initialize LiveData } public LiveData<Authorization> getAuthorizationResult() { //getter exposing LiveData return mAuthorization; } public void logoutCurrentUser(){ //toggle login status if (this.mAuthorization != null){ AppExecutors.getInstance().getDiskIO().execute(()->{ Authorization mAuthorizationObj = this.mAuthorization.getValue(); mAuthorizationObj.setLoggedIn(false); mAuthorizationDAO.logoutCurrentUser(mAuthorizationObj); //update LiveData and changes will be broadcast to all listeners }); } }}AuthorizationViewModel.javapublic class AuthorizationViewModel extends AndroidViewModel { private AuthorizationRepository mAuthorizationRepository; public AuthorizationViewModel(@NonNull Application application) { super(application); this.mAuthorizationRepository = new AuthorizationRepository(application); } public LiveData<Authorization> getLoggedInUserAuth() { //exposes LiveData to the Activity or Fragment return mAuthorizationRepository.getAuthorizationResult(); } public void logoutCurrentUser(){ //allows activity or fragment to toggle login status this.mAuthorizationRepository.logoutCurrentUser(); }}AppActivity.javapublic class AppActivity extends AppCompatActivity { public AuthorizationViewModel mAuthorizationViewModel; public @Nullable Authorization mAuthorization; private NavController mNavController; private NavHostFragment mNavHostFragment; private BottomNavigationView mBottomNavigationView; private boolean mIsLoggedIn; private ActivityAppBinding mBinding; private boolean mIsTokenExpired; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mBinding = DataBindingUtil.setContentView(this, R.layout.activity_app); mNavHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.app_nav_host_fragment); mNavController = mNavHostFragment.getNavController(); mBottomNavigationView = findViewById(R.id.nav_bottom_nav_view); NavigationUI.setupWithNavController(mBottomNavigationView, mNavController); mAuthorizationViewModel = ViewModelProviders.of(this).get(AuthorizationViewModel.class); mAuthorizationViewModel.getLoggedInUserAuth().observe(this, new Observer<Authorization>() { //Observe changes to Authorization LiveData exposed by getLoggedInUserAuth() @Override public void onChanged(@Nullable Authorization authorization) { mBinding.setViewModel(authorization); mIsLoggedIn = authorization == null? false: authorization.isLoggedIn(); mIsTokenExpired = authorization == null ? true : authorization.isTokenExpired(); if(!mIsLoggedIn || mIsTokenExpired){ if (authorization != null){ Log.i("CurrentAuth", "tokenExpiresAt?: "+ authorization.getExp()); } mNavController.navigate(R.id.start_login); //every time authorization is changed, we check if valid else we react by prompting user to login } } }); }}LogoutFragment.javapublic class LogoutFragment extends Fragment { private AuthorizationViewModel mAuthorizationViewModel; private Authorization mAuth; private FragmentLogoutBinding mBinding; public LogoutFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { mAuthorizationViewModel = ViewModelProviders.of(getActivity()).get(AuthorizationViewModel.class); mAuthorizationViewModel.getLoggedInUserAuth().observe(getActivity(), new Observer<Authorization>() { @Override public void onChanged(Authorization authorization) { mAuth = authorization; } }); // Inflate the layout for this fragment mBinding = DataBindingUtil.inflate(inflater,R.layout.fragment_logout,container,false); View view = mBinding.getRoot(); mBinding.setViewModel(mAuth); return view; } @Override public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) { super.onViewCreated(view, savedInstanceState); new AlertDialog.Builder(getContext()) .setTitle(R.string.title_logout_fragment) .setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialogInterface, int i) { mAuthorizationViewModel.logoutCurrentUser(); //toggle login status, this will mutate LiveData by updating the database record then UI will react and call login fragment } }) .setNegativeButton(R.string.no, new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialogInterface, int i) { dialogInterface.cancel(); Navigation.findNavController(view).popBackStack(); } }) .setOnDismissListener(new DialogInterface.OnDismissListener() { @Override public void onDismiss(DialogInterface dialogInterface) { } }) .show(); }} -
慕标琳琳
mAuthorization在类中创建一个getter方法AuthorizationRepositorypublic MutableLiveData<Authorization> getAuthorizationResult() { return mAuthorization;}然后像下面这样修改你的AuthorizationViewModel类public void init() { mAuthorizationRepository.getLoggedInUser();}public LiveData<Authorization> getLoggedInUserAuth() { return mAuthorizationRepository.getAuthorizationResult();} -
智慧大石
为时已晚，但可能会帮助某人。我这样做时遇到了同样的问题MyDao myDao;private LiveData<List<T>> liveList;//in constructor of repo after initializing myDao; this.liveList = myDao.getAllData();//somewhere in repo for(T t : liveList.getValue()){/*computation*/}这就是我解决它的方法MyDao myDao;//in constructor of repo don't do this because called on main thread this.list = myDao.getAll();//in constructor of repo initialize your Dao (in this case myDao)//somewhere in repo (must not be on main thread) for(T t : myDao.getAll()){/*computation*/} //do this on background thread在我的道@Query("SELECT * FROM myTable")List<T> getAll();@Query("SELECT * FROM myTable")LiveData<List<T>> getAllData();或者，如果您在其他地方（而不是存储库）访问 liveList，那么您必须为相同的地方设置一个观察者
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