I try to make sample login page with two fields (username, password) and save button with android architecture component, using android data binding, validating the data in viewmodel and from view model I make call to repository for remote server call as mentioned in official doc, remote server return me userid with success so how can I start new fragment from view model using this success? I learn something about singleLiveEvent and EventObserver, but I'm not able to find there clear usage example:
LoginViewModel
private MutableLiveData<String> snackbarStringSingleLiveEvent= new MutableLiveData<>();
#Inject
public LoginViewModel(#NonNull AppDatabase appDatabase,
#NonNull JobPortalApplication application,
#NonNull MyApiEndpointInterface myApiEndpointInterface) {
super(application);
loginRepository = new LoginRepository(application, appDatabase, myApiEndpointInterface);
snackbarStringSingleLiveEvent = loginRepository.getLogin(username.get(), password.get(), type.get());
}
public MutableLiveData<String> getSnackbarStringSingleLiveEvent() {
return snackbarStringSingleLiveEvent;
}
Repository
public SingleLiveEvent<String> getLogin(String name, String password, String type) {
SingleLiveEvent<String> mutableLiveData = new SingleLiveEvent<>();
apiEndpointInterface.getlogin(name, password, type).enqueue(new Callback<GenericResponse>() {
#Override
public void onResponse(Call<GenericResponse> call, Response<GenericResponse> response) {
mutableLiveData.setValue(response.body().getMessage());
}
#Override
public void onFailure(Call<GenericResponse> responseCall, Throwable t) {
mutableLiveData.setValue(Constant.FAILED);
}
});
return mutableLiveData;
}
Login Fragment
private void observeViewModel(final LoginViewModel viewModel) {
// Observe project data
viewModel.getSnackbarStringSingleLiveEvent().observe(this, new Observer<String>() {
#Override
public void onChanged(String s) {
}
});
}
How can I use EventObserver in above case? Any practical example?
Check out below example about how you can create single LiveEvent to observe only one time as LiveData :
Create a class called Event as below that will provide our data once and acts as child of LiveData wrapper :
public class Event<T> {
private boolean hasBeenHandled = false;
private T content;
public Event(T content) {
this.content = content;
}
public T getContentIfNotHandled() {
if (hasBeenHandled) {
return null;
} else {
hasBeenHandled = true;
return content;
}
}
public boolean isHandled() {
return hasBeenHandled;
}
}
Then declare this EventObserver class like below so that we don't end up placing condition for checking about Event handled every time, everywhere :
public class EventObserver<T> implements Observer<Event<T>> {
private OnEventChanged onEventChanged;
public EventObserver(OnEventChanged onEventChanged) {
this.onEventChanged = onEventChanged;
}
#Override
public void onChanged(#Nullable Event<T> tEvent) {
if (tEvent != null && tEvent.getContentIfNotHandled() != null && onEventChanged != null)
onEventChanged.onUnhandledContent(tEvent.getContentIfNotHandled());
}
interface OnEventChanged<T> {
void onUnhandledContent(T data);
}
}
And How you can implement it :
MutableLiveData<Event<String>> data = new MutableLiveData<>();
// And observe like below
data.observe(lifecycleOwner, new EventObserver<String>(data -> {
// your unhandled data would be here for one time.
}));
// And this is how you add data as event to LiveData
data.setValue(new Event(""));
Refer here for details.
Edit for O.P.:
Yes, data.setValue(new Event("")); is meant for repository when you've got response from API (Remember to return same LiveData type you've taken in VM instead of SingleLiveEvent class though).
So, let's say you've created LiveData in ViewModel like below :
private MutableLiveData<Event<String>> snackbarStringSingleLiveEvent= new MutableLiveData<>();
You provide value to this livedata as Single Event from repository like below :
#Override
public void onResponse(Call<GenericResponse> call, Response<GenericResponse> response) {
mutableLiveData.setValue(new Event(response.body().getMessage())); // we set it as Event wrapper class.
}
And observe it on UI (Fragment) like below :
viewModel.getSnackbarStringSingleLiveEvent().observe(this, new EventObserver<String>(data -> {
// your unhandled data would be here for one time.
}));
Event.java
public class Event<T> {
private T content;
private boolean hasBeenHandled = false;
public Event(T content) {
this.content = content;
}
/**
* Returns the content and prevents its use again.
*/
public T getContentIfNotHandled() {
if (hasBeenHandled) {
return null;
} else {
hasBeenHandled = true;
return content;
}
}
/**
* Returns the content, even if it's already been handled.
*/
public T peekContent() {
return content;
}
}
EventObserver.java
public class EventObserver<T> implements Observer<Event<? extends T>> {
public interface EventUnhandledContent<T> {
void onEventUnhandledContent(T t);
}
private EventUnhandledContent<T> content;
public EventObserver(EventUnhandledContent<T> content) {
this.content = content;
}
#Override
public void onChanged(Event<? extends T> event) {
if (event != null) {
T result = event.getContentIfNotHandled();
if (result != null && content != null) {
content.onEventUnhandledContent(result);
}
}
}
}
Example, In ViewModel Class
public class LoginViewModel extends BaseViewModel {
private MutableLiveData<Event<Boolean>> _isProgressEnabled = new MutableLiveData<>();
LiveData<Event<Boolean>> isProgressEnabled = _isProgressEnabled;
private AppService appService;
private SchedulerProvider schedulerProvider;
private SharedPreferences preferences;
#Inject
LoginViewModel(
AppService appService,
SchedulerProvider schedulerProvider,
SharedPreferences preferences
) {
this.appService = appService;
this.schedulerProvider = schedulerProvider;
this.preferences = preferences;
}
public void login(){
appService.login("username", "password")
.subscribeOn(schedulerProvider.executorIo())
.observeOn(schedulerProvider.ui())
.subscribe(_userLoginDetails::setValue,
_userLoginDetailsError::setValue,
() -> _isProgressEnabled.setValue(new Event<>(false)),
d -> _isProgressEnabled.setValue(new Event<>(true))
)
}
}
In Login Fragment,
viewModel.isProgressEnabled.observe(this, new EventObserver<>(hasEnabled -> {
if (hasEnabled) {
// showProgress
} else {
// hideProgress
}
}));
Using Event and EventObserver class we can achieve the same like SingleLiveEvent class but if you are thinking a lot of boilerplate code just avoid this method. I hope it would help you and give some idea about why we are using SingleEvent in LiveData.
I understand that Google gives the guidelines to use LiveData between the ViewModel and UI but there are edge cases where using LiveData as a SingleLiveEvent is like reinventing the wheel. For single time messaging between the view model and user interface we can use the delegate design pattern. When initializing the view model in the activity we just have to set the activity as the implementer of the interface. Then throughout our view model we can call the delegate method.
Interface
public interface Snackable:
void showSnackbarMessage(String message);
UI
public class MyActivity extends AppCompatActivity implements Snackable {
private MyViewModel myViewModel;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.my_layout);
this.myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
this.myViewModel.setListener(this);
}
#Override
public void showSnackbarMessage(String message) {
Toast.makeText(this, "message", Toast.LENGTH_LONG).show();
}
}
View Model
public class MyViewModel extends AndroidViewModel {
private Snackable listener;
public MyViewModel(#NonNull Application application) {
super(application);
}
public void setListener(MyActivity activity){
this.listener = activity;
}
private void sendSnackbarMessage(String message){
if(listener != null){
listener.showSnackbarMessage(message);
}
}
private void anyFunctionInTheViewModel(){
sendSnackbarMessage("Hey I've got a message for the UI!");
}
}
Related
Why i should pass the LiveData to the activity to get the value of allQuestions. I am a little bit confused.
I tried to get the List of allQuestions in QuestionViewModel from the repo directly but it didn't work.
This is what's in my brain.
public class QuestionViewModel extends AndroidViewModel {
private Repository repo;
private List<QuestionModel> allQuestions;
public QuestionViewModel(#NonNull Application application) {
super(application);
repo = new Repository(application);
allQuestions = repo.getAllQuestions().getValue(); //This line did not work for me (︶︹︺)
}}
What i did do to get it work.
ViewModel
public class QuestionViewModel extends AndroidViewModel {
private Repository repo;
private LiveData<List<QuestionModel>> allQuestions;
public List<QuestionModel> CurrentListOfQues;
public QuestionViewModel(#NonNull Application application) {
super(application);
repo = new Repository(application);
allQuestions = repo.getAllQuestions();
}
public LiveData<List<QuestionModel>> GetAllQues() {
if (allQuestions == null) {
allQuestions = new LiveData<List<QuestionModel>>() {
#Override
public void observe(#NonNull LifecycleOwner owner, #NonNull Observer<? super List<QuestionModel>> observer) {
super.observe(owner, observer);
}
};
}
return allQuestions;
}
public void setCurrent(List<QuestionModel> CurrentListOfQues) {
if (this.CurrentListOfQues == null) {
this.CurrentListOfQues = CurrentListOfQues;
}
}
Question Activity
QVM = new ViewModelProvider(this).get(QuestionViewModel.class);
QVM.GetAllQues().observe(this, new Observer<List<QuestionModel>>() {
#Override
public void onChanged(List<QuestionModel> allQuestions) {
QVM.setCurrent(allQuestions);
}
});
Is this the right way to get the list in my viewmodel? Why allQuestions = repo.getAllQuestions().getValue(); is always null
I'm trying to insert a object (repo) into the database and pass it to the next fragment.
I get the id of the inserted object and then I try to get the entity by the id, so the object includes the id.
onLoaded() should set the object in the ViewModel so I can use it in the next fragment.
The problem: Sometimes onLoaded seems to be called late or not at all. And then I can't add more information in the second fragment.
How can I make sure 'onLoaded' is called before I use the second fragment?
Or is there a better approach I should use? Maybe only passing the id in the ViewModel?
Fragment 1:
private void actionCloneRepo(#NonNull View view) {
TextInputEditText edittext_username = view.findViewById(R.id.edittext_username);
TextInputEditText edittext_token = view.findViewById(R.id.edittext_token);
TextInputEditText edittext_git_name = view.findViewById(R.id.edittext_git_name);
TextInputEditText edittext_git_email = view.findViewById(R.id.edittext_git_email);
Repo repo = new Repo(
Objects.requireNonNull(edittext_url.getText()).toString(),
Objects.requireNonNull(edittext_username.getText()).toString(),
Objects.requireNonNull(edittext_token.getText()).toString(),
Objects.requireNonNull(edittext_git_name.getText()).toString(),
Objects.requireNonNull(edittext_git_email.getText()).toString()
);
repoViewModel = new ViewModelProvider(requireActivity()).get(RepoViewModel.class);
int id = (int) repoViewModel.insert(repo);
repoViewModel.getRepoById(id).observe(getViewLifecycleOwner(), this::onLoaded);
NavHostFragment.findNavController(AddProject1Fragment.this)
.navigate(R.id.action_AddProject1Fragment_to_AddProject2Fragment);
}
private void onLoaded(Repo repo){
repoViewModel.setCurrentRepo(repo);
}
ViewModel:
public class RepoViewModel extends AndroidViewModel {
private final RepoRepository repoRepository;
private final LiveData<List<Repo>> allRepos;
private Repo currentRepo;
public RepoViewModel(Application application) {
super(application);
repoRepository = new RepoRepository(application);
allRepos = repoRepository.getAllRepos();
}
public Repo getCurrentRepo() {
return currentRepo;
}
public void setCurrentRepo(Repo currentRepo) {
this.currentRepo = currentRepo;
}
public LiveData<Repo> getRepoById(int id) {
return repoRepository.getRepoById(id);
}
public LiveData<List<Repo>> getAllRepos() {
return allRepos;
}
public long insert(Repo repo) {
return repoRepository.insert(repo);
}
public void update(Repo repo) {
repoRepository.update(repo);
}
public void delete(Repo repo) {
repoRepository.delete(repo);
}
}
I am developing an application with the help of Model View Presenter pattern.
I make use of Retrofit and so I have a ApiClient and ApiInterface with endpoints. I implement the interface in a RemoteDataSource class which I call in the Repository class.
My questions is - how do I make use of an Interactor class to make the repository communicate with the Presenter?
Here is my code until now:
ApiInterface
public interface ApiInterface {
#GET("?")
Call<ArrayList<Movie>> getMoviesByTitle(#Query("t") String title,#Query("apiKey") String apiKey);
}
RemoteDataSource class
private static MovieRemoteDataSource instance;
private final ApiInterface service;
public MovieRemoteDataSource(ApiInterface movieApi) {
service = ApiClient.createService(ApiInterface.class);
}
public static MovieRemoteDataSource getInstance(ApiInterface movieApi) {
if (instance == null) {
instance = new MovieRemoteDataSource(movieApi);
}
return instance;
}
#Override
public void getMovies(String title, String apiKey, final LoadMovieCallBack callback) {
service.getMoviesByTitle(title,apiKey).enqueue(new Callback<ArrayList<Movie>>() {
#Override
public void onResponse(Call<ArrayList<Movie>> call, Response<ArrayList<Movie>> response) {
ArrayList<Movie> movies = response.body();// != null ? //response.body().getTitle() : null;
if (movies != null && !movies.isEmpty()) {
callback.onMoviesLoaded(movies);
} else {
callback.onDataNotAvailable();
}
}
#Override
public void onFailure(Call<ArrayList<Movie>> call, Throwable t) {
callback.onError();
}
});
}
DataSource interface with a callback
public interface MovieDataSource {
interface LoadMovieCallBack{
void onMoviesLoaded(ArrayList<Movie> movies);
void onDataNotAvailable();
void onError();
}
void getMovies(String title, String apiKey,LoadMovieCallBack callback);
}
Repository
private MovieRemoteDataSource movieRemoteDataSource;
public MoviesRepository() {//ApiInterface movieApi) {
//this.service = ApiClient.createService(ApiInterface.class);
}
public static MoviesRepository getInstance(ApiInterface service) {
if (instance == null) {
instance = new MoviesRepository();
}
return instance;
}
public void getMovies(String title, String apiKey ) {
movieRemoteDataSource.getMovies(title,apiKey,this);
}
In MoviesRepository you should declare a function with Callback. Your Presenter
should implement MovieDataSource.LoadMovieCallBack and pass it when you call MoviesRepository
public void getMovies(String title, String apiKey,MovieDataSource.LoadMovieCallBack callback) {
movieRemoteDataSource.getMovies(title,apiKey,callback);
}
Here is Google MVP already done for todo app sample, you can refer it. But now it deprecated because Google recommends MVVM
here is my problem:
i have used MVVM/Repository design pattern like this:
Activity -(Observes)-> ViewModel's LiveData -> Repository -> WebService API (GET Resource)
i have another calls for UPDATING Resource to WebService.
Problem:
after changing resource on the server. how i can make the Resource livedata to update itself with new servers data
i want to force it fetch data from server again because some other data may have been changed.
and i dont want to use local database (Room) and change it because my server data might be changed. and they need to fetch each time.
The Only solution passed my Mind was to create a Livedata Source (as dataVersion) to it.
and increment it after every update like this (pseudo code):
dataVersion = new MutableLiveData();
dataVersion.setValue(0);
// my repository get method hasnt anything to do with the dataVersion.
myData = Transformation.switchmap(dataVersion, versionNum -> { WebServiceRepo.getList() });
and how dataVersion should get updated in ViewModel.
You could extend MutableLiveData to give it manual fetch functionality.
public class RefreshLiveData<T> extends MutableLiveData<T> {
public interface RefreshAction<T> {
private interface Callback<T> {
void onDataLoaded(T t);
}
void loadData(Callback<T> callback);
}
private final RefreshAction<T> refreshAction;
private final Callback<T> callback = new RefreshAction.Callback<T>() {
#Override
public void onDataLoaded(T t) {
postValue(t);
}
};
public RefreshLiveData(RefreshAction<T> refreshAction) {
this.refreshAction = refreshAction;
}
public final void refresh() {
refreshAction.loadData(callback);
}
}
Then you can do
public class YourViewModel extends ViewModel {
private RefreshLiveData<List<Project>> refreshLiveData;
private final GithubRepository githubRepository;
private final SavedStateHandle savedStateHandle;
public YourViewModel(GithubRepository githubRepository, SavedStateHandle savedStateHandle) {
this.githubRepository = githubRepository;
this.savedStateHandle = savedStateHandle;
refreshLiveData = Transformations.switchMap(savedStateHandle.getLiveData("userId", ""), (userId) -> {
githubRepository.getProjectList(userId);
});
}
public void refreshData() {
refreshLiveData.refresh();
}
public LiveData<List<Project>> getProjects() {
return refreshLiveData;
}
}
And then repository can do:
public RefreshLiveData<List<Project>> getProjectList(String userId) {
final RefreshLiveData<List<Project>> liveData = new RefreshLiveData<>((callback) -> {
githubService.getProjectList(userId).enqueue(new Callback<List<Project>>() {
#Override
public void onResponse(Call<List<Project>> call, Response<List<Project>> response) {
callback.onDataLoaded(response.body());
}
#Override
public void onFailure(Call<List<Project>> call, Throwable t) {
}
});
});
return liveData;
}
This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 3 years ago.
Unable to get particular record from database getting this output.
My output is :
java.lang.RuntimeException: Unable to start activity ComponentInfo{appwork.com.example/appwork.com.example.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe(android.arch.lifecycle.LifecycleOwner, android.arch.lifecycle.Observer)' on a null object reference
I am using room database, Modelview, Repository and Dao file to get Live data but unable to get particular record from database via following things
<pre><code>
public class MainActivity extends AppCompatActivity {
private NoteViewModel noteViewModel;
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
noteViewModel =
ViewModelProviders.of(this).get(NoteViewModel.class);
noteViewModel.getNote("test7").observe(this, new Observer<Note>()
{
#Override
public void onChanged(#Nullable Note note)
{
Toast.makeText(MainActivity.this,
"found title is : " +note.getTitle(),
Toast.LENGTH_LONG).show();
}
});
}
}
public class NoteViewModel extends AndroidViewModel
{
private LiveData<Note> note;
public NoteViewModel(#NonNull Application application)
{
super(application);
// note = repository.getNote("");
}
public LiveData<Note> getNote(String search)
{
repository.getNote(search);
if(note != null)
{
return note;
}
return null;
}
}
public class NoteRepository {
private NoteDao noteDao;
private LiveData<Note> note;
public NoteRepository(Application application)
{
NoteDatabase noteDatabase =
NoteDatabase.getInstance(application);
noteDao = noteDatabase.noteDao();
}
public LiveData<Note> getNote(String search)
{
new SearchNoteAsyncTask(noteDao).execute(search);
if(note != null)
{
return note;
}
return null;
}
public static void asyncFinished(LiveData<Note> results)
{
note = results;
}
public static class SearchNoteAsyncTask extends AsyncTask<String, Void, LiveData<Note>>
{
private NoteDao noteDao;
private LiveData<Note> note;
private SearchNoteAsyncTask(NoteDao noteDao)
{
this.noteDao = noteDao;
}
public LiveData<Note> doInBackground(String... search)
{
return noteDao.getNote(search[0]);
}
public void onPostExecute(LiveData<Note> result)
{
asyncFinished(result);
}
}
}
#Dao
public interface NoteDao{
#Query("Select * from note_table where title =:search Order By priority DESC")
LiveData<Note> getNote(String search);
}
I am getting response in Repository call but unable to get value from public
static class SearchNoteAsyncTask extends AsyncTask<String, Void, LiveData<Note>>
Any working example will be great!
thanks
Problem is that you are returning a different reference of Livedata to your View in -
public LiveData<Note> getNote(String search){
repository.getNote(search);
if(note != null)
{
return note; // this is the livedata reference that UI is observing
}
return null;
}
And when you get the response from repo you change the reference of livedata -
public static void asyncFinished(LiveData<Note> results) {
note = results;
}
But your UI is listening to previous LiveData.
Instead of changing the reference, you should change the value in original note livedata.
That would look something like -
public static void asyncFinished(LiveData<Note> results) {
note.setValue(results.getValue); // not the preferred way though, but it will help you understand why your current implementation doesn't work.
}
And best would be to use Transformation on the original livedata -
public LiveData<Note> getNote(String search){
return Transformations.map(repository.getNote(search),
value -> note.setValue(value));
}