LiveData observe() not called

Question

My observer callback is not being called, can you point out what am I doing wrong ? My application is really simple since I'm just testing this architecture

MainActivity code

viewModel = ViewModelProviders.of(this).get(UpcomingMoviesViewModel.class);
viewModel.getUpcomingMovies().observe(this, upcomingMovies -> {
        // NOT CALLED !!!
        for(UpcomingMovies movie : upcomingMovies) {
            Log.d("Upcoming", "Movie - " + movie.movie.title);
        }
    });

ViewModel code

private LiveData<List<UpcomingMovies>> mUpcomingMovies;
private MovieRepository movieRepository;

public UpcomingMoviesViewModel(@NonNull Application application ) {
    super(application);
    movieRepository = MovieRepository.getsInstance(application); 
}

public LiveData<List<UpcomingMovies>> getUpcomingMovies() {
    mUpcomingMovies = movieRepository.getUpcomingMovies();
    return mUpcomingMovies;
}

And my repository method

public LiveData<List<UpcomingMovies>> getUpcomingMovies(){
    if(movieDao.loadUpcomingMovies().getValue() == null) {
        webService.getUpcomingMovies(api_key).
                enqueue(new Callback<MovieResults>() {

                    @Override
                    public void onResponse(Call<MovieResults> call, Response<MovieResults> response) {
                        MovieResults results = response.body();
                        if(results == null) return;
                        new insertAsyncTask(movieDao).execute(results.movies.toArray(new Movie[results.movies.size()]));
                    }

                    @Override
                    public void onFailure(Call<MovieResults> call, Throwable t) {
                        Log.d("Retrofit error", t.getMessage());
                    }
                });
    }
    return movieDao.loadUpcomingMovies();
}

private static class insertAsyncTask extends AsyncTask<Movie, Void, Void> {

    private final MovieDao mAsyncTask;

    insertAsyncTask(MovieDao dao){
        mAsyncTask = dao;
    }

    @Override
    protected Void doInBackground(Movie... movies) {
        long [] results = mAsyncTask.insertMovies(Arrays.asList(movies));
        long [] results2 = mAsyncTask.insertUpcomingMovies(Arrays.asList(movies));
        return null;
    }
}

Both results and results2 return 20 numbers, which I attribute it to the fact I receive 20 movies. What have I done wrong here ? Thanks for ur time

Prashanth Verma · Accepted Answer · 2018-08-29T15:16:43.027

2

Implement LifeCyclebserver to viewModel and add this in your activity:

getLifecycle().addObserver(viewModel);

edited Aug 29 '18 at 15:16

answered Aug 29 '18 at 14:25

Prashanth Verma

588
3
11

1

`postValue` is a method from `MutableLiveData` not `LiveData` – Greggz Aug 29 '18 at 14:28
Yea, initialize your mUpcomingMovies to MutableLiveData. I forgot to add that code. You are also sure that data is coming till viewModel? – Prashanth Verma Aug 29 '18 at 14:29
Throws exception saying `mUpcomingMovies` is null – Greggz Aug 29 '18 at 14:32
Initialize like this: private MutableLiveData> mUpcomingMovies = new MutableLiveData>() – Prashanth Verma Aug 29 '18 at 14:34
I'm using this line `mUpcomingMovies.postValue(movieRepository.getUpcomingMovies().getValue());` . It's still not getting called – Greggz Aug 29 '18 at 14:37
Did you add this: getLifecycle().addObserver(viewModel) in the activity? – Prashanth Verma Aug 29 '18 at 14:38
No never saw that in the tutorials. Also that doesn't compile – Greggz Aug 29 '18 at 14:46
You have added this in the dependencies right? 'android.arch.lifecycle:extensions:1.1.1' – Prashanth Verma Aug 29 '18 at 14:52
Yes I had the 1.1.0, but I updated it. Still the same error. It expects a `LifecycleObserver` not a `AndroidViewModel` – Greggz Aug 29 '18 at 14:55
Actually my viewModel implements LifecyleObserver, that why i said that. You are also sure that data is coming till viewModel? – Prashanth Verma Aug 29 '18 at 14:56
It worked.. What made it work was implementing LifecycleObserver. It wasnt working with mutableLiveData. Why is the implementation necessary do u know ? Thanks :) – Greggz Aug 29 '18 at 15:03
Because we need to observe the life cycle state changes right (which is one of the best things about these components) – Prashanth Verma Aug 29 '18 at 15:09
Seems to me it should be automatic, I've never seen in any example this obligatory implementation. Anyways, thanks! No need to refer MutableLiveData in your answer. Just update it with the Lifecycle stuff – Greggz Aug 29 '18 at 15:13
I mentioned MutableLiveData because using postValue we can directly put into MainThread instead of us switching threads if at all you are switching threads – Prashanth Verma Aug 29 '18 at 15:18
I'm not, only when doing `Inserts` in my repo. Thanks good to know :) – Greggz Aug 29 '18 at 15:22
Can we use `viewLifecycleOwner` api? – IgorGanapolsky Feb 25 '20 at 16:49

score 0 · Answer 2 · answered Oct 27 '20 at 13:02

0

Remember your viewModel needs to have a constructor that takes in no parameters

answered Oct 27 '20 at 13:02

oyamo

41
1
3

2

Not sure if that answers the question. The OP did not get a NullReferenceException, so it seems their ViewModel instantiated properly. (I'm guessing OP derived their ViewModel from `AndroidViewModel`). – S.L. Barth is on codidact.com Oct 27 '20 at 13:13

score 0 · Answer 3 · answered Jan 18 '22 at 21:49

For Java in the source and destination fragments, call;

viewModel = new ViewModelProvider(getActivity()).get(YourViewModel.class);

Not

viewModel = new ViewModelProvider(this).get(YourViewModel.class);

getActivity instead of this allows you to reference the same activity

LiveData observe() not called

3 Answers3