Return an autogenerated id from Room using AsyncTask and onPostExecute using MVVM

Question

I am trying to get the autogenerated id from a Room database, but am struggling with how to implement using MVVM with all the layers. I am new to Android programming. I believe onPostExecute is the key, but do not know how I get the id back to the UI.

I have this class:

@Entity
public class Scenario {

    @PrimaryKey (autoGenerate = true)
    private int id;
    private String name;

    public Scenario(String name) {this.name = name;}

    public void setId(@NonNull int id) {this.id = id;}
    @NonNull
    public int getId() {return id;}
    public String getName() {return name;}
}

And this DAO (only showing insert related code):

@Dao
public interface ScenarioDao {

    @Insert
    Long insert(Scenario scenario);

}

With this Repository (only showing insert related code):

public class ScenarioRepository {

    private ScenarioDao scenarioDao;
    private LiveData<List<Scenario>> allScenarios;


    public ScenarioRepository(Application application) {

        AppDatabase database = AppDatabase.getInstance(application);
        scenarioDao = database.scenarioDao();
        allScenarios = scenarioDao.getAllScenarios();
    }


    public void insert (Scenario scenario) {
        new InsertScenarioAsyncTask(scenarioDao).execute(scenario);
    }

    private static class InsertScenarioAsyncTask extends AsyncTask<Scenario, Void, Long> {
        private ScenarioDao scenarioDao;

        private InsertScenarioAsyncTask (ScenarioDao scenarioDao) {
            this.scenarioDao = scenarioDao;
        }

        //protected Void doInBackground(Scenario... scenarios) {
        @Override
        protected Long doInBackground(Scenario... scenarios) {
            return scenarioDao.insert(scenarios[0]);
        }

       //protected void onPostExecute(Void aVoid) {
        @Override
        protected void onPostExecute(Long insertId) {
            super.onPostExecute(insertId);
            //onScenarioInserted(insertId, scenario);
        }
    }
}

And this ViewModel:

public class ScenarioDetailViewModel extends AndroidViewModel {

    private ScenarioRepository repository;

    public ScenarioDetailViewModel(@NonNull Application application) {
        super(application);
        repository = new ScenarioRepository(application);
    }

    public void insert(Scenario scenario) {
        repository.insert(scenario);
    }
}

Called by this function in a fragment which is called from a button OnClickListener:

private void saveScenario() {

    String name = editTextName.getText().toString();

    if (name.isEmpty()) {
        Toast.makeText(getActivity(), "Please enter scenario name.", Toast.LENGTH_SHORT).show();
        return;
    }

    Scenario scenario = new Scenario(name);
    scenarioDetailViewModel.insert(scenario);

}

UPDATED Thanks to the comments below.

    public class ScenarioRepository {

    private ScenarioDao scenarioDao;
    private LiveData<List<Scenario>> allScenarios;


    public ScenarioRepository(Application application) {

        AppDatabase database = AppDatabase.getInstance(application);
        scenarioDao = database.scenarioDao();
        allScenarios = scenarioDao.getAllScenarios();
    }


    public void insert (MutableLiveData<Long> id, Scenario scenario) {
        new InsertScenarioAsyncTask(scenarioDao).execute(id, scenario);
    }

    private static class InsertScenarioAsyncTask extends AsyncTask<Scenario, Void, Long> {
        private ScenarioDao scenarioDao;
    private MutableLiveData<Long> id;

        private InsertScenarioAsyncTask (MutableLiveData<Long> id, ScenarioDao scenarioDao) {
            this.scenarioDao = scenarioDao;
    this.id=id;
        }

        //protected Void doInBackground(Scenario... scenarios) {
        @Override
        protected Long doInBackground(Scenario... scenarios) {
            return scenarioDao.insert(scenarios[0]);
        }

       //protected void onPostExecute(Void aVoid) {
        @Override
        protected void onPostExecute(Long insertId) {
    id.postValue(insertId);
            super.onPostExecute(insertId);
        }

        @Override
        protected void onCancelled() {
            id.postValue(new Long(0));
            super.onCancelled();
        }
    }
}

and in the fragment:

    scenarioDetailViewModel = ViewModelProviders.of(this).get(ScenarioDetailViewModel.class);

    final Observer<Long> scenarioIdObserver = new Observer<Long>() {
        @Override
        public void onChanged(Long aLong) {
            Log.d(TAG, "onChanged, scenarioId is " + aLong);
        }
    };

    scenarioDetailViewModel.getCurrentScenarioId().observe(getActivity(),scenarioIdObserver);

And this in the ViewModel:

public class ScenarioDetailViewModel extends AndroidViewModel {

    private static final String TAG="ScenarioDetailViewModel";
    private ScenarioRepository repository;
    private MutableLiveData<Long> currentId;

    public ScenarioDetailViewModel(@NonNull Application application) {
        super(application);
        currentId = new MutableLiveData<Long>();
        repository = new ScenarioRepository(application);
    }

    public MutableLiveData<Long> getCurrentId() {
        return currentId;
    }

    public void insert(Scenario scenario) {
        Log.d(TAG,"inserting scenario " + scenario.getName());
        repository.insert(currentId,scenario);
    }

    public void update(Scenario scenario) {
        repository.update(scenario);
    }

    public void delete(Scenario scenario) {
        repository.delete(scenario);
    }
}

Answer 1

You can use a LiveData inside AsyncTask
ScenarioRepository would look like this:

public MutableLiveData<Long> insert (Scenario scenario) {
        MutableLiveData<Long> liveData = new MutableLiveData()
        new InsertScenarioAsyncTask(liveData, scenarioDao).execute(scenario);
        return liveData ;
}

ِYour InsertScenarioAsyncTask:

private static class InsertScenarioAsyncTask extends AsyncTask<Scenario, Void, Long> {
        private ScenarioDao scenarioDao;
        private MutableLiveData<Long> liveData;

        private InsertScenarioAsyncTask (MutableLiveData<Long> liveData, ScenarioDao scenarioDao) {
            this.liveData = liveData;
            this.scenarioDao = scenarioDao;
        }

        //protected Void doInBackground(Scenario... scenarios) {
        @Override
        protected Long doInBackground(Scenario... scenarios) {
            return scenarioDao.insert(scenarios[0]);
        }

       //protected void onPostExecute(Void aVoid) {
        @Override
        protected void onPostExecute(Long insertId) {
            liveData.postValue(insertId);
            super.onPostExecute(insertId);
            //onScenarioInserted(insertId, scenario);
        }

         @Override
         protected void onCancelled() {
            liveData.postValue(0);
            super.onCancelled();
         }
    }

in your fragment:

liveData.observe(this, insertedId -> {
// insertedId is the id of the inserted scenario
// if value is 0 then the task was cancelled
    if(insertedId == 0){

    }else{

    }
});