Room Database: Getting SELECT MAX() Twice After Creating New RecyclerView Item

I have a RecyclerView list of CardView items that is working properly. Upon creation of a new CardView that is inserted into the database, I would like to fire a Toast that inform

Solution 1:

The view model operations you call within onClickSave are asynchronous:

public void onClickSave(View v) {
    mViewModel.insertCard(card1);
    mViewModel.getMax().observe(this, value -> { newMax = value; makeText(AddorUpdateCardActivity.this, "TEXT", .LENGTH_LONG).show();});
}

The implementation of LiveData records the data version as well as the last version seen by the observer.

Therefore insertCard starts to operate on a worker thread while you start observing getMax from the main thread with a newly created observer. Thus you'll receive the current value as well as the new value after the database was updated.

Instead you could observe it only once in onCreate() and wait for the updates triggered by the database:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mViewModel = new ViewModelProvider(this).get(cardViewModel.class);
    mViewModel.getMax().observe(this, value -> { newMax = value; makeText(AddorUpdateCardActivity.this, "TEXT", .LENGTH_LONG).show();});
}

public void onClickSave(View v) {
    mViewModel.insertCard(card1);
}

Solution 2:

The Room Insert operation inside AsyncTask takes a while before the maxCount variable is updated. Since you are showing the Toast inside a button click, the message is displayed right away without receiving the updated value from LiveData.

Move the Toast message inside the obverve() method so that it gets triggered only after a LiveData change.

mViewModel.getMax().observe(this, value -> {
        newMax = value;
        Toast.makeText(AddorUpdateCardActivity.this, "card #" + newMax + " was saved to the list", Toast.LENGTH_LONG).show();
    });

At this point, the code should be working but you'll get multiple LiveData events for a single Insert. This is happening because you have used 2 separate instances of Dao for Insert and Query operation.

public cardRepository(Application application) {
    RoomDatabase db = RoomDatabase.getDatabase(application);
    cardDao = db.cardDao(); // <---------- Instance #1
    getMax = cardDao.getMax();
}

public LiveData<Integer> getMax() {
    return getMax;  
}

 public void insertCard(Card newcard) {
    new InsertAsyncTask(quickcardDao).execute(newcard);
}

private static class InsertAsyncTask extends AsyncTask<Card, Void, Integer> {

    private CardDao asyncTaskDao;

    InsertAsyncTask(CardDao dao) {
        asyncTaskDao = dao; // <---------- Instance #2
    }

    @Override
    protected Integer doInBackground(final Card... params) {

        asyncTaskDao.insertCard(params[0]);
        return null;
    }
}

To resolve it use the same Dao instance everywhere:

public cardRepository(Application application) {
        RoomDatabase db = RoomDatabase.getDatabase(application);
        cardDao = db.cardDao();
    }

    public LiveData<Integer> getMax() {
        return cardDao.getMax();  
    }

     public void insertCard(Card newcard) {
        AsyncTask.execute(() -> cardDao.insertCard(newcard));
    }

Solution 3:

Because of using AsyncTask to insert card to database, that function take some time to complete and you show your toast, instantly! Change your activity to this:

AddorUpdateCardActivity
...
private int newMax = -1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mViewModel = new ViewModelProvider(this).get(cardViewModel.class);

    mViewModel.getMax().observe(this, integer2 -> {
        newMax = integer2;
        Toast.makeText(AddorUpdateCardActivity.this, "card #" + newMax + " was saved to the list", Toast.LENGTH_LONG).show();
        hideProgressBar();
    });
}

public void onClickSave(View v) {

    //set card data
    // then insert data in database
    mViewModel.insertCard(card1);

    showProgressBar();
}