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Android: Infinite loop with LiveData<Boolean>

This gets called when a button is clicked
#Override
public void onFavoriteIconClicked() {
viewModel.isFavoriteExist(test.getId()).observe(getViewLifecycleOwner(), new Observer<Boolean>() {
#Override
public void onChanged(Boolean aBoolean) {
viewModel.isFavoriteExist(test.getId()).removeObserver(this);
if (aBoolean) {
binding.addToFavorite.setImageResource(R.drawable.non_fav);
viewModel.delete(test);
} else if (getActivity() != null) {
Test test2 = new Test(test.getId(), test.getName());
viewModel.insert(test2);
binding.addToFavorite.setImageResource(R.drawable.fav);
}
}
});
}
If the test object exists in the Favorite database, I have to delete it. After deleting, it calls this again (since it observed a chane) and inserts it again.
It keeps looping infinitely. Is there a better way to implement this or stop this?

It seems like some business logic has entered your view (Activity) class.
Since LiveData & Room are meant to be used when receiving updates about Database changes is needed, and your use of the DB is not requiring constant updates, I would suggest going with a more direct approach.
First, Remove the use of LiveData from your Database. Use simple return values.
Your view (Activity/Fragment) can then tell the view model that a button was clicked.
#Override
public void onFavoriteIconClicked() {
viewModel.onFavoriteClicked()
}
The view will observe the view model in order to receive the correct icon to show.
Something like:
viewModel.favoriteIcon.observe(getViewLifecycleOwner(), new Observer<Integer>() {
#Override
public void onChanged(Integer iconResId) {
binding.addToFavorite.setImageResource(iconResId)
}
}
Now the viewModel can handle the logic (or better add a Repository layer - See Here)
Upon click, Check if entry exist in DB.
If exists: remove it from DB and set favoriteIcon value:
favoriteIcon.setValue(R.drawable.non_fav)
If doesn't exist: Add it to DB and set favoriteIcon value.
favoriteIcon.setValue(R.drawable.fav)
For a good tutorial about using Room & LiveData - as well as doing so using the View/ViewModel/Repository pattern, check this link

Android Java - Race Condition in OnCreate with two Observers and making lists

sorry if this is a convoluted question. Working on creating an app for a college course and I'm running into (what appears to be) a race condition in my OnCreate method.
TL;DR - sometimes my spinner populates and I can get an index from it. Sometimes it's not populated yet when trying to get a specific index. Details and code below.
The app is a "course scheduler" for a college student.
I'm creating an Activity that displays existing course information and allows you to edit it. In the OnCreate method for this Activity, I am filling a spinner for "Mentors" for the course and a spinner for which "Term" the course belongs in. This information is being pulled from a Room DB.
I have a seperate activity for a new course and for editing a course. For the "new course" activity, everything works fine. I getAllMentors() or getAllTerms() successfully and fill the spinner list.
For the "Edit Course" Activity, there's an extra step involved and it seems to be causing me some issues.
When editing a course, I pass the intent from the originating Activity with all the necessary EXTRAS. This is successful.
In OnCreate for EditCourseActivity, I do the following:
I get the mentorID from the EXTRA that's passed in from the originating Activity.
I access my MentorViewModel and call my getAllMentors() method which returns LiveData> of all mentors in the db.
because it returns LiveData, I use an observer and loop through the LiveData adding the Name of each mentor to a List and the
entire mentor to a List.
I populate my spinner with the information in List full of mentor names.
then I do a for loop, looping through List looking for one that has the same id as what I grabbed form the EXTRA in step 1.
If I find a match in that list, I call a getMentorName() method to snag their name as a string.
I have a methond getIndex(spinner, string) that will loop through the provided spinner, trying to find a match for the string that's
passed in (mentors name) that I grabbed that should match the ID of
the mentor assigned to the course. This method returns index location
of the matched string in the spinner.
I set the spinner selection to the index found.
I do basically the same process for term.
Me being a new developer, I'm not used to OnCreate running the code synchronously.
Because of this, it appears that I have a race condition somewhere between populating the List of mentor names that populates the spinner, and calling my getIndex() method.
Sometimes the spinner is populated and getIndex works properly and sets the correct mentor. Sometimes the spinner is empty and my getIndex() returns -1 (which it should do in a no-find situation) that populates the spinner with the first item in the list (once it's populated).
protected void onCreate(Bundle savedInstanceState) {
//////////////////////////Handling Mentor spinner menu/////////////////////////////////////////////////
int mentorId = courseData.getIntExtra(EXTRA_COURSE_MENTOR_ID, -1);
final ArrayAdapter<String> sp_CourseMentorAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, mentorNameList);
sp_CourseMentorAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
sp_CourseMentor.setAdapter(sp_CourseMentorAdapter);
final MentorViewModel mentorViewModel = ViewModelProviders.of(this).get(MentorViewModel.class);
//Mentor test = mentorViewModel.getMentorById(mentorId);
mentorViewModel.getAllMentors().observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
if (mentorList != null) {
for (Mentor m : mentorList) {
mentorNameList.add(m.getMentor_name());
mentorListMentor.add(m);
}
}
sp_CourseMentorAdapter.notifyDataSetChanged();
}
});
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
Is there a way to get them to run asynchronously? Somehow to get the observer doing my getAllMentors() to complete first and populate the spinner, THEN have the for loop run?
Or a better way to handle this?
Thanks in advance.

Room always runs the code on a separated thread, not the Main/UI thread. You can change that behavior with
allowMainThreadQueries()
after initializating your database. This will make the query run first, populate your list and then run your for-loop code. I do not recommend this approach, since it is a bad practice to make queries on the UI thread.
You have two options:
Change your foor loop to a function and call it after adding the values from the observer:
mentorViewModel.getAllMentors().observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
if (mentorList != null) {
for (Mentor m : mentorList) {
mentorNameList.add(m.getMentor_name());
mentorListMentor.add(m);
}
lookForMentor();
}
}
});
private void lookForMentor() {
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
}
Put the for inside the observer, change the Room DAO to return a List and use LiveData on your own viewmodel:
MentorViewModel.java:
MentorViewModel extends ViewModel {
private MutableLiveData<List<Mentor>> _mentorsLiveData = new MutableLiveData<List<Mentor>>();
public LiveData<List<Mentor>> mentorsLiveData = (LiveData) _mentorsLiveData;
void getAllMentors(){
//room db query
_mentorsLiveData.postValue(mentorsList);
}
}
EditActivity.java:
mentorsViewModel.getAllMentors();
mentorViewModel.mentorsLiveData.observe(this, new Observer<List<Mentor>>() {
#Override
public void onChanged(#Nullable List<Mentor> mentorList) {
mentorsListMentor.addAll(mentorList);
sp_CourseMentorAdapter.notifyDataSetChanged();
for(Mentor m: mentorListMentor){
if (m.getMentor_id()==mentorId){
String test = m.getMentor_name();
int spinnerSelectionM2 = getIndexM(sp_CourseMentor, test);
sp_CourseMentor.setSelection(spinnerSelectionM2);
}
}
}
}
});

Why this method runs before the other method finishes?

I am kinda new to Android programming. I am building a simple application and I have a "follow-unfollow" concept on it. What I simply want is, if the current user follows the user he/she is exploring, I want him/her to see "unfollow" button. If not following, there should be a "follow" button. On my UserProfileActivity class I have a method called onPrepareOptionsMenu() and inside this method I can set the buttons.
#Override
public boolean onPrepareOptionsMenu(Menu menu) {
MenuItem follow = menu.findItem(R.id.action_follow);
MenuItem unfollow = menu.findItem(R.id.action_unfollow);
Bundle bundle = this.getIntent().getExtras();
if(isFollowing(bundle.getString("userid")) == true){
follow.setVisible(false);
unfollow.setVisible(true);
}
else{
follow.setVisible(true);
unfollow.setVisible(false);
}
return super.onPrepareOptionsMenu(menu);
}
Also, I have another method called isFollowing() and it returns a boolean "true" if current user follows the other user, it returns "false" if not. It is the simplest way that I have thought to solve this issue.
public boolean isFollowing(String userID){
isFollowingResult = false;
firebaseDatabase = FirebaseDatabase.getInstance();
databaseReference = firebaseDatabase.getReference();
final DatabaseReference followingData = databaseReference.child("followingData");
followingData.child(currentUser.getUid()).child(userID).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot.exists())
isFollowingResult = true;
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
return isFollowingResult;
}
When I run this, and when I click on a user's profile onPrepareOptionsMenu() method is called and inside it isFollowing() method is called. The problem is, it does not wait for the isFollowing() method to run and finish running and it immediately sees it as "false" and always shows "follow" button. Any suggestion would be appreciated.
Thanks.

Make follow and unfollow the fields of the class. Then change their visibility in onDataChange(DataSnapshot) method.

There are multiple issues with the code.
Firstly, your isFollowing() function is setting up the listener, on the data field, but it'll only get called when the data changes. In this case, you may only want to read the data once:
https://firebase.google.com/docs/database/android/read-and-write#read_data_once
Secondly, the use of a listener implies asynchronicity. Meaning, you'll need to wait until you get the callback later in order to get the value you want.
The ideal solution in order to maintain responsiveness of your app is to maintain a local "copy" of the value in your database with a listener that constantly updates that value. That way, you can query the state of your variable quickly (since it's stored/replicated locally) and still be up to date with your database (with the listener).
This will also prevent each "read" from going all the way to the service and back and also remove the need for your UI to wait to render correctly (accurately).

After clearing the Recyclerview's Data(ArrayList), onLoadMore event of EndlessRecyclerOnScrollListener is not firing

I need to clear the data of recyclerview because I want to show the fresh data from the server, I dont know how to clear the recyclerview without affecting the infinite scrolling

Just clear your list your list in Adapter and call notifyDataSetChanged method.
public class RecyclerViewAdapter extends RecyclerView.Adapter<RelativeAdapter.ViewHolder> {
private List<model> mArrayList = new ArrayList<>();
public void clearList(){
mArrayList.clear();
notifyDataSetChanged();
}
}
just call the clearList() method when you want to clear list.

I got a solution, the issue was I did not reset EndlessRecyclerOnScrollListener counter variable which was basically calculating the logic for reaching last cardview of recyclerview.
There is another plugin which basically gives you method to reset counter variable
https://github.com/codepath/android_guides/wiki/Endless-Scrolling-with-AdapterViews-and-RecyclerView

For loop being missed out in Android

I have an activity in Android that takes the result of a web service and puts it into an adapter to display a list of items. I've created a listener interface for this called OnLoadCompleteListener and my listener for this activity looks like
mListener = new OnLoadCompleteListener() {
#Override
public void onPostExecute(RootWebService service) {
if(service instanceof WebService) {
//gets the number of items
int total = ((WebService) service).getCount();
for(int i=0; i<total; ++i) {
//Log.d("ITEM", "inserting " + i);
Item item = ((WebService) service).get(i);
//our adapter class automatically receives items
mAdapter.addItem(item);
}
}
}
};
Now this is the bizarre bit: the listener is getting hit, total is being set to 12 (the number of items I asked for) and then the for loop is being bypassed altogether. The adapter remains empty and the screen displays pretty much nothing.
However, if the commented out Log.d(...) call is made active, the loop works.
Why? The callback is being run from the UI thread, the getCount() method is returning the correct value, and this adapter has worked before. What on earth is happening?
By the way, mAdapter is an instance of a subclass of Adapter I wrote for the purpose, and the addItem(Item) method looks like this:
public synchronized void addItem(Item item) {
mItems.add(item);
notifyDataSetChanged();
}
mItems is exactly what you would expect.

I believe you need to do something like invalidate or something along those lines which pretty much just refreshes the data in the adapter.
I think its this.
myListView.getAdapter().notifyDataSetChanged();
If that doesn't work it could be...
myListView.invalidate();