How to listen for changes in android studio using firebase

Question

I am having a hard time reading changes on my firebase. What I am trying to do is for every change when the start button is triggered a dialog will show saying, "This has already been executed."

I tried using addListenerForSingleValueEvent but it seems that the dialog only showed when all of the children have changed.

 startBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            currentWeek = programTrackers.get(0).getWeek();
            DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("ProgramTracker")
                    .child(GlobalUser.getmUser().getiD())
                    .child(programTrackers.get(0).getProgramId());
            databaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    final int childCount = (int) dataSnapshot.getChildrenCount();
                    int count = 0;
                    for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                        ProgramTracker programTracker = snapshot.getValue(ProgramTracker.class);
                        if (programTracker.getProgramExerciseWeek() == currentWeek) {
                            programTrackChecker.add(programTracker);


                        }
                        count++;
                    }
                    if (count == childCount) {
                        boolean isFinished = false;
                        //last iteration
                        for (int i = 0; i < programTrackChecker.size(); i++) {
                            if (programTrackChecker.get(i).isProgramExerciseFinished()) {
                                isFinished = true;
                            } else
                                isFinished = false;
                        }
                        if (isFinished) {
                            AlertDialog.Builder builder = new AlertDialog.Builder(DayExerciseActivity.this);
                            builder.setMessage("You have already completed all exercises. Are you sure you want to do it again?");
                            builder.setPositiveButton("Yes", (dialogInterface, i) -> {
                                startActivity(new Intent(DayExerciseActivity.this, DoProgramActivity.class)
                                        .putExtra("programTrackers", programTrackers)
                                        .putExtra("exerciseNum", String.valueOf(0)));

                            });
                            builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialogInterface, int i) {
                                    dialogInterface.dismiss();
                                }
                            });
                            builder.show();


                        } else
                            startActivity(new Intent(DayExerciseActivity.this, DoProgramActivity.class)
                                    .putExtra("programTrackers", programTrackers)
                                    .putExtra("exerciseNum", String.valueOf(0)));

                    }

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });

Answer 1

You need to change this:

databaseReference.addListenerForSingleValueEvent(new ValueEventListener

Into this:

databaseReference.addValueEventListener(new ValueEventListener

From the docs:

public abstract void onDataChange (DataSnapshot snapshot)

This method will be called with a snapshot of the data at this location. It will also be called each time that data changes