Detect value change with firebase ValueEventListener

Question

I am trying to detect value change from my Firebase Database. Here is my code for initializing the ValueEventListener:

valueEventListener = new ValueEventListene         


        

Answer 1

There is nothing built to the Firebase Realtime Database to tell you what specific data under the snapshot has changed in the onDataChange method.

If you want to know what specific property has change, you'll need to:

1. Keep the snapshot that you get in onDataChange in a field.
2. When onDataChange gets called, compare the data in the new snapshot with the data in the field.

Say that you have a reference on a node in your JSON, and under that node is a status property, and you want to both listen to the entire node, and detect if the status has changed.

You'd do that with something like:

// Add a field to your class to keep the latest snapshot
DataSnapshot previousSnapshot;

// Then add your listener
databaseReference.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        bool wasActive = false;
        if (previousSnapshot != null && previousSnapshot.child("status").exists()) {
            wasActive = dataSnapshot.child("status").getValue(Boolean.class);
        }
        boolean isActive = dataSnapshot.child("status").getValue(Boolean.class);

        if (isActive <> wasActive) {
            ... the user's status changed
        }

        previousSnapshot = dataSnapshot;
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        throw databaseError.toException(); // never ignore errors
    }
});