How do I wait for firebase queries to complete?

Question

I have multiple buttons in my android application and when clicked it has to fetch data from Firebase database and the fetched data is added into a list which has to be passed as an argument to another class constructor . But the fetching of data happens in another thread by default since firebase queries are asynchronous . How do I make sure that list has been added with data from other thread before passing it to constructor ?

Answer 1

It's simple use oncompletelistener you can refer this example:

db=FirebaseFirestore.getInstance();
                db.collection("Order").document(TableListFragment.tableno)
                        .update(
                                "Items", FieldValue.arrayUnion("ABCD"),
                                "Quantity", FieldValue.arrayUnion("34")
                        ).addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task) {
                        Toast.makeText(getContext(),"Item Added",Toast.LENGTH_LONG).show();
                    }
                }).addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                   Log.i("onFailure:",e.toString());
                    }
                });