Nested Firestore asynchronous listeners in Android

Question

So I have Activities documents in several Days collections and I need to combine all of the Activities in a list. I thought I should loop the collections and then loop the Activities and ended up with the code below which I don't know whether it's the best way to combine multiple collections. Even worse I don't know WHEN my list is ready to be used with all the asynchronous calls. Any advice? Thanks!

db.collection("calendar").get()
        .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    for (DocumentSnapshot ds : task.getResult().getDocuments()) {
                        ds.getReference().collection("thingstodo").get()
                                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                                    @Override
                                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                                        for (DocumentSnapshot ds : task.getResult().getDocuments()) {
                                            ScheduledItem item = ds.toObject(ScheduledItem.class);
                                            itemsList.add(item);
                                        }

                                    }
                                });

                    }
            }
        });

Answer 1

The get() method you're using on CollectionReference (which is a subclass of Query) returns a Task that becomes resolved when the document is ready. Instead of adding a listener to that individual Task, collect all the tasks into a List, and pass that to Tasks.whenAllComplete() to respond when the entire set is complete. You can then examine the results of all the tasks there, as needed.