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);
                                        }

                                    }
                                });

                    }
            }
        });

HondaGuy · Accepted Answer · 2018-03-20 05:33:52Z

3

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.

edited Mar 20, 2018 at 5:33

HondaGuy

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answered Dec 25, 2017 at 23:56

Doug Stevenson

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2 Comments

Marcos Over a year ago

Thanks for that, Taskmaster! :) but this is too advanced for me. Would you mind pointing out to some example? Thanks

Marcos Over a year ago

do you mean by using .get().getResult()? Can I build a task with this QuerySnapshot? What about the nested CollectionReference?

Marcos · Accepted Answer · 2017-12-27 18:21:06Z

Managed to code what @Doug had suggested. Thanks taskmaster! This is so much better:

db.collection("calendar").get()
    .continueWith(new Continuation<Task<QuerySnapshot>, Task<?>>() {
        @Override
        public Task<?> then(@NonNull Task<Task<QuerySnapshot>> task) throws Exception {
            List<Task<QuerySnapshot>> tasks = new ArrayList<Task<QuerySnapshot>>();
            for (DocumentSnapshot ds : task.getResult().getResult())
                tasks.add(ds.getReference().collection("thingstodo").get());

            return Tasks.whenAllSuccess(tasks);
        }
    })
    .addOnCompleteListener(new OnCompleteListener<Task<?>>() {
        @Override
        public void onComplete(@NonNull Task<Task<?>> task) {
            List<QuerySnapshot> lists = (ArrayList<QuerySnapshot>)task.getResult().getResult();
            for (QuerySnapshot qs : lists)
                for (DocumentSnapshot ds: qs) {
                    ScheduledItem item = ds.toObject(ScheduledItem.class);
                    //add to list including day
                    itemsList.add(item);

                }

            //list ready to be used!
        }
    });

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