Android 中的嵌套 Firestore 异步侦听器

Question

所以我在几天集合中有活动文档，我需要将所有活动合并到一个列表中。我想我应该循环集合，然后循环活动，最后得到下面的代码，我不知道这是否是组合多个集合的最佳方式。更糟糕的是，我不知道我的列表何时准备好用于所有异步调用。有什么建议吗？谢谢！

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

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() 整组完成后响应。然后，您可以根据需要检查那里所有任务的结果。

Answer 2

设法按照@Doug 的建议编写代码。感谢题主！这好多了：

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!
        }
    });

Android 中的嵌套 Firestore 异步侦听器

Nested Firestore asynchronous listeners in Android

android

asynchronous

listeners

firebase

google-cloud-firestore