如何将永久侦听器附加到Firestore查询? [英] How to attach a permanent listener to a Firestore query?

问题描述

我正在尝试在Firestore中实现分页.我已经创建了一个EventListener对象，如下所示:

I'm trying to implement pagination in Firestore. I have created an EventListener object like so:

EventListener<QuerySnapshot> listener = (querySnapshot, e) -> {
    if (e != null) return;

    for (DocumentChange documentChange : querySnapshot.getDocumentChanges()) {
        //Get data
    }
    lastVisible = querySnapshot.getDocuments().get(querySnapshot.size() - 1);
};

我也有这样的查询:

Query query = usersRef.orderBy("name", ASCENDING).limit(4);

在我的onCreate中，我这样做:

In my onCreate I do this:

query.addSnapshotListener(MainActivity.this, listener);

要在RecyclerView中实现分页，我使用:

To implement pagination in my RecyclerView, I use:

RecyclerView.OnScrollListener onScrollListener = new RecyclerView.OnScrollListener() {
    @Override
    public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {}

    @Override
    public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
        query = query.startAfter(lastVisible);
        query.addSnapshotListener(MainActivity.this, listener);
    }
};
recyclerView.addOnScrollListener(onScrollListener);

表示当用户达到限制时，我要加载下一个用户.但是，我没有正确的索引.根据弗兰克·范·普菲伦(Frank van Puffelen)的 answer ，我正在使用多个侦听器，这确实是我在做的事情，因为我两次使用了query.addSnapshotListener().我的问题是，如何在不附加新侦听器的情况下将第二个查询替换为第一个查询?如何始终使用相同的侦听器?

Meaning that when the user reaches the limit, I want to load next users. However, I don't get the right indexes. According to Frank van Puffelen's answer, I'm using multiple listeners, which is indeed what I'm doing, since I'm using query.addSnapshotListener() twice. My question is, how to replace the first query, with the second one without attaching a new listener? How to always use the same listener?

推荐答案

不可能简单地替换侦听器的查询.在向查询添加侦听器时，它将永久使用该查询，直到删除侦听器为止.

It's not possible to simply replace the query for a listener. When you add a listener to a query, it will forever use that query until the listener is removed.

如果要开始获取新查询的结果，则必须重复该过程并将侦听器添加到该新查询.如果需要，可以使用相同的侦听器对象，但不能简单地交换"到其他对象.一个新的查询.

If you want to start getting results for a new query, you will have to repeat the process and add a listener to that new query. You can use the same listener object if you want, but you can't simply "swap in" a new query.

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