java - Firestore 分页

Question

我对如何使用 Firestore 正确对查询进行分页有疑问。

通过将下一个查询放入上一个查询的 OnSuccessListener 中，就像 Firestore 页面上的示例一样，是否会不可避免地总是触发一次加载所有页面的链式 react ？这难道不是我们想通过分页来避免的事情吗？

// Construct query for first 25 cities, ordered by population
Query first = db.collection("cities")
        .orderBy("population")
        .limit(25);

first.get()
    .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
        @Override
        public void onSuccess(QuerySnapshot documentSnapshots) {
            // ...

            // Get the last visible document
            DocumentSnapshot lastVisible = documentSnapshots.getDocuments()
                    .get(documentSnapshots.size() -1);

            // Construct a new query starting at this document,
            // get the next 25 cities.
            Query next = db.collection("cities")
                    .orderBy("population")
                    .startAfter(lastVisible)
                    .limit(25);

            // Use the query for pagination
            // ...
        }
    });

来源:https://firebase.google.com/docs/firestore/query-data/query-cursors

这是我的方法。我知道在真实的应用程序中我应该使用 RecyclerView，但我只想在 TextView 上测试它。我想在每次点击按钮时加载 3 个文档。将 lastResult 存储为成员，然后检查它是否不为 null，以查看它是否是第一个查询是否有意义？

public void loadMore(View v) {
    Query query;
    if (lastResult == null) {
        query = notebookRef.orderBy("priority")
                .orderBy("title")
                .limit(3);
    } else {
        query = notebookRef.orderBy("priority")
                .orderBy("title")
                .startAfter(lastResult)
                .limit(3);
    }
    query.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
        @Override
        public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
            String data = "";

            for (QueryDocumentSnapshot documentSnapshot : queryDocumentSnapshots) {
                Note note = documentSnapshot.toObject(Note.class);
                note.setDocumentId(documentSnapshot.getId());

                String documentId = note.getDocumentId();
                String title = note.getTitle();
                String description = note.getDescription();
                int priority = note.getPriority();

                data += "ID: " + documentId
                        + "\nTitle: " + title + "\nDescription: " + description
                        + "\nPriority: " + priority + "\n\n";
            }

            if (queryDocumentSnapshots.size() > 0) {
                data += "_____________\n\n";
                textViewData.append(data);


                lastResult = queryDocumentSnapshots.getDocuments()
                        .get(queryDocumentSnapshots.size() - 1);
            }
        }
    });
}

Answer 1

您必须在回收器 View / ListView 中使用 ScrollListener。开始时，您将获取 25 个数据限制，一旦用户再次滚动到页面末尾，您必须使用限制进行新的 firestore 调用(无论您保留什么)。但此时您必须在查询中继续使用 startAt() 。 startAt() 的输入将是第一次获取的数据中的最后一个键。这只是基本概述。您可以引用this查询链接。

您可以使用 firestore 在 recyclerview/listview 中创建分页，如下所示:

基本上遵循以下步骤:

1) 打开 Activity/Fragment 时，您的第一个查询将获取 25 个数据限制

Query first = db.collection("cities")
        .orderBy("population")
        .limit(25);

first.get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
        @Override
        public void onSuccess(QuerySnapshot documentSnapshots) {
            // add data to recyclerView/listview

            // Get the last visible document
            DocumentSnapshot lastVisible = documentSnapshots.getDocuments()
                    .get(documentSnapshots.size() -1);
        }
    });

2) 现在覆盖适配器的 onScrollListener

boolean isEndChildResults = false;
    mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
                @Override
                public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                    super.onScrollStateChanged(recyclerView, newState);
                    if (newState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL) {
                        isScrolling = true;
                    }
                }
   @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            currentVisibleItem = linearLayoutManager.getChildCount();
            totalItem = linearLayoutManager.getItemCount();
            scrolledItem = linearLayoutManager.findFirstVisibleItemPosition();
            if (isScrolling && (currentVisibleItem + scrolledItem == totalItem) && !isEndChildResults && documentSnapshot != null) {
                isScrolling = false;
                mProgressBarScroll.setVisibility(View.VISIBLE);

                FirebaseFirestore firebaseFirestore = FirebaseFirestore.getInstance();

                Query query = firebaseFirestore.collection(...).document(...).limit(25).orderBy(...).startAt(lastVisible);
                query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {

                        if (task.isSuccessful()) {
                          // add data to recyclerView/listview
                          lastVisible = documentSnapshots.getDocuments().get(documentSnapshots.size() -1);


                     if (task.getResult().size() < postPerPageLimit) {
                       // if your result size is less than your query size which means all the result has been displayed and there is no any other data to display 
                                    isEndChildResults = true;
                                }
                            }

                        }
                    }
                });

          if(isEndChildResults){
         // show snackbar/toast
            }
         }

*lastVisible documentSnapshot 将在每次滚动时发生变化，并且将从 lastVisible 快照中获取数据