java - 当用户滚动到 RecyclerView 底部时，使用 ScrollListener 从 Firebase 实时数据库提取数据时出现问题

Question

我有一个应用程序，它从 Firebase 实时数据库中提取对象数据，将其添加到 ArrayList，然后将其添加到 RecyclerView。我在 RecyclerView 上使用了 onScrollListener，这样当用户滚动到 RecyclerView 底部时，就会调用一个函数从 Firebase 中提取更多数据，将其添加到 ArrayList 并通知 RecyclerViewAdapter 数据集更改。

当我运行我的应用程序时，它会在我的 RecyclerView 中显示初始数据拉取，然后立即开始调用该函数以连续从我的数据库中获取更多数据，直到没有更多数据可供拉取并且我的 RecyclerView 随之更新数据;无论我是否滚动(以任何方向/任何幅度)，都会发生这种情况。

理想情况下，我希望应用程序仅在用户滚动到 RecyclerView 底部时才提取数据，并且一次提取一个数据。我很难调试这个问题，因为逻辑似乎是正确的；任何帮助将不胜感激。

相关全局变量

RecyclerView rvVertical;
RecyclerViewAdapter rvaVertical;
long mTotalChildren = 0;
int mLastKey = 0;

ChildEventListener childEventListenerMain, childEventListenerPager;
boolean Loading = false;

private ArrayList<BlogObject> blogArray = new ArrayList<>();
private ArrayList<BlogObject> tempList = new ArrayList<>();

查看已加载

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    final View view = inflater.inflate(R.layout.socialmediatab, container, false);

    rvVertical = (RecyclerView) view.findViewById(R.id.bloglistRecyclerView);
    Loading = true;
    Log.d(TAG, "onCreateView - Loading = " + Loading);
    firstEventListener();
    regAdapterVertical();

    addOnScrollListener();
    return view;
}

第一个被调用的函数。初始化从 Firebase 中第一次提取的数据

private void firstEventListener() {

    FirebaseDatabase.getInstance().getReference().child("blogs").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            mTotalChildren = dataSnapshot.getChildrenCount();
        }
        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) { }
    });

    final Query BlogQuery = FirebaseDatabase.getInstance().getReference().child("blogs").orderByChild("createdAt").limitToLast(3);

    childEventListenerMain = new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot ds, String s) {
            Loading = true;
            Log.d(TAG, "First get blogs instance - Loading = " + Loading);
            BlogObject addblog = ds.getValue(BlogObject.class);

            if (addblog != null) {
                BlogObject addBlog = new BlogObject(ds.getValue(BlogObject.class).getImage(), ds.getValue(BlogObject.class).getTitle(), ds.getValue(BlogObject.class).getTimeStamp(), ds.getValue(BlogObject.class).getCreatedAt());
                blogArray.add(addBlog);
                rvaVertical.notifyDataSetChanged();
                mLastKey = blogArray.get(0).getCreatedAt();
                rvVertical.scrollToPosition(blogArray.size() - 1);
            }
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) { }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) { }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) { }

        @Override
        public void onCancelled(DatabaseError databaseError) { }
    };

    BlogQuery.addChildEventListener(childEventListenerMain);
    ValueEventListener BlogChildSingleListener = new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            Loading = false;
        }
        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) { }
    };

    BlogQuery.addListenerForSingleValueEvent(BlogChildSingleListener);
}

将滚动监听器添加到我的回收器 View 中。理想情况下，当且仅当用户滚动到 RecyclerView 底部时，才应该从 Firebase 获取更多数据

private void addOnScrollListener() {
    rvVertical.addOnScrollListener(new RecyclerView.OnScrollListener() {
        @Override
        public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
        }

        @Override
        public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
            Log.d(TAG, "Just Entered onScrolled and Loading = " + Loading);
            super.onScrolled(recyclerView, dx, dy);

            if(!rvVertical.canScrollVertically(1)) {
                LinearLayoutManager linearLayoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();
                if (blogArray.size() >= 3 && !Loading && linearLayoutManager.findLastCompletelyVisibleItemPosition() == blogArray.size() - 1 && blogArray.size() < mTotalChildren) {
                    Loading = true;
                    getMoreBlogs();
                    Log.d(TAG, "Right After getMoreBlogs gets called. Loading = " + Loading);
                }
            }
        }
    });
}

通过第一次提取数据来初始化适配器

private void regAdapterVertical() {
    LinearSnapHelper linearSnapHelper = new SnapHelperOneByOne();
    linearSnapHelper.attachToRecyclerView(rvVertical);
    rvaVertical = new RecyclerViewAdapter(blogArray);
    rvVertical.setAdapter(rvaVertical);
    rvVertical.setLayoutManager((RecyclerView.LayoutManager) new LinearLayoutManager(getActivity(), LinearLayoutManager.VERTICAL, false));
}

这是我从 Firebase 提取更多数据的函数。它在我的滚动监听器中被调用

public void getMoreBlogs() {
    tempList.clear();

    Query getMoreQuery = FirebaseDatabase.getInstance().getReference().child("blogs").orderByChild("createdAt").endAt(mLastKey).limitToLast(3);
    childEventListenerPager = new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot ds, String s) {
            Loading = true;
            BlogObject blogObject = ds.getValue(BlogObject.class);
            if (blogObject != null) {
                BlogObject addBlog = new BlogObject(ds.getValue(BlogObject.class).getImage(), ds.getValue(BlogObject.class).getTitle(), ds.getValue(BlogObject.class).getTimeStamp(), ds.getValue(BlogObject.class).getCreatedAt());
                tempList.add(addBlog);
            }
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) { }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) { }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) { }

        @Override
        public void onCancelled(DatabaseError databaseError) { }
    };

    getMoreQuery.addChildEventListener(childEventListenerPager);

    ValueEventListener addtoAdapter = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            tempList.remove(tempList.size() - 1);
            blogArray.addAll(0, tempList);
            mLastKey = blogArray.get(0).getCreatedAt();
            rvaVertical.notifyDataSetChanged();
            Loading = false;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    };

    getMoreQuery.addListenerForSingleValueEvent(addtoAdapter);
}
}

这是我的加载 Logcat( boolean 值)，我在整个程序中跟踪它，因为我相信这是问题的主要部分。我已将其附在下面:

2019-05-30 10:26:14.590 24930-24970/com.yourgesture.newsocialmediatab I/FirebaseAuth: [FirebaseAuth:] Loading module via FirebaseOptions.
2019-05-30 10:26:14.956 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: onCreateView - Loading = true
2019-05-30 10:26:16.399 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: First get blogs instance - Loading = true
2019-05-30 10:26:16.406 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: First get blogs instance - Loading = true
2019-05-30 10:26:16.534 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Just Entered onScrolled and Loading = false
2019-05-30 10:26:16.539 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Right After getMoreBlogs gets called. Loading = true
2019-05-30 10:26:16.663 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Just Entered onScrolled and Loading = false
2019-05-30 10:26:16.666 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Right After getMoreBlogs gets called. Loading = true
2019-05-30 10:26:16.826 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Just Entered onScrolled and Loading = false
2019-05-30 10:26:16.828 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Right After getMoreBlogs gets called. Loading = true
2019-05-30 10:26:16.976 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Just Entered onScrolled and Loading = false
2019-05-30 10:26:16.977 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Right After getMoreBlogs gets called. Loading = true
2019-05-30 10:26:17.229 24930-24930/com.yourgesture.newsocialmediatab D/Social Media Tab: Just Entered onScrolled and Loading = false

Answer 1

在你的firstEventListener()中，

getReference().child("博客")实际上会获取整个子数据，这将被视为读取操作，即使您没有使用它。

因此您的方法不会保存任何数据。

让我分享一种方法:

private void getUsers(String nodeId) {
    Query query;

    if (nodeId == null)
        query = FirebaseDatabase.getInstance().getReference()
                .child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
                .orderByKey()
                .limitToFirst(mPostsPerPage);
    else
        query = FirebaseDatabase.getInstance().getReference()
                .child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
                .orderByKey()
                .startAt(nodeId)
                .limitToFirst(mPostsPerPage);

    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            UserModel user;
            List<UserModel> userModels = new ArrayList<>();
            for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                userModels.add(userSnapshot.getValue(UserModel.class));
            }

            mAdapter.addAll(userModels);
            mIsLoading = false;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            mIsLoading = false;
        }
    });
}

三个新变量在上述函数中很重要:mPostsPerPage:MainActivity.java类中定义的Integer常量，指定每页查询多少条记录。

mIsLoading:它是 MainActivity 中定义的 boolean 值，用于跟踪 Firebase 数据库查询是否正在进行。

Consts.FIREBASE_DATABASE_LOCATION_USERS:它是在 Consts.java 类中定义的常量，其中仅存储“users”节点的位置

引用号:https://blog.shajeelafzal.com/2017/12/13/firebase-realtime-database-pagination-guide-using-recyclerview/