SWIFT Firebase 实时数据库分页双重提取问题

Question

当我使用以下分页方法时，有时我会获取双重结果，有时分页不起作用。我想知道问题是什么以及 Firebase 实时数据库分页的最佳实践是什么。

我的用户数据结构：

"users" 
|- autoId1
     |- name: "abc"
     |- age: "22"
     |- ...
|- autoId2
     |- name: "xyz"
     |- age: "29"
     |- ...
|- autoId3
     |- name: "bbb"
     |- age: "20"
     |- ...

我有一个班级的集合，每个班级都有学生人数：

"class"
   |- "autoId" //this is the autoId for class
     |- autoId1: 1  //these are students autoId
     |- autoId2: 1
     |- autoId3: 1

当我在课堂上获取学生时，我有以下代码：

private var currentKey = nil
private func fetchUsersAtClass() {
    guard let currentUid = Auth.auth().currentUser?.uid else {return}
    if currentKey == nil {
          //classId is the autoId for a class
        CLASS_USERS.child(classId).queryLimited(toLast: 5).observeSingleEvent(of: .value) { (snapshot) in
         guard let first = snapshot.children.allObjects.first as? DataSnapshot else {return}
         guard let allObjects = snapshot.children.allObjects as? [DataSnapshot] else {return}
            allObjects.forEach({ (snapshot) in
                let uid = snapshot.key
               //fetch users from the users collection based on the [userId] fetched from class collection
                Database.fetchUser(with: uid, completion: { (user) in
                    self.users.append(user)
                    self.users.sort(by: { (p1, p2) -> Bool in
                        return p1.credit > p2.credit
                    })
                    self.tableView.reloadData()
                })
            })
            self.currentKey = first.key
        }
    } else {
        CLASS_USERS.child(classId).queryOrderedByKey().queryEnding(atValue: currentKey).queryLimited(toLast: 4).observeSingleEvent(of: .value, with: { (snapshot) in
            guard let first = snapshot.children.allObjects.first as? DataSnapshot else { return }
            guard let allObjects = snapshot.children.allObjects as? [DataSnapshot] else { return }
            
            allObjects.forEach({ (snapshot) in
                let uid = snapshot.key
                if uid != self.currentKey {
                    Database.fetchUser(with: uid, completion: { (user) in
                        self.users.append(user)
                        self.users.sort(by: { (p1, p2) -> Bool in
                            return p1.credit > p2.credit
                        })
                        self.tableView.reloadData()
                    })
                }
            })
            self.currentKey = first.key
        })
    }
}

对于触发分页，我有：

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
        if users.count > 4 {
            if indexPath.item == users.count - 1 {
                fetchUsersAtClass()
            }
        } 
}

Answer 1

由于您使用的是.queryOrderedByKey().queryEnding(atValue: currentKey)，因此实际上预期带有currentKey 的节点会同时针对前一页和当前页进行检索。您必须在应用程序代码中删除它。

最近向 API 添加了一个新的 queryEnding(beforeValue:) 方法，它允许您不获取重叠条目。