使用ordeBy()和endBefore()时,Firestore降序分页返回0个结果

我正在研究Firebase Firestore,还有什么比另一个聊天应用更好的示例项目要创建。

我如何想象它会起作用:

1. 当用户输入片段时,将获取最新消息
2. 更新 oldestTimestamp ,将邮件添加到回收器视图
3. 当用户滚动到顶部时 fecthMore 并获取比当前最旧消息旧的20条消息

4. 更新 旧时间戳 ,将邮件添加到回收器视图

   阅读后 order-limit-data 和 query-cursors 我想出了那些解决办法,可惜没有一个奏效。

我用过 whereLessThanOrEqualTo("timestamp", previousOldestMessageTimestamp) 但这返回了0条消息

之后,我试着用 .orderBy("timestamp", Query.Direction.DESCENDING) .endBefore(previousOldestMessageTimestamp) 结果和以前一样

然后我试着将查询简化为 .orderBy("timestamp") .endBefore(previousOldestMessageTimestamp) 它返回了一些消息,但在列表中是整个对话中最早的消息

我应该做些什么才能让它正常工作(就像每个聊天应用一样)?

你也能解释一下 whereLessThanOrEqualTo() 和 orderBy() + endBefore()

这里有一些背景

private var previousOldestMessageTimestamp: Long = -1

private fun firstFetch(){
    //get oldest messages
    //update previousOldestMessageTimestamp to oldest message timestamp
}

 private fun fetchMore() {
    val conversationRef = FirebaseFirestore.getInstance()
                                           .document("conversations")
                                           .collection("some_id")

    conversationRef
            .whereLessThan("timestamp", previousOldestMessageTimestamp) 
            //^ 0 messages

            .orderBy("timestamp", Query.Direction.DESCENDING)
            .endBefore(previousOldestMessageTimestamp)
            //^ returns 0 messages 

            .orderBy("timestamp") 
            .endBefore(previousOldestMessageTimestamp)
            //^ returns messages but starts with oldest one,
            //not what I want

            .limit(20)
            .get().addOnCompleteListener { it: Task<QuerySnapshot!>
                val messages: List<Message> = it.result.map { it.toObject(Message::class.java) }
                println("I fetched ${messages.size} messages : $messages")

                if (messages.isNotEmpty()) {
                    previousOldestMessageTimestamp = *oldestMessage*.timestamp
                }
            }
}