java - Android - Firestore 使用 startAfter() 和 limit() 返回一个空查询

Question

我正在尝试使用 Firebase Firestore startAfter() 和 limit() 查询方法实现分页系统。第一个查询成功返回，但第二个查询返回空快照。

• 这是我的 getNextPage() 方法:

  fun getNextPage(paginationSize : Long) : TrendingRepository {
  database.collection("app")
      .document("data")
      .collection("offers")
      .orderBy("discount")
      .startAfter(lastVisible)
      .limit(paginationSize)
      .get().addOnSuccessListener { snapshot ->
  
          Log.i("TrendingRepo", "pagination size : $paginationSize")
          val newList = ArrayList<Offer>()
  
          if (!snapshot.isEmpty) {
              lastVisible = snapshot.documents[snapshot.size() - 1]
          }
  
          for (document in snapshot) {
              val item = document.toObject(Offer::class.java)
              newList.add(item)
              Log.i("TrendingRepo", "at position: ${newList.indexOf(item)} got item: ${item.id}")
          }
  
          successListener?.onSuccess(newList)
  
      }.addOnFailureListener {
          failureListener?.onFailure(it.localizedMessage)
      }
  
  return this
  }
  

• 这是我的 Logcat:

TrendingRepo: pagination size : 48 // first try

TrendingRepo: at position: 0 got item: 0pqcRzSd06WWlNNmcolu

TrendingRepo: at position: 1 got item: 7I7wiSYt5yEBWwN08bqJ

...

TrendingRepo: at position: 45 got item: 4B3dEPhFLqhKrYpLWYE7

TrendingRepo: at position: 46 got item: 4ddLqiGe8ReXW8SKq2Q6

TrendingRepo: at position: 47 got item: 4uVnnGNAmKvGUUHcV01n

TrendingRepo: pagination size : 48 // second try

//no more logging, data is empty

Answer 1

可能会出现项目小于分页大小的情况，所以这里是代码

private var lastVisible: DocumentSnapshot? = null
private var isLastPage: Boolean = false
private var isDocEmpty: Boolean = false

var ref: Task<QuerySnapshot>? = null

 if (lastVisible != null) {
ref = database.collection("app").document("data").collection("offers").orderBy("discount").startAfter(lastVisible).limit(paginationSize).get()
 } else {
ref = database.collection("app").document("data").collection("offers").orderBy("discount").limit(paginationSize).get()
 }


 ref.addOnSuccessListener { documents ->

            hideProgress()
            isDocEmpty = documents.isEmpty



            if (!isDocEmpty) {
                lastVisible = documents.last()
                isLastPage = documents.size() < paginationSize
            }

            isLoading = false
        }
            .addOnFailureListener { exception ->
                Log.w("TAG", "Error getting documents: ", exception)
                isLoading = false
                hideProgress()
            }

希望这对您有帮助。