火力基地检索元素 [英] Firebase & Retrieving Elements

问题描述

我正在尝试从Firebase读取x元素，但是我有一种误解……

I'm trying to read x amount of elements from Firebase, but I have a feeling I'm misunderstanding something...

DataSnapshot返回正确的子计数，但是当我尝试遍历这些子计数时，该循环永远不会执行.

DataSnapshot returns the correct child count, however when I try to loop through the children, the loop never executes.

注意:Kotlin中的代码

Note: Code in Kotlin

fun list(count: Int, callback: ListCallback) {
    val playersRef = firebase.child("players")
    val queryRef = playersRef.orderByChild("rank").limitToFirst(count)
    queryRef.addListenerForSingleValueEvent(object : ValueEventListener {

        override fun onCancelled(error: FirebaseError?) {
            Log.e("firebase", error!!.message)
        }

        override fun onDataChange(snapshot: DataSnapshot?) {
            val children = snapshot!!.children
            // This returns the correct child count...
            Log.i("firebase", children.count().toString())
            val list = ArrayList<Entry>()
            // However, this loop never executes
            children.forEach {
                val e = Entry()
                e.name = it.child("name").value as String
                e.rank = it.child("rank").value as Long
                e.wins = it.child("wins").value as Long
                e.losses = it.child("losses").value as Long
                Log.i("firebase", e.toString())
                list.add(e)
            }
            callback.onList(list)
        }
    })
}

推荐答案

这对我有用:

val firebase: Firebase = Firebase("https://stackoverflow.firebaseio.com/34378547")

fun main(args: Array<String>) {
    list(3)
    Thread.sleep(5000)
}

fun list(count: Int) {
    val playersRef = firebase.child("players")
    val queryRef = playersRef.orderByChild("rank").limitToFirst(count)
    queryRef.addListenerForSingleValueEvent(object : ValueEventListener {

        override fun onCancelled(error: FirebaseError?) {
            println(error!!.message)
        }

        override fun onDataChange(snapshot: DataSnapshot?) {
            val children = snapshot!!.children
            // This returns the correct child count...
            println("count: "+snapshot.children.count().toString())
            children.forEach {
                println(it.toString())
            }
        }
    })
}

输出:

count: 2
DataSnapshot { key = -K6-Zs5P1FJLk4zSgNZn, value = {wins=13, name=fluxi, rank=1, losses=1} }
DataSnapshot { key = -K6-ZtdotHkkBzs5on9X, value = {wins=10, name=puf, rank=2, losses=42} }

更新

在评论中，有人讨论了为什么snapshot.children.count()有效，而children.count()无效.该问题是由两个事实引起的:

Update

In the comments there was some discussion about why snapshot.children.count() works, while children.count() doesn't. The problem is caused by two facts:

1. Firebase的DataSnapshot.getChildren()返回一个Iterable，它只能向前迭代(Iterable的协定也是如此).
2. 科特林count()在Iterable上循环计数.

1. Firebase's DataSnapshot.getChildren() returns an Iterable, which can be iterated forward only (as is the contract of an Iterable).
2. The Kotlin count() loops over the Iterable to count its items.

因此，在完成Kotlin的count()之后，Iterable在序列的末尾.随后的for循环不再循环.在我的代码段中，我分别调用snapshot.children以获得单独的迭代器以获取计数.

So after Kotlin's count() is done, the Iterable is at the end of the sequence. The subsequent for loop has nothing to loop over anymore. In my snippet I call snapshot.children separately to get a separate iterator to get the count.

了解Kotlin如何实现count()时，最好使用Firebase内置的childrenCount:

Knowing how Kotlin implements count() it is better to use Firebase's built-in childrenCount:

println("count: "+snapshot.childrenCount)

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