在Firebase中迭代快照子级

Question

我有一个Firebase资源，它包含几个对象，我想使用Swift对它们进行迭代。我期望的工作如下(根据Firebase文档)

https://www.firebase.com/docs/ios-api/Classes/FDataSnapshot.html#//api/name/children

var ref = Firebase(url:MY_FIREBASE_URL)
ref.observeSingleEventOfType(.Value, withBlock: { snapshot in
  println(snapshot.childrenCount) // I got the expected number of items
    for rest in snapshot.children { //ERROR: "NSEnumerator" does not have a member named "Generator"
       println(rest.value)     
     }
 })

因此，对Firebase返回的NSEnumerator对象进行Swift迭代似乎存在问题。

帮助是非常受欢迎的。

Answer 1

如果我正确地阅读了文档，这就是你想要的：

var ref = Firebase(url: MY_FIREBASE_URL)
ref.observeSingleEvent(of: .value) { snapshot in
    print(snapshot.childrenCount) // I got the expected number of items
    for rest in snapshot.children.allObjects as! [FIRDataSnapshot] {
       print(rest.value)     
    }
}

更好的办法可能是：

var ref = Firebase(url: MY_FIREBASE_URL)
ref.observeSingleEvent(of: .value) { snapshot in
    print(snapshot.childrenCount) // I got the expected number of items
    let enumerator = snapshot.children
    while let rest = enumerator.nextObject() as? FIRDataSnapshot {
       print(rest.value)     
    }
}

第一个方法要求NSEnumerator返回所有对象的数组，然后可以按照通常的方式进行枚举。第二个方法一次从NSEnumerator中获取一个对象，并且可能更高效。

在这两种情况下，所枚举的对象都是FIRDataSnapshot对象，因此需要进行强制转换，以便可以访问value属性。

使用for-in 循环的：

自从在斯威夫特1.2天内写出最初的答案以来，语言已经发生了变化。现在可以使用直接与枚举数和case let一起工作的case let循环来分配类型：

var ref = Firebase(url: MY_FIREBASE_URL)
ref.observeSingleEvent(of: .value) { snapshot in
    print(snapshot.childrenCount) // I got the expected number of items
    for case let rest as FIRDataSnapshot in snapshot.children {
       print(rest.value)     
    }
}

Answer 2

我刚刚把上面的答案转换为Swift 3:

ref = FIRDatabase.database().reference()    
ref.observeSingleEvent(of: .value, with: { snapshot in
       print(snapshot.childrenCount) // I got the expected number of items
       for rest in snapshot.children.allObjects as! [FIRDataSnapshot] {
           print(rest.value)
           }
})

更好的办法可能是：

    ref = FIRDatabase.database().reference() 
    ref.observeSingleEvent(of: .value, with: { snapshot in
            print(snapshot.childrenCount) // I got the expected number of items
            let enumerator = snapshot.children
            while let rest = enumerator.nextObject() as? FIRDataSnapshot {
                print(rest.value)
            }
        })

Answer 3

这是相当可读的，工作良好：

var ref = Firebase(url:MY_FIREBASE_URL)
ref.childByAppendingPath("some-child").observeSingleEventOfType(
  FEventType.Value, withBlock: { (snapshot) -> Void in

      for child in snapshot.children {

        let childSnapshot = snapshot.childSnapshotForPath(child.key)
        let someValue = childSnapshot.value["key"] as! String
      }
})