迭代 Firebase 中的子快照

Iterate over snapshot children in Firebase(迭代 Firebase 中的子快照)

问题描述

I have a Firebase resource that contains several objects and I would like to iterate over them using Swift. What I expected to work is the following (according to the Firebase documentation)
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)     
     }
 })

So it seems there is a problem with Swift iterating over the NSEnumerator object returned by Firebase.

Help is really welcome.

If I read the documentation right, this is what you want:

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)     
    }
}

A better way might be:

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)     
    }
}

The first method requires the NSEnumerator to return an array of all of the objects which can then be enumerated in the usual way. The second method gets the objects one at a time from the NSEnumerator and is likely more efficient.

In either case, the objects being enumerated are FIRDataSnapshot objects, so you need the casts so that you can access the value property.

Using for-in loop:

Since writing the original answer back in Swift 1.2 days, the language has evolved. It is now possible to use a for in loop which works directly with enumerators along with case let to assign the type:

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)     
    }
}

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