Iterate over snapshot children in Firebase

Question

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.

vacawama · Accepted Answer · 2018-08-04T17:41:02.457

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)     
    }
}

score 20 · Answer 2 · edited Feb 24 '17 at 17:02

I have just converted the above answer to 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)
           }
})

A better way might be:

    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)
            }
        })

Thanks it works but `print(rest.value)` prints `Optional(0)` instead it should be printing a interger. How can i make `print(rest.value)` to print a integer? — Parag Kadam, Mar 05 '17 at 13:42

score 16 · Answer 3 · edited Feb 26 '17 at 10:38

16

This is pretty readable and works fine:

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
      }
})

edited Feb 26 '17 at 10:38

Cœur

37,241
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answered May 21 '15 at 19:13

Dan

4,197
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and if i want to remove the someValue from firebase database just that key, what i do? – Kika_Fortez Aug 17 '16 at 12:10
for SDK v2, do we have anyway to make a promise callback in forloop make sure we done all the stuffs before doing other things – atom2ueki Sep 05 '16 at 10:01
There is no reason to call `childSnapshotForPath`, each child is already a child snapshot. – phatmann Feb 07 '18 at 20:11

Edward · Answer 4 · 2017-05-24T14:51:08.160

7

   ref = FIRDatabase.database().reference().child("exampleUsernames")    
   ref.observeSingleEvent(of: .value, with: { snapshot in

       for rest in snapshot.children.allObjects as! [FIRDataSnapshot] {

           guard let restDict = rest.value as? [String: Any] else { continue }
           let username = restDict["username"] as? String
       }
   })

edited May 24 '17 at 14:51

answered Dec 18 '16 at 17:29

Edward

2,864
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score 6 · Answer 5 · edited Jan 01 '21 at 15:56

Firebase 4.0.1

       Database.database().reference().child("key").observe(.value) { snapshot in
            if let datas = snapshot.children.allObjects as? [DataSnapshot] {
                let results = datas.flatMap({
                  ($0.value as! [String: Any])["xxx"]
                })
                print(results)
            }
       }

Firebase 7.3.0

       Database.database().reference().child("key").observe(.value) { snapshot in
            if let datas = snapshot.children.allObjects as? [DataSnapshot] {
                let results = datas.compactMap({
                  ($0.value)
                })
                print(results)
            }
       }

If you have multiple keys/values, and want to return an array with dictionary elements, declare an array:

var yourArray = [[String: Any]]()

then change block body to this:

     let children = snapshot.children
     while let rest = children.nextObject() as? DataSnapshot, let value = rest.value {
          self.yourArray.append(value as! [String: Any])
      }

Iterate over snapshot children in Firebase

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