How can I fetch a list of data from firebase?

Question

I am following a to-do list app tutorial from YouTube. I want to fetch a list of data from applications.

My code is like:

func fetchAllItems(completion: @escaping([TodoItem]) -> Void) {
  guard let uid = Auth.auth().currentUser?.uid else { return }

  var allItems = [TodoItem]()

  ref.child("users").child(uid).child("applications").observe(.childAdded) { snapshot in

    fetchSingleItem(id: snapshot.key) { item in
      allItems.append(item)
      print(allItems)
      completion(allItems)
    }
  }
}
    
func fetchSingleItem(id: String, completion: @escaping(TodoItem) -> Void) {
    guard let uid = Auth.auth().currentUser?.uid else { return }

    ref.child("users").child(uid).child("applications").child(id).observeSingleEvent(of: .value) { snapshot in

      guard let dictionary = snapshot.value as? [String: Any] else {
        return
      }
      let todoItem = TodoItem(keyID: id, dictionary: dictionary)
      print(todoItem)
      completion(todoItem)
    }
  }
}

The struct of TodoItem is like:

struct TodoItem {
  var title: String
  var isComplete: Bool
  init(keyID: String, dictionary: [String: Any]) {
    self.title = dictionary["title"] as? String ?? ""
    self.isComplete = dictionary["isComplete"] as? Bool ?? false
  }
}

But when I print allItems, only the first one shows in console. Can anyone help me with this?