How to create an id?

Question

Swift

Tell me plz. How to create an iteration of id in the idPosts array, but at the same time, after the application exited, the value was saved, and not reset.
Structure:

users: {
  "asd2123123adsa": [
    name: "Ivan",
    lastname: "Ivanov" 
    idPosts: [
      id_1: "123123213",
      id_2: "543547643",
      ...
      id_n: "785621232"
    ]
  ],

  "dg43531234234": [
    name: "Petr",
    lastname: "Petrov" 
    idPosts: [
      id_1: "223123213",
      id_2: "343547643",
      ...
      id_n: "185621232"
    ]
  ]
}

Answer 1

To add a new element to idPosts:

let currentUserID = // Setup user id
let idPostsRef = FIRDatabase.database().reference().child("users/\(key)/idPosts")
// Getting last postID key
idPostsRef.observeSingleEvent(of: .value, with: { snapshot in
            if let dictionary = snapshot.value as? [String: AnyObject] {
                let lastPostId = dictionary.keys.sorted()[dictionary.count - 1]
                
                let substringStartIndex = lastPostId.index(lastPostId.startIndex, offsetBy: +2)
                guard let lastPostNumber = Int(lastPostId.substring(from: substringStartIndex)) else { return }
                let index = lastPostNumber + 1
                idPostsRef.child("id\(index)").setValue(post.id)
        } else {
                idPostsRef.child("id0").setValue(post.id)                    
        }
        
})