How to append data to array properly? Swift4

Question

I've been trying to retrieve some data from firebase and store it in arrays. But I've got a problem when I try to append the data.

I can store the data into arrays only in the closure and outside of the closure, i all the sudden lose all the values inside of the arrays. Why does this happen? Please help me to find what I'm missing...

        var names = [String]()
        var txts = [String]()
        var imgUrls = [String]()

Database.database().reference().child("Posts").child("dummy").observe(.value) { (snapshot) in
                guard let snapshots = snapshot.children.allObjects as? [DataSnapshot] else {
                    return
                }
                for snap in snapshots {
                    let autoId = snap.key as String
                    Database.database().reference().child("Posts").child("dummy").child(autoId).observe(.value, with: { (snapshot) in
                        guard let data = snapshot.value as? [String: Any] else {return}

                        //name
                        guard let name =  data["name"] as? String else {return}
                        self.names.append(name)
                        //image
                        guard let imgUrl = data["image"] as? String else {return}
                        self.imgUrls.append(imgUrl)
                        //txt
                        guard let txt = data["text"] as? String else {return}
                        self.txts.append(txt)

                        print(self.names) //this returns all data from firebase
                        print(self.imgUrls) //this returns all data from firebase
                        print(self.txts) //this returns all data from firebase
      }
                        print(self.names) //this returns empty array
                        print(self.imgUrls) //this returns empty array
                        print(self.txts) //this returns empty array
    }

You are missing the fact that the closure is asynchronous. Look for "Swift + Closure + Async". Also, don't use 3 arrays, use only one with a custom struct that will hold a imageUrl, name and text properties. Theses three infos are sync'ed, they means nothing else. If you remove one item of self.names array, you need to do the same action for the 2 others, right? What if you want to shuffle them? Use one single array. — Larme
– Larme, Commented Oct 18, 2018 at 12:10
I have one comment regarding your use if the guard statement insufferable the for loop. In your case if an error if a nil value is detected in a snap you will exit the method without iterating through the other remaining snaps. I think if let is better suited. Or simply replace return with continue so your forloop can continue with the next object. — OverD
– OverD, Commented Oct 18, 2018 at 13:27

Umar Maikano · Accepted Answer · 2021-02-26 18:04:31Z

Could it be because you are assigning the data inside a closure which could happen in a later time. So basically the last print statement occurs before the closure statements.

I will recommend you use didSet to understand the sequence. For example:

var names: [String] = [String]() { 
    didSet {
      print(names)  // this is print each time the value is set
    }
}
var txts: [String] = [String]() {
   didSet {
    print(txts)
   }
}
var imgUrls: [String] = [String]() {
   didSet {
       print(imgUrls)
   }
}

Database.database().reference().child("Posts").child("dummy").observe(.value) { (snapshot) in 
     guard let snapshots = snapshot.children.allObjects as? [DataSnapshot] else { return }

  for snap in snapshots {
                let autoId = snap.key as String
                Database.database().reference().child("Posts").child("dummy").child(autoId).observe(.value, with: { (snapshot) in
                    guard let data = snapshot.value as? [String: Any] else {return}

                    //name
                    guard let name =  data["name"] as? String else {return}
                    self.names.append(name)
                    //image
                    guard let imgUrl = data["image"] as? String else {return}
                    self.imgUrls.append(imgUrl)
                    //txt
                    guard let txt = data["text"] as? String else {return}
                    self.txts.append(txt)
}



}

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