How to read data from firebase and append it to an array - swift

Question

I have a userImages collection and I am trying to read data from it and appending it to my imgUrls array.
This is my code where I read the data from the database and try appending it to my array. Unfortunately, I keep getting an error because the array is apparently empty.
override func viewDidLoad() {
var ref: DatabaseReference!
let userID = Auth.auth().currentUser?.uid //holds the current user uid
ref = Database.database().reference()
var imgUrls = [String]() //array to hold the image urls from the userImages collection
ref.child("userImages").child(userID!).observeSingleEvent(of: .value) { (snapshot) in //read from userImages collection only from the subcollection where the
guard let dict = snapshot.value as? [String: Any] else { return } //document Id equals the current user uid. Create a dictionary from the
//snapshot values
let values = dict.values //holds the values from the dictionary
for value in values { //for loop to go through each value from the dictionary
imgUrls.append((value as? String)!) //and append to the imgUrls array
}
}
testLabel.text = imgUrls[0] //I used this to test, but i get an error saying the array is empty
}
I posted a question before, but it was so convoluted I decided to delete it and repost it simpler.
Any help is much appreciated!

Answer 1

the reason you are not getting anything in your testLabel.text is because:
ref.child("userImages").child(userID!).observeSingleEvent(of: .value) { (snapshot) in //read from userImages collection only from the subcollection where the
...
}
is an asynchronous function called. That is it will be done sometimes in the future.
but your:
testLabel.text = imgUrls[0] //I used this to test, but i get an error saying the array is empty
is outside of this call. So the results in "imgUrls" are not available yet.
Put this line inside the function or wait until it has finished before using the results.

Answer 2

You try to use the array before the observeSingleEvent closure is executed. All observation calls to Firebase are asynchronous. This means that you test code is executed before the closure and the array is still empty.
ref.child("userImages").child(userID!).observeSingleEvent(of: .value) { snapshot in
// this closure is executed later
guard let dict = snapshot.value as? [String: Any] else { return }
let values = dict.values
for value in values {
imgUrls.append((value as? String)!)
}
// your test code should bee here
}
// this is executed before closure and the array is empty
testLabel.text = imgUrls[0]
For this reason you get the error. You need to add your test code into the closure.
However, there is another catch. Asynchronous calls (their closures) are executed on the background thread.
ref.child("userImages").child(userID!).observeSingleEvent(of: .value) { snapshot in
// this is executed on the background thread
}
However, all user interface calls must be executed on the main thread. Also wrap your test code by calling the main thread, otherwise you won't see the result in the user interface.
DispatchQueue.main.async {
// this code is executed on the main thread
// all UI code must be executed on the main thread
}
After editing, your code might look like this:
override func viewDidLoad() {
var ref: DatabaseReference!
let userID = Auth.auth().currentUser?.lid
ref = Database.database().reference()
var imgUrls = [String]()
ref.child("userImages").child(userID!).observeSingleEvent(of: .value) { snapshot in
guard let dict = snapshot.value as? [String: Any] else { return }
let values = dict.values
for value in values {
imgUrls.append((value as? String)!)
}
DispatchQueue.main.async {
testLabel.text = imgUrls[0]
}
}
}