Assign value of a Firestore document to a variable - swift

Question

I am trying to read the value of a Firestore document. I have tried doing it two different ways, but each fails.
In the first one, an error is thrown on the return line: Unexpected non-void return value in void function. I found out why this happened, and so, I implemented the second way.
import UIKit
import Firestore
func readAvailableLists(forUser user: String) -> [String] {
let db = Firestore.firestore()
db.collection("userslist").document(user).getDocument { (document, err) in
if let document = document, document.exists {
return UserInformationDocument(dictionary: document.data()!)?.lists!
} else {
print("Document does not exist")
}
}
}
In the second method, I assign the UserInformationDocument(dictionary: document.data()!)?.lists! to a variable and return that variable at the end of the function (see code below). However, when I do this, it the function returns an empty array. What surprises me is that the print return the correct value, but after long after the function has executed the return statement. Is it because it is an async demand? And if so, how should I fix this?
import UIKit
import Firestore
func readAvailableLists(forUser user: String) -> [String] {
let db = Firestore.firestore()
var firestoreUserDocument: [String] = []
db.collection("userslist").document(user).getDocument { (document, err) in
if let document = document, document.exists {
firestoreUserDocument = (UserInformationDocument(dictionary: document.data()!)?.lists!)!
print((UserInformationDocument(dictionary: document.data()!)?.lists!)!)
} else {
print("Document does not exist")
}
}
return firestoreUserDocument
}

Answer 1

The Firebase call is an asynchronous function. It takes extra time to execute because it's talking to a server (as you've noted) - as a result, the completion block (the block that defines document and err in your example) happens at a different time, outside of the rest of the body of the function. This means you can't return a value from inside it, but you can pass another closure through to it, to execute later. This is called a completion block.
func readAvailableLists(forUser user: String, completion: #escaping ([String]?, Error?) -> Void) -> [String] {
let db = Firestore.firestore()
db.collection("userslist").document(user).getDocument { (document, err) in
if let document = document, document.exists {
// We got a document from Firebase. It'd be better to
// handle the initialization gracefully and report an Error
// instead of force unwrapping with !
let strings = (UserInformationDocument(dictionary: document.data()!)?.lists!)!
completion(strings, nil)
} else if let error = error {
// Firebase error ie no internet
completion(nil, error)
}
else {
// No error but no document found either
completion(nil, nil)
}
}
}
You could then call this function elsewhere in your code as so:
readAvailableLists(forUser: "MyUser", completion: { strings, error in
if let strings = strings {
// do stuff with your strings
}
else if let error = error {
// you got an error
}
})