Why can I not modify a variable declared outside a closure?

Question

Every time I test my code, an empty string is returned to the pickerUI rather than the college name. Why is that? When debugging, docData is set correctly but changes back to an empty string after the closure.

var ans = "";
var pickerData = [Any?]();
let db = Firestore.firestore();

override func viewDidLoad() {
    super.viewDidLoad();
    let docRef = db.collection("colleges").document("UMD");
    var docData = "";
    docRef.getDocument {  ( document, error) in
        if error == nil {
            docData = document!.get("Name") as! String;

        } else{

        }
    }
    pickerData.append(docData);
    picker.delegate = self
    picker.dataSource = self
}

Because `getDocument` works asynchronously. Code doesn't run necessarily in order of the lines. And – unrelated – `Any?` is the worst type in Swift. — vadian, May 18 '20 at 19:39
http://www.programmingios.net/returning-a-value-from-asynchronous-code/ — matt, May 18 '20 at 19:53

score 0 · Accepted Answer · edited Jun 20 '20 at 09:12

0

It's because data is loaded from Firestore asynchronously, and your main code continues to run while that is happening. It's easiest to see by placing a few log statements:

let docRef = db.collection("colleges").document("UMD")
print("Before starting to get data")
docRef.getDocument {  ( document, error) in
    print("Got data")
}
print("After starting to get data")

When you run this code, it prints:

Before starting to get data

After starting to get data

Got data

This is probably not the order you expected, but it does completely explain why your code doesn't work. By the time your pickerData.append(docData) runs, the docData = document!.get("Name") as! String hasn't run yet.

For this reason, any code that needs data from the database needs to be inside the closure, or be called from there:

let docRef = db.collection("colleges").document("UMD")
var docData = ""
docRef.getDocument {  ( document, error) in
    if error == nil {
        docData = document!.get("Name") as? String ?? "No Name"

        pickerData.append(docData)
        picker.delegate = self
        picker.dataSource = self
    } else{

    }
}

Also see:

Asign value of a Firestore document to a variable
Using Firestore DB, how can I break out of a for loop inside of a snapshot listener when a certain condition is met?, which also shows using a custom closure to keep your own code outside of the getDocument closure.
Storing asynchronous Cloud Firestore query results in Swift, showing using a dispatch group to keep the code

edited Jun 20 '20 at 09:12

Community

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answered May 18 '20 at 19:42

Frank van Puffelen

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Downvoter: can you please explain why you downvoted? – Frank van Puffelen May 18 '20 at 20:53
Great answer! Not sure why it was downvoted. You don't need the ; after each line. Also safely unwrapping the optionals is probably a good idea `guard let doc = document else { return }` and then `let docData = doc.get("Name") as? String ?? "No Name"` – Jay May 19 '20 at 20:03
Hey Jay. Good point on the semicolons, I removed them. I'm leaving the hard-cast, as the closure is guaranteed to *either* get an error *or* a document. The hard-cast can only fail if that guarantee isn't met, which... uhm... isn't what guarantees are about. :) – Frank van Puffelen May 19 '20 at 20:59
Agreed! I wonder why the [docs](https://firebase.google.com/docs/reference/swift/firebasefirestore/api/reference/Type-Definitions.html#firdocumentsnapshotblock) show both parameters of the FIRDocumentSnapshotBlock as optional - I guess if there *is* an error then the snapshot could be nil? Anyhoo.. this part though `.get("Name")` needs it as if a document doesn't have a Name field, it will crash. – Jay May 19 '20 at 21:16
Oh, I don't like this API signature very much. Flutter has a single wrapper object for this type of async data, so that is *never* null. But I prefer two callbacks even better, like RTDBs `onDataChanged` and `onCancelled` (in Android, but similar logic for all platforms). Agreed on guarding the other cast, so I updated it in my answer. I'm clearly a polyglot, which means that I'm equally non-idiomatic in every language. :-D – Frank van Puffelen May 19 '20 at 21:58

Why can I not modify a variable declared outside a closure?

1 Answers1