How to solve this closure completion handler issue?

Question

I have been stuck for almost two days death staring into these functions and want to know why I can't get my desired result.

Here's the first function:

   func getTheSchoolID(completion: @escaping ((String?)-> ())) {
    db.collection("school_users").whereField("userID", isEqualTo: user!.uid).getDocuments { (querySnapshot, err) in
        if let err = err {
            print("There was an error fetching the documents: \(err)")

        } else {
            self.skoolID = querySnapshot!.documents.map { document in
                return SchoolID(schoolID: (document.get("school_id") as! String))
            }
        }
    }

    let fixedID = "\(skoolID)"
    let substrings = fixedID.dropFirst(28).dropLast(3)
    let realString = String(substrings)
    completion(realString)

}

This was the function in an answer given to my last StackOverflow question so I used it and sort of understand the closure completion handler concept.

This next block of code is what I want to happen but doesn't happen.

    getTheSchoolID { [weak self] (realString) in
        if let id = realString {
            DispatchQueue.main.async {
                self?.schoolIDTextF.text = id

            }
        }
    }

Now when i check the textfield in the Viewcontroller it's still empty.

The collection path is fine the field is correct I'll even add a screenshot of my firestore database for reference.

If anyone can help that would be great. I'll even send a $10 Paypal payment if you give me the answer and it works. That's how much it means to me. Thanks.

Answer 1

getDocuments works asynchronously, you have to call completion inside the closure

func getTheSchoolID(completion: @escaping (String?) -> Void) {
    db.collection("school_users").whereField("userID", isEqualTo: user!.uid).getDocuments { (querySnapshot, err) in
        if let err = err {
            print("There was an error fetching the documents: \(err)")
            completion(nil)
        } else {
            self.skoolID = querySnapshot!.documents.map { document in
                return SchoolID(schoolID: document.get("school_id") as! String)
            }
            let fixedID = "\(self.skoolID)"
            let substrings = fixedID.dropFirst(28).dropLast(3)
            let realString = String(substrings)
            completion(realString)
        }
    }
}

And you are using too many parentheses.

And the map expression looks pretty weird.