(Swift) How to make async code, synchronously?

Question

I am making an application using Swift.
I want to take data from FireStore, but the problem is that the application goes on to the next job, before finishing retrieving data.
So I tried to use this "DispatchSemaphore" code, but this stops the application from working...
I want to know how to wait the getDocument closure to finish its task, and move to next.

func changeTerms(uniqueName: String) {
        print("colRef: \(colRef.document(uniqueName).path)")
        let semaphore = DispatchSemaphore(value: 0)
        colRef.document(uniqueName).getDocument { snapShot, err in
            if let err = err {
                self.makeAlerts(title: "Error", message: err.localizedDescription, buttonName: "OK")
            } else {
                if let doc = snapShot, doc.exists {
                    if let data = doc.data() {
                        if let terms = data[K.Fstore.data.attributes] as? [String] {
                            var temp: [String] = []
                            for attIdx in self.rankedAttributes {
                                temp.append(terms[attIdx])
                            }
                            self.attributeTerms = temp
                            print("attributeTerms: \(self.attributeTerms)")
                            self.showTitle()
                            self.showTerm()
                        }
                    }
                }
            }
            semaphore.signal()
        }
        semaphore.wait()
    }

The closure could start the next document get before it ends. — Ptit Xav
– Ptit Xav, Commented Apr 13, 2022 at 19:45
How to make async code, synchronously? Don't, please learn how asynchronous data processing works. — vadian
– vadian, Commented Apr 13, 2022 at 19:50

HunterLion · Accepted Answer · 2022-04-13 22:05:05Z

I ran into the same issue with Firebase, and I found the solution using withCheckedContinuation(). You can learn more about it here.

Your code could be:

let temp: [String] = await withCheckedContinuation { continuation in
        colRef.document(uniqueName).getDocument { snapShot, err in
            if let err = err {
                self.makeAlerts(title: "Error", message: err.localizedDescription, buttonName: "OK")
            } else {
                if let doc = snapShot, doc.exists {
                    if let data = doc.data() {
                        if let terms = data[K.Fstore.data.attributes] as? [String] {
                            var temp: [String] = []
                            for attIdx in self.rankedAttributes {
                                temp.append(terms[attIdx])
                            }
                            self.attributeTerms = temp
                            print("attributeTerms: \(self.attributeTerms)")
                            self.showTitle()
                            self.showTerm()
                        }
                    }
                }
            }
            continuation.resume(returning: temp)
        }
}

The execution will wait for the variable temp to get a value, before it moves on.

This is still an asynchronous code, running in the background, but it will run sequentially.

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