Swift DispatchGroup导致错误：[Firebase / Firestore] [I-FST000001]无法到达Cloud Firestore后端

Question

I am using Firebase & Firestore to read in data using:

var itemData: [String:Any]?
let docRef = db.collection("store").document(documentID)

let group = DispatchGroup()
group.enter()
DispatchQueue.global().async {
    docRef.getDocument { (document, error) in
        if let document = document, document.exists {
            print("Document data: \(document.data() ?? ["key":"__data__"])")
            itemData = document.data()
        } else {
            print("Document does not exist")
        }
        group.leave()
    }
}
group.wait()
print("ITEM_DATA: \(itemData)")
return itemData

when I remove all references of any DispatchGroup, group.enter(), etc... the code runs okay, albeit not in the order that I want it to (ie it runs the print("ITEM_DATA: \\(itemData)") line with itemData as nil and only later prints out the result of print("Document data: \\(document.data() ?? ["key":"__data__"])") ). So I know that there isn't anything wrong with the actual request to Firebase.

However, when I try to fix the issue of into printing/returning itemData before I write to it, with the DispatchGroup as in the code above, when I run the function, the program gives me the error code:

[Firebase/Firestore][I-FST000001] Could not reach Cloud Firestore backend. Backend didn't respond within 10.000000 seconds.
This typically indicates that your device does not have a healthy Internet connection at the moment. The client will operate in offline mode until it is able to successfully connect to the backend.

The program never connects to Firestore after I get this message. I've tried this with simulators and actual devices both with and without wifi, but to no avail. I know it's not a problem with my internet connection because it works when I remove all the stuff with DispatchGroup, but I still can't figure out what the issue is or how to get around this problem.

Answer 1

Try entering and leaving the Dispatch Group being in a same Queue

let group = DispatchGroup()
DispatchQueue.global().async {
    group.enter()
    docRef.getDocument { (document, error) in
        if let document = document, document.exists {
            print("Document data: \(document.data() ?? ["key":"__data__"])")
            itemData = document.data()
        } else {
            print("Document does not exist")
        }
        group.leave()
    }
}
group.wait()
print("ITEM_DATA: \(itemData)")

In case you are wondering what have I changed in your method?

Take a look at enter statement moving inside DispatchQueue async closure

What might be causing the issue?

Entering a Dispatch Group being in different queue and leaving dispatch group while being in a different queue might be causing the issue

EDIT 1:

Hey, just tested your code and got it to work

let group = DispatchGroup()
    DispatchQueue.global().async {
        group.enter()
        sleep(10)
        debugPrint("Hello")
        group.leave()
    }
    group.notify(queue: DispatchQueue.main) {
        print("ITEM_DATA: )")
    }

Here is the out put sequence from console

1. "Hello"
2. ITEM_DATA: )

I know I am not using wait() and wait() should also work but little busy with my work so can't test with wait right now

EDIT 2:

Just realised that OP is intending to return a value from a async function using Dispatch group hence updating my answer for the same

You can't have a return statement from a async call no matter what logic you apply. Dispatch group will not help you here, all that you need here is a closure or block

func test(onCompletion completion: @escaping ([String:Any]) -> ()) {
    DispatchQueue.global().async {
        docRef.getDocument { (document, error) in
            if let document = document, document.exists {
                completion(document.data())
            }
        }
    }
}

And call it as

    self.test {[weak self] (data) in
        debugPrint(data)
    } 

Hope it helps