ios - Is Firebase getDocument run line by line?

Question

I'm a swift and Firebase beginner, I met a problem that confuses me a lot. We all know the following code

var a = [Int]()
for i in 0..2{
    a.append(i)
    print(a)
print(a)

should give me the output

[0]
[0,1]
[0,1,2]
[0,1,2]

And it is true as the result. However, when I work on getDocument(), and I write a similar logic code, the output is strange! The code is the following.

override func viewDidLoad() {
    super.viewDidLoad()
    let docSecond = getThirtyData()
    getDataContent(thirtyData: docSecond)
}
func getThirtyData()->Array<Int>{
    var querySecond = [Int]()
    var docSecond = [Int]()
    let postDoc = db.collection("announcement")
    postDoc.getDocuments { (querySnapshot, err) in
        if let err = err{
            print("Error getting documents: (err)")
        }else{
            for document in querySnapshot!.documents{
                //print(document.data()["postTimeSecondInt"] as! Int)
                querySecond.append(document.data()["postTimeSecondInt"] as! Int)
            }
            for _ in 1...querySecond.count{
                let max = querySecond.max()!
                docSecond.append(max)
                let index = querySecond.firstIndex(of: max)
                querySecond.remove(at: index!)
            }
            print(docSecond)
            print("123")
        }
    }
    print(docSecond)
    print("456")
    return docSecond
}

func getDataContent(thirtyData: [Int]){
    print(thirtyData)
    print("789")
}

I thought the result might come as same logic which is "123" print first, then "456", then "789". However, the result is like below.

[]
456
[]
789
[1588428987, 1588428980, 1588427392]
123

Seems like it run the code line below simultaneously with the for loop. Can anyone explain why this happens?

Answer 1

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