即使有DispatchGroup问题也要从数组读取

Question

我在从Firebase集合中获取字符串并将其显示在集合视图中时遇到问题。

基本上，我相信firebase函数对于程序的执行速度不够快，所以我将其放入也不能够解决问题的DispatchGroup中

 //arrays of names and descriptions
    var names:[String] = []
    var descriptions: [String] = []

这是firebase函数，用于从firebase检索名称和描述

func firebase()
{
    //connection to firebase for the names and descriptions
    let db = Firestore.firestore()

    db.collection(test).getDocuments { (snapshot, err) in

        if let err = err {

            print("Error getting documents: \(err)")
        } else {
            for document in snapshot!.documents {
                let name = document.get("Name") as! String
                let description = document.get("Description") as! String
                //Add names and descriptions to the arrays
                self.names.append(name)
                self.descriptions.append(description)
            }
            for x in self.names{
                print(x)
            }
            for y in self.descriptions{
                print(y)
            }
        }
    }
}

这里是ViewDidLoad函数：当我不尝试打印名称[0]

时，它可以完美工作

override func viewDidLoad() {
    super.viewDidLoad()
    let myGroup = DispatchGroup()
    myGroup.enter()
    firebase()

    // When your task completes
    myGroup.leave()
    myGroup.notify(queue: DispatchQueue.main) {
        // do your remaining work
        print(self.names[1])
        self.setupBottomControls()
        self.setupTopControls()
        self.setupButton()

        self.setupImages()

        self.collectionView?.backgroundColor = .white
        self.collectionView?.register(PageCell.self, forCellWithReuseIdentifier: "cellId")

        self.collectionView?.isPagingEnabled = true
    }
}

我不断收到错误：

Fatal error: Index out of range

Answer 1

您正在滥用DispatchGroup。在这种情况下是不合适的。

只需添加完成处理程序

func firebase(completion: @escaping () -> Void)
{
    //connection to firebase for the names and descriptions
    let db = Firestore.firestore()

    db.collection(test).getDocuments { (snapshot, err) in

        if let err = err {

            print("Error getting documents: \(err)")
        } else {
            for document in snapshot!.documents {
                let name = document.get("Name") as! String
                let description = document.get("Description") as! String
                //Add names and descriptions to the arrays
                self.names.append(name)
                self.descriptions.append(description)
            }
            for x in self.names{
                print(x)
            }
            for y in self.descriptions{
                print(y)
            }
            completion()
        }
    }
}

并使用它

override func viewDidLoad() {
    super.viewDidLoad()

    firebase() { [unowned self] in
         DispatchQueue.main.async {
            // do your remaining work
            print(self.names[1])
            self.setupBottomControls()
            self.setupTopControls()
            self.setupButton()

            self.setupImages()

            self.collectionView?.backgroundColor = .white
            self.collectionView?.register(PageCell.self, forCellWithReuseIdentifier: "cellId")

            self.collectionView?.isPagingEnabled = true
        }
    }
}