Swift iOS - 如何从管理循环中取消 DispatchGroup()

Question

我遍历几个Url ，将它们转换为Data ，然后将数据发送到Firebase Storage ，然后在一切完成后将收集到的信息发送到Firebase Database

我使用 DispatchGroup() 的.enter()开始循环，一旦我将数据发送到 Storage 并获得值 url 字符串absoluteString我使用.leave()开始下一次迭代。

我意识到在循环过程中，有几个点可能会发生错误：

1. 一旦进入UrlSession
2. 一旦进入 Storage 的.putData函数
3. 一旦进入 Storage 的.downloadURL(completion:...完成处理程序
4. 如果最终 downloadURL 的?.absoluteString为零

如果我在其中任何一点出现错误，我会显示一个警报函数showAlert( ) 来显示警报，并使用session.invalidateAndCancel()取消 UrlSession。 我取消了一切，因为我希望用户重新开始。

由于 DispatchGroup() 留在.enter() ，我如何取消 DispatchGroup() 以停止循环？

var urls = [URL]()
var picUUID = UUID().uuidString
var dict = [String:Any]()

let session = URLSession.shared
let myGroup = DispatchGroup()
var count = 0

for url in urls{

    myGroup.enter()
    session.dataTask(with: url!, completionHandler: {
            (data, response, error) in

            if error != nil { 
                self.showAlert() // 1st point of error
                return 
            }

            DispatchQueue.main.async{
                self.sendDataToStorage("\(self.picUUID)_\(self.count).jpg", picData: data)
                self.count += 1
            }
    }).resume()

    myGroup.notify(queue: .global(qos: .background) {
        self.sendDataFromDictToFirebaseDatabase()
        self.count = 0
        self.session.invalidateAndCancel()
   }
}

func sendDataToStorage(_ picId: String, picData: Data?){

    dict.updateValue(picId, forKey:"picId_\(count)")

    let picRef = storageRoot.child("pics")
    picRef.putData(picData!, metadata: nil, completion: { (metadata, error) in

        if error != nil{
            self.showAlert()  // 2nd point of error
            return
        }

        picRef?.downloadURL(completion: { (url, error) in

            if error != nil{
                self.showAlert()  // 3rd point of error
                return
            }

            if let picUrl = url?.absoluteString{

               self.dict.updateValue(picUrl, forKey:"picUrl_\(count)")
               self.myGroup.leave() //only leave the group if a Url string was obtained
            }else{
               self.showAlert()  // 4th point of error
            }
        })
    })
}

func showAlert(){
    // the DispatchGroup() should get cancelled here
    session.invalidateAndCancel()
    count = 0
    UIAlertController...
}

func sendDataFromDictToFirebaseDatabase(){
}

Answer 1

在问题下方的评论中，@rmaddy 说“无论成功与否，您都需要leave ”。 我这样做了，但循环仍然运行并且sendDataFromDictToFirebaseDatabase()仍然触发事件，尽管有错误。

我能找到的唯一解决方法是将循环放在一个带有完成处理程序的函数中，并使用bool来决定sendDataFromDictToFirebaseDatabase()是否应该触发：

var urls = [URL]()
var picUUID = UUID().uuidString
var dict = [String:Any]()

let session = URLSession.shared
let myGroup = DispatchGroup()
var count = 0
var wasThereAnError = false // use this bool to find out if there was an error at any of the error points

func loopUrls(_ urls: [URL?], completion: @escaping ()->()){
    
    for url in urls{
        
        myGroup.enter()
        session.dataTask(with: url!, completionHandler: {
            (data, response, error) in
            
            if error != nil {
                self.showAlert() // 1st point of error. If there is an error set wasThereAnError = true
                return
            }
            
            DispatchQueue.main.async{
                self.sendDataToStorage("\(self.picUUID)_\(self.count).jpg", picData: data)
                self.count += 1
            }
        }).resume()
        
        myGroup.notify(queue: .global(qos: .background) {
            completion()
        }
    }
}

// will run in completion handler
func loopWasSuccessful(){
    
    // after the loop finished this only runs if there wasn't an error
    if wasThereAnError == false {
        sendDataFromDictToFirebaseDatabase()
        count = 0
        session.invalidateAndCancel()
    }
}

func sendDataToStorage(_ picId: String, picData: Data?){
    
    dict.updateValue(picId, forKey:"picId_\(count)")
    
    let picRef = storageRoot.child("pics")
    picRef.putData(picData!, metadata: nil, completion: { (metadata, error) in
        
        if error != nil{
            self.showAlert()  // 2nd point of error. If there is an error set wasThereAnError = true
            return
        }
        
        picRef?.downloadURL(completion: { (url, error) in
            
            if error != nil{
                self.showAlert()  // 3rd point of error. If there is an error set wasThereAnError = true
                return
            }
            
            if let picUrl = url?.absoluteString{
                
                self.dict.updateValue(picUrl, forKey:"picUrl_\(count)")
                self.myGroup.leave() // leave group here if all good on this iteration
            }else{
                self.showAlert()  // 4th point of error. If there is an error set wasThereAnError = true
            }
        })
    })
}

func showAlert(){
    wasThereAnError = true // since there was an error set this to true
    myGroup.leave() // even though there is an error still leave the group
    session.invalidateAndCancel()
    count = 0
    UIAlertController...
}

func sendDataFromDictToFirebaseDatabase(){
}

并使用它：

@IBAction fileprivate func postButtonPressed(_ sender: UIButton) {    

    wasThereAnError = false // set this back to false because if there was an error it was never reset

    loopUrls(urls, completion: loopWasSuccessful)
}