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

Question

我循环遍历几个Urls，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().enter()来启动循环，一旦我将数据发送到 Storage 并获得absoluteString我.leave()用来开始下一次迭代的值 url 字符串。

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功能
3. 一旦进入 Storage 的.downloadURL(completion:...完成处理程序
4. 并且如果最终的 downloadURL?.absoluteString为 nil

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

由于 DispatchGroup() 挂在 处.enter()，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

在下面的评论中，问题@rmaddy 说“你需要调用leave是否成功”。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入带有完成处理程序的函数内，并使用 abool来决定是否应该sendDataFromDictToFirebaseDatabase()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)
}