何时何地在 Swift 中关闭 UIAlertController？

Question

我正在调用一个执行 URLSession 的方法，但在它执行任何操作之前，会呈现一个 UIAlertController 阻塞 UI，直到获得来自请求的某种响应。逻辑告诉我，在主线程上调用该方法的完成块中取消 UIAlertController 将是最佳选择。我认为这是错误的吗？显然如此，所呈现的UIAlertController 确实会显示，但永远不会关闭。帮忙？

块：

getCostandIV { output in

            let cost = output["ask"] as! NSNumber
            let IV = output["IV"] as! NSNumber

            self.enteredCost = cost.stringValue
            self.enteredIV = IV.stringValue

            DispatchQueue.main.async {

                self.progress.dismiss(animated: true, completion: nil)
                self.tableView.reloadSections(IndexSet(integer: 1), with: UITableView.RowAnimation.none)
                self.canWeSave()

            }

        }

功能：

 func getCostandIV (completionBlock: @escaping (NSMutableDictionary) -> Void) -> Void {

    DispatchQueue.main.async {


        self.progress = UIAlertController(title: "Retrieving ask price and volatility...", message: nil, preferredStyle: UIAlertController.Style.alert)
        self.present(self.progress, animated: true, completion: nil)

    }

    guard let url = URL(string: "https://api.tdameritrade.com/v1/marketdata/chains?apikey=test&symbol=\(symbol)&contractType=\(type)&strike=\(selectedStrike)&fromDate=\(selectedExpiry)&toDate=\(selectedExpiry)") else {
        return
    }

    let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let dataResponse = data,
            error == nil else {
                //print(error?.localizedDescription ?? "Response Error")

                DispatchQueue.main.async {


                        self.presentedViewController?.dismiss(animated: true, completion: {

                        let alert = UIAlertController(title: "There was an error retrieving ask price and volatility.", message: "Please try again later.", preferredStyle: .alert)
                        alert.addAction(UIAlertAction(title: "OK", style: .default))
                        self.present(alert, animated: true)

                    })

                }

                return }
        do{
            //here dataResponse received from a network request
            let jsonResponse = try JSONSerialization.jsonObject(with:
                dataResponse, options: [])
            //                //print(jsonResponse) //Response result

            guard let jsonDict = jsonResponse as? NSDictionary else {
                return
            }
            //                //print(jsonDict)

            var strikeMap : NSDictionary = [:]

            if self.type == "CALL" {
                strikeMap = jsonDict["callExpDateMap"] as! NSDictionary

            } else {
                strikeMap = jsonDict["putExpDateMap"] as! NSDictionary

            }

            self.strikes.removeAllObjects()

            let inner = strikeMap.object(forKey: strikeMap.allKeys.first ?? "<#default value#>") as! NSDictionary
            let innerAgain = inner.object(forKey: inner.allKeys.first ?? "<#default value#>") as! NSArray
            let dict : NSDictionary = innerAgain[0] as! NSDictionary

            let dict2 = ["ask" : dict["ask"] as! NSNumber, "IV" : dict["volatility"] as! NSNumber] as NSMutableDictionary



            completionBlock(dict2)


        } catch let parsingError {
            print("Error", parsingError)
        }
    }
    task.resume()
}

编辑：使用self.presentedViewController?.dismiss(animated: true, completion: nil) 并不能解决问题。另外，self.progress的dismiss函数的完成块也没有被调用。

编辑 2： 回调中的关闭代码之前的presentViewController 为nil，即使在关闭之前在警报控制器上调用了present？

Answer 1

使用这个，为了解除你的警报，你应该在异步块中添加解除方法，并为此设置计时器，你应该告诉异步块从现在到 5 秒开始异步，然后做一些事情：

        alert.addAction(UIAlertAction(title: "ok", style: .default,
                                      handler: nil))
        viewController.present(alert, animated: true, completion: nil)

        // change to desired number of seconds (in this case 5 seconds)
        let when = DispatchTime.now() + 5
        DispatchQueue.main.asyncAfter(deadline: when){
            // your code with delay
            alert.dismiss(animated: true, completion: nil)
        }

Answer 2

如果你多次调用getCostandIV方法，第二个alert就不会出现，self.progress会有unpresented alert的引用。

改变

self.progress.dismiss(animated: true, completion: nil)

到

self.presentedViewController?.dismiss(animated: true, completion: nil)

Answer 3

只有在一切顺利的情况下，您的警报才会被解除。 我建议你把你的功能改成这样：

 func getCostandIV (completionBlock: @escaping (NSMutableDictionary?, Error?) -> Void) -> Void

并确保在您的guard 语句失败或抛出错误时调用您的completionBlock。在您当前的代码中，警报仅在网络请求失败时才解除，但在解析 JSON 时出现问题时不会解除。