ios - 如果 segue.identifier == "showChildFiles"当 segue 标识符看起来正确时，Swift 3 准备 segue 不通过

Question

我正在创建一个应用程序，家长可以在其中登录并查看有关其本地托儿中心的信息。但是，为登录运行 View Controller 的代码无法正常工作。一旦用户按下登录按钮，我希望我的代码使用标识符“ShowChildFiles”执行 segue。但是在 prepare(for segue: UIStoryboardSegue, sender: Any?) 函数中，我检查了正确的 segue 标识符 (if segue.identifier == "ShowChildFiles") 但是它没有传递这个 if 语句。我在失败时打印出标识符，它返回“ShowChildFiles”。

这是代码

import Foundation
import UIKit

class MyChildLoginViewController: UIViewController {

    @IBOutlet weak var usernameTextField: UITextField!

    @IBOutlet weak var passwordTextField: UITextField!

    @IBAction func loginButtonPressed(_ sender: Any) {

        var request = URLRequest(url: URL(string: "myLogin.php")!)
        request.httpMethod = "POST"
        let postString = "userName=" + usernameTextField.text! + "&password=" + passwordTextField.text!
        request.httpBody = postString.data(using: .utf8)
        let task = URLSession.shared.dataTask(with: request) { data, response, error in
            guard let data = data, error == nil else {                                                 // check for fundamental networking error
                print("error=\(error)")
                return
            }

            if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {           // check for http errors
                print("statusCode should be 200, but is \(httpStatus.statusCode)")
                print("response = \(response)")
            }

            let responseString = String(data: data, encoding: .utf8)
            print("responseString = \(responseString)")

            if(responseString == "true"){
                //Login successful
                DispatchQueue.main.async(//Not sure why but this has to be executed asynchronously
                    execute: {
                        self.performSegue(withIdentifier: "ShowChildFiles", sender: Any?.self)
                    })

            }else if(responseString == "false"){
                //incorrect password
            }else if(responseString == "incorrect username"){
                //We dont recognise this username
            }else{
                //something went wrong
            }
        }
        task.resume()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        let tap: UITapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(MyChildLoginViewController.dismissKeyboard))//Dismisses keyboard when screen tapped
        view.addGestureRecognizer(tap)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

     // MARK: - Navigation
     // In a storyboard-based application, you will often want to do a little preparation before navigation
     override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
     // Get the new view controller using segue.destinationViewController.
     // Pass the selected object to the new view controller.

        if  segue.identifier == "ShowChildFiles" ,
            let destination = segue.destination as? ChildFilesTableViewController {
            print("preparing for segue")
            destination.passedString = self.getFamID()
        }else{
            print(segue.identifier!)
            print("not going in the correct place")
        }

     }

    func getFamID() -> String{

        let cs: [Character] = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o","p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z", " "]
        /*var cs = Set(chars.characters)*/
        var user = [Character](usernameTextField.text!.lowercased().characters)
        var famID = ""
        for index in 0 ... user.count - 1 {
            if(!cs.contains(user[index])){
                famID = famID + String(user[index])
            }
        }
        return famID

    }
    //Calls this function when the tap is recognized.
    func dismissKeyboard() {
        //Causes the view (or one of its embedded text fields) to resign the first responder status.
        view.endEditing(true)
    }
}

Answer 1

因为您似乎已经通过日志证明 segue 标识符就是您认为的那样，"ShowChildFiles" — 事实上，它几乎不可能是其他情况，因为那是您使用的标识符首先要执行 segue — 必须得出结论，if 条件的失败部分是第二部分，let destination = segue.destination as? ChildFilesTableViewController。显然，segue 的目的地不是 ChildFilesTableViewController。 (为什么不将 print(type(of:segue.destination)) 作为日志的一部分，然后找出答案？)