swift - UIAlertView 没有出现

Question

大家好，当应用程序启动时，我无法让 LoginAlert 出现……应用程序构建成功，但警报 View 没有出现。我错过了什么？

提前感谢您提供任何和所有见解!

//  TimelineTableViewController.swift


import UIKit

class TimelineTableViewController: UITableViewController {


    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }


    override func viewDidAppear(animated: Bool) {
        if ((PFUser.currentUser()) != nil){
            var loginAlert:UIAlertController = UIAlertController(title: "Sign Up / Login",message: "Please Sign up or Login",
                preferredStyle: UIAlertControllerStyle.Alert)

            loginAlert.addTextFieldWithConfigurationHandler({
                textfield in
                textfield.placeholder = "Your username"
            })

            loginAlert.addTextFieldWithConfigurationHandler({
                textfield in
                textfield.placeholder = "Your password"
                textfield.secureTextEntry = true
             })

            loginAlert.addAction(UIAlertAction(title: "Login", style: UIAlertActionStyle.Default, handler:{
                alertAction in
                let textFields:NSArray = loginAlert.textFields! as NSArray
                let usernameTextfield:UITextField = textFields.objectAtIndex(0) as UITextField
                let passwordTextfield:UITextField = textFields.objectAtIndex(1) as UITextField

                PFUser.logInWithUsernameInBackground(usernameTextfield.text, password: passwordTextfield.text){
                    (user:PFUser!, error:NSError!)->Void in
                    if ((user) != nil){
                        println("Login successful")
                    }else{
                        println("Login Failed")
                    }

                }

            }))

            loginAlert.addAction(UIAlertAction(title: "Sign Up", style: UIAlertActionStyle.Default, handler:{
                alertAction in
                let textFields:NSArray = loginAlert.textFields! as NSArray
                let usernameTextfield:UITextField = textFields.objectAtIndex(0) as UITextField
                let passwordTextfield:UITextField = textFields.objectAtIndex(1) as UITextField

                var poster:PFUser = PFUser()
                poster.username = usernameTextfield.text
                poster.password = passwordTextfield.text

                poster.signUpInBackgroundWithBlock{
                    (success:Bool!, error:NSError!)->Void in
                    if !(error != nil){
                        println("Sign Up Successful")
                    }else{
                        let errorString = error.userInfo!["error"] as String
                        println(errorString)
                    }
                }

            }))

            self.presentViewController(loginAlert, animated: true, completion: nil)

        }
    }


    override func viewDidLoad() {
        super.viewDidLoad()

        // Uncomment the following line to preserve selection between presentations
        // self.clearsSelectionOnViewWillAppear = false

        // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
        // self.navigationItem.rightBarButtonItem = self.editButtonItem()
    }

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

    // MARK: - Table view data source

    override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
        // #warning Potentially incomplete method implementation.
        // Return the number of sections.
        return 0
    }

    override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        // #warning Incomplete method implementation.
        // Return the number of rows in the section.
        return 0
    }

Answer 1

使 loginAlert 成为一个类变量。 presentViewController 不保留对 alertController 的引用。因为它现在在一个局部变量中，它会在 viewDidAppear 结束时被删除。