自 iOS8 起，苹果就建议告警框使用 UIAlertController 来代替 UIAlertView 和 UIActionSheel。下面总结了一些常见的用法。

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    override func viewDidAppear(_ animated: Bool){
        super.viewDidAppear(animated)
        
        let alertController = UIAlertController(title: "系统提示",
                        message: "您确定要离开hangge.com吗？", preferredStyle: .alert)
        let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
        let okAction = UIAlertAction(title: "好的", style: .default, handler: {
            action in
            print("点击了确定")
        })
        alertController.addAction(cancelAction)
        alertController.addAction(okAction)
        self.present(alertController, animated: true, completion: nil)
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

2，除了弹出，还可以使用从底部向上滑出的样式
（注意：如果上拉菜单中有“取消”按钮的话，那么它永远都会出现在菜单的底部，不管添加的次序是如何）

let alertController = UIAlertController(title: "保存或删除数据", message: "删除数据将不可恢复",
                                        preferredStyle: .actionSheet)
let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
let deleteAction = UIAlertAction(title: "删除", style: .destructive, handler: nil)
let archiveAction = UIAlertAction(title: "保存", style: .default, handler: nil)
alertController.addAction(cancelAction)
alertController.addAction(deleteAction)
alertController.addAction(archiveAction)
self.present(alertController, animated: true, completion: nil)

3，按钮使用“告警”样式（文字颜色变红，用来来警示用户）

let okAction = UIAlertAction(title: "好的", style: .destructive, handler: nil)

4，添加任意数量文本输入框（比如可以用来实现个登陆框）

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    override func viewDidAppear(_ animated: Bool){
        super.viewDidAppear(animated)
        
        let alertController = UIAlertController(title: "系统登录",
                            message: "请输入用户名和密码", preferredStyle: .alert)
        alertController.addTextField {
            (textField: UITextField!) -> Void in
            textField.placeholder = "用户名"
        }
        alertController.addTextField {
            (textField: UITextField!) -> Void in
            textField.placeholder = "密码"
            textField.isSecureTextEntry = true
        }
        let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
        let okAction = UIAlertAction(title: "好的", style: .default, handler: {
            action in
            //也可以用下标的形式获取textField let login = alertController.textFields![0]
            let login = alertController.textFields!.first!
            let password = alertController.textFields!.last!
            print("用户名：\(login.text) 密码：\(password.text)")
        })
        alertController.addAction(cancelAction)
        alertController.addAction(okAction)
        self.present(alertController, animated: true, completion: nil)
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

5，使用代码移除提示框

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

6，提示框弹出后，过段时间自动移除
下面样例弹出一个不带按钮的消息提示框，过个两秒钟提示框自动消失。

let alertController = UIAlertController(title: "保存成功!",
                                        message: nil, preferredStyle: .alert)
//显示提示框
self.present(alertController, animated: true, completion: nil)
//两秒钟后自动消失
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2) {
    self.presentedViewController?.dismiss(animated: false, completion: nil)
}

附：扩展 UIAlertController 方便使用

从上面的样例可以发现，每次要弹出提示框都要创建一个 UIAlertController，然后添加 action 按钮，最后再通过对应的视图控制器 present 出来。我们可以对 UIAlertController 进行个扩展，把这些操作做个封装，方便使用。

1，UIAlertController扩展（UIAlertExtension.swift）

import UIKit

extension UIAlertController {
    //在指定视图控制器上弹出普通消息提示框
    static func showAlert(message: String, in viewController: UIViewController) {
        let alert = UIAlertController(title: nil, message: message, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "确定", style: .cancel))
        viewController.present(alert, animated: true)
    }
    
    //在根视图控制器上弹出普通消息提示框
    static func showAlert(message: String) {
        if let vc = UIApplication.shared.keyWindow?.rootViewController {
            showAlert(message: message, in: vc)
        }
    }
    
    //在指定视图控制器上弹出确认框
    static func showConfirm(message: String, in viewController: UIViewController,
                            confirm: ((UIAlertAction)->Void)?) {
        let alert = UIAlertController(title: nil, message: message, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "取消", style: .cancel))
        alert.addAction(UIAlertAction(title: "确定", style: .default, handler: confirm))
        viewController.present(alert, animated: true)
    }
    
    //在根视图控制器上弹出确认框
    static func showConfirm(message: String, confirm: ((UIAlertAction)->Void)?) {
        if let vc = UIApplication.shared.keyWindow?.rootViewController {
            showConfirm(message: message, in: vc, confirm: confirm)
        }
    }
}

2，使用样例

下面分别调用两种提示框：

//弹出普通消息提示框
UIAlertController.showAlert(message: "保存成功!")

//弹出确认选择提示框
UIAlertController.showConfirm(message: "是否提交?") { (_) in
    print("点击了确认按钮!")
}

运行效果如下：