这是示例代码：

import UIKit
import MessageUI

class ViewController: UIViewController, MFMailComposeViewControllerDelegate {

@IBAction func showEmail(sender : AnyObject) {
    var emailTitle = "Test Email"
    var messageBody = "This is a test email body"
    var toRecipents = ["a.nakhimov@gmail.com"]
    var mc: MFMailComposeViewController = MFMailComposeViewController()
    mc.mailComposeDelegate = self
    mc.setSubject(emailTitle)
    mc.setMessageBody(messageBody, isHTML: false)
    mc.setToRecipients(toRecipents)

    self.presentViewController(mc, animated: true, completion: nil)
}

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

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

func mailComposeController(controller:MFMailComposeViewController, didFinishWithResult result:MFMailComposeResult, error:NSError) {
    switch result.value {
    case MFMailComposeResultCancelled.value:
        NSLog("Mail cancelled")
    case MFMailComposeResultSaved.value:
        NSLog("Mail saved")
    case MFMailComposeResultSent.value:
        NSLog("Mail sent")
    case MFMailComposeResultFailed.value:
        NSLog("Mail sent failure: %@", [error.localizedDescription])
    default:
        break
    }
    self.dismissModalViewControllerAnimated(true)
    // self.dismissViewControllerAnimated(true, completion: nil)
}

}

当我按下按钮时，功能 showEmail 被执行并显示用于发送电子邮件的表格。如果单击“发送”，则一切正常-发送邮件，然后执行函数
mailComposeController 。NSLog显示标签 “邮件已发送”， 并再次出现初始屏幕。

如果我在发送邮件的对话框中，单击 “取消” 按钮，则对话框不会消失，函数 mailComposeController
不起作用，两个按钮-“发送”和“取消”，变为灰色，因此保持不变。

怎么了