Swift 尝试获取键盘帧时出现下标错误？

我正在尝试从键盘通知中获取键盘帧。但是我得到的错误是[NSObject:AnyObject]没有任何成员资格下标

我正在使用以下代码

  if keyboardNotification != nil {

        let userInfo = keyboardNotification!.userInfo


        let animationDuration: NSTimeInterval = (userInfo[UIKeyboardAnimationDurationUserInfoKey] as NSNumber).doubleValue()

        let keyboardScreenBeginFrame = (userInfo[UIKeyboardFrameBeginUserInfoKey] as NSValue).CGRectValue()

        let keyboardScreenEndFrame = (userInfo[UIKeyboardFrameEndUserInfoKey] as NSValue).CGRectValue()
    }

notification.userInfo是[NSObject:AnyObject]

那么，试试这个

if let userInfo = keyboardNotification.userInfo {

    let animationDuration        = (userInfo[UIKeyboardAnimationDurationUserInfoKey] as! NSNumber).doubleValue()
    let keyboardScreenBeginFrame = (userInfo[UIKeyboardFrameBeginUserInfoKey] as! NSValue).CGRectValue()
    let keyboardScreenEndFrame   = (userInfo[UIKeyboardFrameEndUserInfoKey] as! NSValue).CGRectValue()

}