Swift 键盘显示时Tableview滚动内容

我有一个带有文本字段和文本视图的表视图.我已经实现了苹果示例代码https://developer.apple.com/library/ios/documentation/StringsTextFonts/Conceptual/TextAndWebiPhoneOS/KeyboardManagement/KeyboardManagement.html中建议的代码

@IBOutlet var myTableView: UITableView
func keyboardWasShown (notification: NSNotification)
{
    println("keyboard was shown")
    var info = notification.userInfo
    var keyboardSize = info.objectForKey(UIKeyboardFrameBeginUserInfoKey).CGRectValue().size

    myTableView.contentInset = UIEdgeInsetsMake(0, 0, keyboardSize.height, 0)
    myTableView.scrollIndicatorInsets = myTableView.contentInset
}

func keyboardWillBeHidden (notification: NSNotification)
{
    println("keyboard will be hidden")
    myTableView.contentInset = UIEdgeInsetsZero
    myTableView.scrollIndicatorInsets = UIEdgeInsetsZero
}
  override func viewDidLoad() {

    super.viewDidLoad()

    NSNotificationCenter.defaultCenter().addObserver(self, selector: "keyboardWasShown:", name: UIKeyboardDidShowNotification, object: nil)
    NSNotificationCenter.defaultCenter().addObserver(self, selector: "keyboardWillBeHidden:", name: UIKeyboardWillHideNotification, object: nil)

}

当我点击滚动视图的"文本"时，它会刚好位于屏幕顶部上方，但当我松开键盘时，它仍会向上滚动.就像第一次之后不能修改insets属性一样.我犯了什么错？