UIPickerView隐藏时选择第一行

我做了一个工具栏 UIPickerView :

override func viewDidLoad() {
    pickerToolbar.barStyle = UIBarStyle.Default
    pickerToolbar.translucent = true
    pickerToolbar.tintColor = UIColor(red: 0.0, green: 112/255, blue: 186/255, alpha: 1.0)
    pickerToolbar.sizeToFit()

    let doneButton = UIBarButtonItem(title: "Done", style: UIBarButtonItemStyle.Plain, target: self, action: "hidePicker()")
    let spaceButton = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.FlexibleSpace, target: nil, action: nil)
    let cancelButton = UIBarButtonItem(title: "Cancel", style: UIBarButtonItemStyle.Plain, target: self, action: "hidePicker()")

    pickerToolbar.setItems([cancelButton, spaceButton, doneButton], animated: false)
    pickerToolbar.userInteractionEnabled = true

    typePicker.addSubview(pickerToolbar)
}

func hidePicker() {
    typePicker.hidden = true
}

但当我点击“完成”或“取消”按钮时,选择器选择第一行并关闭。我不想要这样的行为,我只想隐藏选择器并保留选定的值。

所以,我有两个问题:

1. 为什么? 普通选择器 表现得像这样?
2. 如何预防?

谢谢你的帮助!