>通过将UITextFieldDelegate添加到类声明中,使视图控制器成为UITextFieldDelegate.
>为您的文本字段和按钮添加IBOutlets.
>在viewDidLoad中,将按钮的isEnabled属性设置为false,并将self设置为textField.delegate.
>实现textField：shouldChangeCharactersInRange：replacementString：method.每次编辑文本字段时都会调用此方法.在那里,检查当前文本字段是否通过调用toInt()转换为Int,并根据需要启用/禁用按钮.

这是代码：

class ViewController: UIViewController,UITextFieldDelegate {

    @IBOutlet weak var textField: UITextField!
    @IBOutlet weak var button: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
        button.isEnabled = false
        textField.delegate = self
    }

    func textField(textField: UITextField,shouldChangeCharactersInRange range: NSRange,replacementString string: String) -> Bool {
        // Find out what the text field will be after adding the current edit
        let text = (textField.text as NSString).stringByReplacingCharactersInRange(range,withString: string)

        if let intVal = text.toInt() {
            // Text field converted to an Int
            button.isEnabled = true
        } else {
            // Text field is not an Int
            button.isEnabled = false
        }

        // Return true so the text field will be changed
        return true
    }
}

编辑：在Swift 3中,enabled属性更改为isEnabled.