限制UItextField的输入位数(比如：电话号码)

1 遵守UITextFieldDelegate代理，并设置代理

self.textField.delegate = self

2 实现以下这个方法

    func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
        let string = (textField.text! as NSString).stringByReplacingCharactersInRange(range, withString:string)
        if (self.textField == textField) {
            if string.characters.count > 10 {
                textField.text = (string as NSString).substringToIndex(11)
                return false
            }
        }
        return true
    }