1.例如四个文本框分别是

class ViewController: UIViewController,UITextFieldDelegate {
@IBOutlet weak var telphone: UITextField!
@IBOutlet weak var email: UITextField!
@IBOutlet weak var username: UITextField!
@IBOutlet weak var passwd: UITextField!
当你输入完第一个电话的时候，手机键盘上会有下一项，点击后，如果不做处理，是不会换到email输入框的其中“UITextFieldDelegate“需要手动加上。

2.在默认的viewDidLoad()下面添加
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
        telphone.delegate = self
email.delegate = self
        username.delegate = self
passwd.delegate = self
}

3.之后写一个函数实现光标聚焦在哪一行
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        if textField == self.telphone {
            telphone.resignFirstResponder()
            email.becomeFirstResponder()
        } else if textField == self.email {
            email.resignFirstResponder()
            username.becomeFirstResponder()
        } else if textField == self.username {
            username.resignFirstResponder()
            passwd.becomeFirstResponder()
        } else if textField == self.passwd {
            passwd.resignFirstResponder()
        }
        return true
    }