doneClick 不能有 UITextField 类型的参数，当它用作 UIBarButtonItem 的目标时代码>.

doneClick cannot have parameter of UITextField type, when it's used as a target to UIBarButtonItem.

您将不得不编辑的不仅仅是该方法.工具栏需要知道它属于哪个 UITextField.因此，创建 UIToolbar 的自定义子类，您将在其中设置其项目，并向其添加一个委托，该委托将在完成和取消按下时发送事件:

You will have to edit more than just that method. The toolbar needs to know to which UITextField it belongs to. So create your custom subclass of UIToolbar, in which you will setup its items, and add a delegate to it which will be sent events on done and cancel pressed:

import UIKit

protocol AccessoryToolbarDelegate: class {
    func doneClicked(for textField: UITextField)
    func cancelClicked(for textField: UITextField)
}

class AccessoryToolbar: UIToolbar {

    fileprivate let textField: UITextField

    weak var accessoryDelegate: AccessoryToolbarDelegate?

    init(for textField: UITextField) {
        self.textField = textField
        super.init(frame: CGRect.zero)

        self.barStyle = .default
        self.isTranslucent = true
        self.tintColor = UIColor(red: 92/255, green: 216/255, blue: 255/255, alpha: 1)
        self.sizeToFit()

        let doneButton = UIBarButtonItem(title: "Done", style: .plain, target: self, action: #selector(doneClicked))
        let spaceButton = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
        let cancelButton = UIBarButtonItem(title: "Cancel", style: .plain, target: self, action: #selector(cancelClicked))
        self.setItems([cancelButton, spaceButton, doneButton], animated: false)
        self.isUserInteractionEnabled = true

        textField.inputAccessoryView = self
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    @objc fileprivate func doneClicked() {
        accessoryDelegate?.doneClicked(for: self.textField)
    }

    @objc fileprivate func cancelClicked() {
        accessoryDelegate?.cancelClicked(for: self.textField)
    }
}

现在使用此代码，您可以将旧代码重写为:

Now with this code you can rewrite your old code to:

import UIKit

class InvoiceViewController: UIViewController, AccessoryToolbarDelegate {
    var thePicker = UIDatePicker()

    @IBOutlet weak var dateIssueTextField: UITextField!
    @IBOutlet weak var dueDateTextField: UITextField!

    func doneClicked(for textField: UITextField) {
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .short
        dateFormatter.timeStyle = .none
        // I am not sure if you want this
        textField.text = dateFormatter.string(from: thePicker.date)

        // or rather this, but that's up to you to finish
        dateFormatter.dateFormat = "yyyy-MM-dd"
        let finalDate: String = dateFormatter.string(from: thePicker.date)
        print(finalDate)

        textField.resignFirstResponder()
    }

    func cancelClicked(for textField: UITextField) {
        textField.resignFirstResponder()
    }

    func setUpTextFieldPicker(textField: UITextField) {
        // DatePicker
        self.thePicker = UIDatePicker(frame:CGRect(x: 0, y: 0, width: self.view.frame.size.width, height: 216))
        self.thePicker.backgroundColor = UIColor.white
        self.thePicker.datePickerMode = UIDatePickerMode.date
        // dateIssueTextField.inputView = thePicker
        // dueDateTextField.inputView = thePicker
        textField.inputView = thePicker

        // ToolBar
        let toolbar = AccessoryToolbar(for: textField)
        // let's not forget to set the delegate
        toolbar.accessoryDelegate = self
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        setUpTextFieldPicker(textField: dateIssueTextField)
        setUpTextFieldPicker(textField: dueDateTextField)
    }
}