Cocoa Swift 用protocol与delegate在两个ViewController间回传数据失败怎么办

• 从FirstViewController跳转至SecondViewController时，未给新创建的SecondViewController实例的delegate属性赋值，代理默认值为nil，后续回调逻辑无法触发
• SecondViewController点击返回按钮时，先执行了页面跳转再调用代理方法，跳转后当前Second实例可能被释放，weak修饰的delegate会自动置空，同时你重新实例化了一个全新的FirstViewController，和之前跳转前的First实例不是同一个对象，就算回调触发也不会修改原有页面的UI

FirstViewController.swift #

//  FirstViewController.swift
import Cocoa

class FirstViewController: NSViewController, DataEnteredDelegate {
    @IBOutlet weak var buttonPasswordState: NSView!
    @IBOutlet weak var label: NSTextField!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func buttonPasswordState(_ sender: Any) {
        guard let controller = self.storyboard?.instantiateController(withIdentifier: "SecondViewController") as? SecondViewController else { return }
        // 给SecondViewController的代理赋值为当前First实例
        controller.delegate = self
        self.view.window?.contentViewController = controller
    }
    
    func userDidEnterInformation(info: String) {
        label.stringValue = info
        // 可添加日志确认回调触发
        print("接收到回传数据: \(info)")
    }
}

SecondViewController.swift #

//  SecondViewController.swift
import Cocoa

protocol DataEnteredDelegate: class {
    func userDidEnterInformation(info: String)
}

class SecondViewController: NSViewController {
    @IBOutlet weak var buttonContinue: NSButton!
    weak var delegate: DataEnteredDelegate? = nil
    @IBOutlet weak var textField: NSTextField!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func buttonContinue(_ sender: Any) {
        // 先执行代理回调传递数据，再执行页面跳转
        delegate?.userDidEnterInformation(info: textField.stringValue)
        // 不需要重新实例化FirstViewController，直接切换回原有页面即可，若为模态弹出场景可直接调用dismiss
        guard let controller = self.storyboard?.instantiateController(withIdentifier: "FirstViewController") as? FirstViewController else { return }
        self.view.window?.contentViewController = controller
    }
}

可在代理调用位置和代理实现位置增加打印日志，若调用代理前打印delegate值为nil，说明跳转前的代理赋值逻辑未生效，可检查Storyboard中ViewController的Identifier是否正确配置。

内容的提问来源于stack exchange，提问作者iosSwift