ios - 委托(delegate)/协议(protocol)将数据从一个 View Controller 传递到另一个

Question

尝试使用协议(protocol)和扩展将数据从一个 View Controller MainScreenVC 传递到另一个 RatesVC，但这不起作用，应用程序每次都崩溃。我清楚地看到第二个 VC 上的代码存在问题(因为在第一个 VC 上执行操作后打印显示正确的数据)但不确定哪里出错了。

StoryBoard and 1st VC Example Second VC

第一个 View Controller

import UIKit

protocol transferNameOfCurrency {
    func currencySelected(nameOfCurrency: String)
}

class MainScreenVC: UIViewController {

    var transferCurrencyDelegate: transferNameOfCurrency?
    var nameOfTheCurrency: String?

    @IBAction func updateRates(_ sender: Any) {
        nameOfTheCurrency = "EUR"
        transferCurrencyDelegate?.currencySelected(nameOfCurrency: 
nameOfTheCurrency)
        print(nameOfTheCurrency)

    }
}

第二个 View Controller

import UIKit

class RatesVC: UIViewController {
    var currencySelected: String?

    override func viewDidLoad() {
        super.viewDidLoad()

        if let push = self.storyboard?.instantiateViewController(withIdentifier: "MainScreenVC") as? MainScreenVC
        {
            push.transferCurrencyDelegate = self
        }

         // Do any additional setup after loading the view.
    }
}

extension RatesVC: transferNameOfCurrency {
    func currencySelected(nameOfCurrency: String) {
       currencySelected = nameOfCurrency
       print(currencySelected)
    }

}

Answer 1

最明显的问题在于:

if let push = self.storyboard?.instantiateViewController(withIdentifier: "MainScreenVC") as? MainScreenVC {
    push.transferCurrencyDelegate = self
}

您必须意识到 instantiateViewController 创建了一个新 View Controller - 它不是对显示在屏幕上的 View Controller 的引用。在该代码中，您刚刚创建了一个全新的 View Controller ，然后将其委托(delegate)设置为 self，除此之外别无其他。

在不知道上下文的情况下，很难提出任何建议 - prepare(for:) segue 可能是您要设置委托(delegate)的地方。无论如何，问题是您必须获得对屏幕上显示的 Controller 的引用，该 Controller 应该对这些事件使用react。

此外，从内存管理方面来说，您真的应该考虑将 delegate 属性设置为 weak 属性以防止内存泄漏。

编辑

所以在看到您在 link 提供的最小工作示例之后，我想我可以提供有关如何将该字符串获取到 SecondVC 的解决方案。

你的第一个带有评论的 View Controller :

import UIKit

class ViewController: UIViewController {

    var newLine: String = "EUR"

    @IBAction func push(_ sender: Any) {
        // here the secondVC does not exist yet, calling delegate.transferWord() here would have no sense
        // performSegue will create that secondVC, but now it does not exist, nor it is set up as the delegate
        self.performSegue(withIdentifier: "ViewController", sender: navigationController)
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let secondVC = segue.destination as? SecondVC, segue.identifier == "ViewController" {
            // at this moment secondVC did not load its view yet, trying to access it would cause crash
            // because transferWord tries to set label.text directly, we need to make sure that label
            // is already set (for experiment you can try comment out next line)
            secondVC.loadViewIfNeeded()
            // but here secondVC exist, so lets call transferWord on it
            secondVC.transferWord(word: newLine)
        }
    }
}

这里不需要委托(delegate)，因为你的 ViewController 是将 SecondVC 推送到 Navigation Controller 的那个 - 这意味着你可以在 prepare 中直接访问它(for:)，如上所示。

现在 SecondVC 非常简单(我省略了不必要的代码):

import UIKit

class SecondVC: UIViewController {

    @IBOutlet weak var label: UILabel!

    func transferWord(word: String) {
        label.text = word
    }
}

Storyboard可以保持原样。