Question

尝试将数据从一个视图控制器MainScreenVC传递到具有协议和扩展名的另一个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（因为打印在第一个VC上操作后显示正确的数据）但不确定错误在哪里。

第一视图控制器

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ViewController

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
}

你必须意识到instantiateViewController创建了一个新的视图控制器 - 它不是对屏幕上显示的视图控制器的引用。在该代码中，您刚刚创建了一个全新的视图控制器，然后将其委托设置为self，但除此之外别无其他。

在不知道上下文的情况下，很难提出任何建议 - prepare(for:) segue可能是您想要设置委托的地方。无论如何，问题是你必须获得对屏幕上显示的控制器的引用，该引用应该是对这些事件的反应。

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

修改

因此，在看到您在link提供的最小工作示例后，我想我可以提供有关如何将该字符串添加到SecondVC的解决方案。

您的第一个带注释的视图控制器：

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) } } }

此处不需要代表，因为ViewController是将SecondVC推送到导航控制器的人 - 这意味着您可以直接在prepare(for:)中访问它，如您所见上方。

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 } }

故事板可以保持原样。

委托/协议将数据从一个视图控制器传递到另一个视图控制器

1 个答案: