[英]how to use delegate to pass data to from one View Controller to another?
[英]Delegate/Protocols Passing data from one view controller to another
试图通过协议和扩展名将数据从一个视图控制器MainScreenVC传递到另一个RatesVC,但这无法正常工作,每次应用程序崩溃。 我清楚地看到第二个VC上的代码有问题(因为在第一个VC上执行操作后打印显示正确的数据),但是不确定哪里出了错误。
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)
}
}
最明显的问题在这里:
if let push = self.storyboard?.instantiateViewController(withIdentifier: "MainScreenVC") as? MainScreenVC {
push.transferCurrencyDelegate = self
}
您必须意识到instantiateViewController
会创建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 prepare(for:)
segue可能是您要设置委托的地方。 无论如何,问题在于您必须获得对屏幕上显示的控制器的引用,该控制器应该对这些事件做出反应。
此外,从内存管理方面,您应该真正考虑将delegate
属性设置为weak
属性,以防止内存泄漏。
编辑
因此,在看到您在link上提供的最小工作示例之后,我想我可以提供有关如何将字符串获取到SecondVC
的解决方案。
您的第一个带有评论的视图控制器:
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)
}
}
}
这里不需要委托,因为ViewController
是将SecondVC
推向Navigation控制器的代表-这意味着您可以直接在prepare(for:)
访问它,如您在上面看到的。
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。
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