致命错误：展开一个可选值UIPageViewController时意外发现nil

Question

所以我想为我的应用程序开发演练屏幕。这是我到目前为止尝试过的。

我有3个视图控制器。

第一视图控制器具有一个UIView和两个按钮。 UIView用于显示滑动内容。第二视图控制器有两个文本字段，我想在其中显示一些信息。（DataViewController）第三视图控制器是UIPageViewController。

这里有一些代码可以更好地理解。

第一个ViewController的代码。

import UIKit

class ViewController: UIViewController{
    let dataSource = ["Heading 1","Heading 2","Heading 3","Heading 4"]
    @IBOutlet weak var welcomeContentView: UIView!
    var currentIndex = 0
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        configureWelcomeSlides()
    }
    func configureWelcomeSlides()
    {
        guard let welcomePageViewController = storyboard?.instantiateViewController(withIdentifier: String(describing: WelcomePageViewController.self)) as? WelcomePageViewController else{
            return
        }
        welcomePageViewController.delegate = self
        welcomePageViewController.dataSource = self

        addChild(welcomePageViewController)
        welcomePageViewController.didMove(toParent: self)
        welcomePageViewController.view.translatesAutoresizingMaskIntoConstraints = false
        welcomeContentView.addSubview(welcomePageViewController.view)

        let views: [String: Any] = ["pageView": welcomePageViewController.view]

        welcomeContentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-0-[pageView]-0-|",
                                                                         options: NSLayoutConstraint.FormatOptions(rawValue: 0),
                                                                         metrics: nil,
                                                                         views: views))
        welcomeContentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-0-[pageView]-0-|",
                                                                         options: NSLayoutConstraint.FormatOptions(rawValue: 0),
                                                                         metrics: nil,
                                                                         views: views))

        guard let startingViewController = detailViewControllerAt(index: currentIndex) else{
            return
        }
        welcomePageViewController.setViewControllers([startingViewController], direction: .forward, animated: true, completion: nil)
    }
    func detailViewControllerAt(index: Int) -> WelcomePageContentController?{

        if index >= dataSource.count || dataSource.count == 0 {
            return nil
        }

        guard let welcomePageContentController = storyboard?.instantiateViewController(withIdentifier: String(describing: WelcomePageContentController.self)) as? WelcomePageContentController
            else {
            return nil
        }
        welcomePageContentController.index = index
        welcomePageContentController.slideHeading!.text = dataSource[index]
        return welcomePageContentController
    }



}
extension ViewController: UIPageViewControllerDelegate, UIPageViewControllerDataSource {

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        return currentIndex
    }
    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return dataSource.count
    }
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        let welcomePageContentController  = viewController as? WelcomePageContentController

        guard var currentIndex2 = welcomePageContentController?.index else {
            return nil
        }
        currentIndex = currentIndex2

        if currentIndex2 == 0{
            return nil
        }
        currentIndex2 -= 1
        return detailViewControllerAt(index: currentIndex2)
    }
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        let welcomePageContentController  = viewController as? WelcomePageContentController

        guard var currentIndex2 = welcomePageContentController?.index else {
            return nil
        }
        if currentIndex2 == dataSource.count {
            return nil
        }
        currentIndex2 += 1
        currentIndex = currentIndex2
        return detailViewControllerAt(index: currentIndex2)
    }

}

DataViewController的代码（在我的情况下，它命名为WelcomePageContentViewController）。

import UIKit

class WelcomePageContentController: UIViewController{

    @IBOutlet weak var slideHeading: UITextView?
    @IBOutlet weak var slideText: UITextView?
    var displayText: String!
    var index : Int!
    override func viewDidLoad() {
        super.viewDidLoad()
        slideHeading!.text = displayText
        slideText!.text = displayText

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


    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destination.
        // Pass the selected object to the new view controller.
    }
    */

}

UIPageViewController的代码（在我的情况下，它命名为WelcomePageViewController）。

import UIKit

class WelcomePageViewController: UIPageViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

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

我在执行viewdidLoad（）方法后从WelcomePageContentController访问文本字段，但我的应用程序仍然崩溃。

错误消息：致命错误：展开一个可选值时意外发现nil。

在此行发生崩溃。welcomePageContentController.slideHeading！.text = dataSource [index]在ViewController类中。

方法：-

func detailViewControllerAt（index：Int）-> WelcomePageContentController？

附有故事板图片。

Xcode版本（版本10.1（10B61））斯威夫特（4.2）

Answer 1

当从情节提要板实例化视图控制器时，实际上尚未加载视图，并且尚未设置IBOutlet。似乎您正在试图强行访问尚未设置的UITextView，这会导致崩溃。

从情节提要实例化视图控制器后立即尝试添加welcomePageContentController.loadViewIfNeeded()：

guard let welcomePageContentController = storyboard?.instantiateViewController(withIdentifier: String(describing: WelcomePageContentController.self)) as? WelcomePageContentController
    else {
    return nil
}

welcomePageContentController.loadViewIfNeeded()

welcomePageContentController.index = index
welcomePageContentController.slideHeading!.text = dataSource[index]

这将加载视图并为您的IBOutlet填充非零值（如果它们确实已连接到情节提要，这将允许您在文本视图中设置文本。）>