使用 UIPageViewController 的一些错误

Question

I tying to use this code to create reader like any ebook readers with using UIPageViewController but I have three errors in code (marked with comments):

1. Expression resolves to an unused property Initializer for conditional
2. binding must have Optional type, not 'TextDataViewController' 'nil' is
3. incompatible with return type 'TextDataViewController'

How to fix it?

This is my code:

class ReadViewController: UIViewController, NSLayoutManagerDelegate {
    
    @IBOutlet weak var contentView: UIView!
    let dataSource = ["1","2","3","4"]
    var currentViewControllerIndex = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        configurePageViewController()
    }
    
    func configurePageViewController() {
        guard let pageViewController = storyboard?.instantiateViewController(withIdentifier: String(describing: TextPageViewController.self)) as? TextPageViewController else {
            return
        }
        
        pageViewController.delegate.self // error: Expression resolves to an unused property 
        pageViewController.dataSource .self // error: Expression resolves to an unused property 

        
        addChildViewController(pageViewController)
        pageViewController.didMove(toParentViewController: self)
        
        pageViewController.view.translatesAutoresizingMaskIntoConstraints = false
        
        contentView.addSubview(contentView)
        
        let views: [String: Any] = ["pageView": pageViewController.view]
        
        contentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-0-[pageView]-0-|",
                                                                 options: NSLayoutConstraint.FormatOptions(rawValue: 0),
                                                                 metrics: nil,
                                                                 views: views))
        
        contentView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-0-[pageView]-0-|",
                                                                 options: NSLayoutConstraint.FormatOptions(rawValue: 0),
                                                                 metrics: nil,
                                                                 views: views))
        
        guard let startingViewController = detailViewControllerAt(index: currentViewControllerIndex) else{  // error: Initializer for conditional binding must have Optional type, not 'TextDataViewController'

            return
        }
        
        pageViewController.setViewControllers([startingViewController]?, direction: .forward, animated: true)
    }
    
    func detailViewControllerAt(index: Int) -> TextDataViewController {
        
        if index >= dataSource.count || dataSource.count == 0 {
            return nil // error: 'nil' is incompatible with return type 'TextDataViewController'
        }
        
        guard let dataViewController = storyboard?.instantiateViewController(withIdentifier: String(describing: TextDataViewController.self)) as? TextDataViewController else {
            return nil // error: 'nil' is incompatible with return type 'TextDataViewController'
        }
        
        dataViewController.index = index
        dataViewController.displayText = dataSource[index]
        
        return dataViewController
    }
    
}

extension ReadViewController: UIPageViewControllerDelegate, UIPageViewControllerDataSource {

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {

        print ("presentationIndex \(currentViewControllerIndex)")

        return currentViewControllerIndex
    }

    func presentationCount(for pageViewController: UIPageViewController) -> Int {

        print ("presentationCount \(dataSource.count)")

        return dataSource.count
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        let dataViewController = viewController as? TextDataViewController

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

        print ("viewControllerBefore \(currentIndex)")

        if (currentIndex == 0) {
            return nil
        }

        currentIndex -= 1

        currentViewControllerIndex = currentIndex

        return detailViewControllerAt(index: currentIndex)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        let dataViewController = viewController as? TextDataViewController

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

        print ("viewControllerBefore \(currentIndex)")

        if currentIndex == dataSource.count-1 {
            return nil
        }

        currentIndex += 1

        currentViewControllerIndex = currentIndex

        return detailViewControllerAt(index: currentIndex)
    }
}

Answer 1

Change your code like below

pageViewController.delegate = self
pageViewController.dataSource = self

func detailViewControllerAt(index: Int) -> TextDataViewController? {

You are trying to return nil but your function return type is not nullable, or Optional in Swift.