Load html text in WKWebView

Question

I use this code to load my html file with text in WKWebView:

do {
   guard let filePath = Bundle.main.path(forResource: "\(readBookNumber)", ofType: "html")
       else { 
           print ("File reading error")
           return
       }
   var content =  try String(contentsOfFile: filePath, encoding: .utf8)
   let baseUrl = URL(fileURLWithPath: filePath)
            
   content.changeHtmlStyle(font: "Iowan-Old-Style", fontSize:  UserDefaults.standard.integer(forKey: "textSize"), fontColor: textColor)
   webView.loadHTMLString(headerString+content, baseURL: baseUrl)
}
catch {
    print ("File HTML error")
}

and this code to load the page where the user stopped reading last time:

self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))

I use code for loading last page in this method:

func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
         self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))
    }
}

At first I used deadline: .now() + 0.1, but that didn't work. Because the last read page was loaded initially, and after a few seconds I see my text on the first page. I change it to deadline: .now() + 0.5 and the text loads fine from the last page read. Its was 700 pages. But now I want to load another text with 1700 pages. And I have same problem like first time. I can change deadline: .now() + 1.0 and my text will load fine. But I think this is not the best solution. I run it on my iPhone X. But maybe if I run it on iPad mini 2 I should change deadline: .now() + 10.0 because iPad mini 2 not very powerful. How to solve the problem?

Update based on @DPrice code:

If I use this code:

override func viewDidLoad() {
    super.viewDidLoad()
    webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)

....
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == "estimatedProgress") {
        if webView.estimatedProgress == 1.0 {
            self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad\(self.readBookNumber)"))
        }
    }
}

I have same bad result like in my code.

But if I use this code:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == "estimatedProgress") {
        if webView.estimatedProgress == 1.0 {
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
                self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad\(self.readBookNumber)"))
            }
        }
    }
}

Everything works fine. And my last page loading fine. But it does not solve the problem in my question.

Answer 1

Here is a modified version of your ViewController class:

import UIKit
import WebKit

class ViewController: UIViewController, UIScrollViewDelegate, WKNavigationDelegate {
    
    @IBOutlet weak var webView: WKWebView!
    @IBOutlet weak var pagesLabel: UILabel!
    
    var readBookNumber = 0
    let headerString = "<meta name=\"viewport\" content=\"initial-scale=1.0\" />"
    var textSize = 3

    var contentSize: CGSize = .zero

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Web View Delegate
        
        webView.scrollView.delegate = self
        webView.navigationDelegate = self
        
        webView.scrollView.isPagingEnabled = true
        webView.scrollView.alwaysBounceVertical = false
        webView.scrollView.showsHorizontalScrollIndicator = true
        webView.scrollView.showsVerticalScrollIndicator = false
        webView.scrollView.panGestureRecognizer.isEnabled = false
        webView.scrollView.pinchGestureRecognizer?.isEnabled = false
        webView.scrollView.bouncesZoom = false
        
        self.webView.isOpaque = false;
        self.webView.backgroundColor = .clear
        
        // Load File
        
        do {
            guard let filePath = Bundle.main.path(forResource: "0", ofType: "html")
                else {
                    print ("File reading error")
                    return
                }
            var content =  try String(contentsOfFile: filePath, encoding: .utf8)
            let baseUrl = URL(fileURLWithPath: filePath)
            
            content.changeHtmlStyle(font: "Iowan-Old-Style", fontSize: 4, fontColor: "black")
            webView.loadHTMLString(headerString+content, baseURL: baseUrl)
            
            // add content size Observer
            webView.scrollView.addObserver(self, forKeyPath: #keyPath(UIScrollView.contentSize), options: .new, context: nil)

        }
        catch {
            print ("File HTML error")
        }
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if (keyPath == #keyPath(UIScrollView.contentSize)) {
            let contentSize = webView.scrollView.contentSize
            if contentSize != self.contentSize {
                self.contentSize = contentSize
                DispatchQueue.main.async {
                    self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))
                }
            }
        }
    }

    // MARK: - webView Scroll View
    
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        self.stoppedScrolling()
    }

    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if !decelerate {
            self.stoppedScrolling()
        }
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        var currentPage = Int((webView.scrollView.contentOffset.x / webView.scrollView.frame.size.width) + 1)
        let pageCount = Int(webView.scrollView.contentSize.width / webView.scrollView.frame.size.width)
        
        if currentPage == 0 {
            currentPage = 1
        } else {
            
        }
        
        if !webView.isHidden {
            pagesLabel.text = "\( currentPage ) из \( pageCount )"
        } else {
            pagesLabel.text = ""
        }
    }
    
    func scrollViewWillBeginZooming(_ scrollView: UIScrollView, with view: UIView?) {
        webView.scrollView.pinchGestureRecognizer?.isEnabled = false
    }

    func stoppedScrolling() {
        let pageToLoad = Int((webView.scrollView.contentOffset.x))
        UserDefaults.standard.set(pageToLoad, forKey: "pageToLoad")
    }
    
    // MARK: - loading webView

    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
    }
    
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        
        // Маленькая задержка, которую мне хотелось бы использовать
        
        /*DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
            self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))
        }*/
        
        // Большая задержка, которую мне приходится использовать

        // don't do this here... we'll do the "auto-scroll" inside the change contentSize Observer
        //DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
        //    self.webView.scrollView.contentOffset.x = CGFloat(UserDefaults.standard.integer(forKey: "pageToLoad"))
        //}
    }

    func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error)  {
    }


}

extension String {
  mutating func changeHtmlStyle(font: String, fontSize: Int, fontColor: String) {
    let style = "<font face='\(font)' size='\(fontSize)' color= '\(fontColor)'>%@"
    self = String(format: style, self)
  }
}

It uses an Observer to watch the contentSize change in the web view's scroll view.

Note that it is called multiple times - with different values - during the load and layout process, but it may do the job for you.

Also note, though, that you'll need to account for changes in the web view size - for example, if the user rotates the device. So... more to do, but this may get you going.

Answer 2

Instead of observing WKWebView.estimatedProgress you should observe UIScrollView.contentSize because you need to scroll to an available position e.g.:

var positionY: CGFloat = 1000
var contentSize = CGSize(width: 0, height: 0)

override func viewDidLoad() {
    super.viewDidLoad()
    ...
    webView?.scrollView.addObserver(self, forKeyPath: #keyPath(UIScrollView.contentSize), options: .new, context: nil)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == #keyPath(UIScrollView.contentSize)) {
        if let contentSize = webView?.scrollView.contentSize, contentSize != self.contentSize {
            self.contentSize = contentSize
            if contentSize.height > positionY {
                webView?.scrollView.setContentOffset(CGPoint(x: 0, y: positionY), animated: true)
            }
        }
    }
}

Answer 3

You can add a property observer and watch the estimated progress of the page load:

override func viewDidLoad() {
    super.viewDidLoad()
    webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)

....
}

and observe when the page is being loaded:

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if (keyPath == "estimatedProgress") {
        if webView.estimatedProgress == 1.0 {
            print ("page loaded")
        }
    }
}

You may be able to predict based on the page number how far into the loading process you need to be before you set your offset.