WKWebView that resizes itself

StackableWebView.swift

import Foundation
import WebKit

// See https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html for more details.
private var observerContext = 0

class StackableWebView: WKWebView {
    
    // Keep track of height which will change when the view is loaded.
    var webViewHeight: CGFloat = 0.0
    
    init(frame: CGRect) {
        let config = WKWebViewConfiguration()
        let preferences = WKPreferences()
        config.preferences = preferences
        
        super.init(frame: frame, configuration: config)
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }
    
    func setup() {
        addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: &observerContext)
        
        scrollView.isScrollEnabled = false
        scrollView.scrollsToTop = false
        scrollView.showsVerticalScrollIndicator = false
        scrollView.showsHorizontalScrollIndicator = false
    }
    
    override var intrinsicContentSize: CGSize {
        var size = super.intrinsicContentSize
        size.height = webViewHeight
        return size
    }
    
    deinit {
        removeObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), context: &observerContext)
    }
}

extension StackableWebView {
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if context == &observerContext {
            if let _ = change?[.newKey] as? Double {
                
                guard self.estimatedProgress > 0.5 else { return }
                
                let js = "document.getElementsByTagName('html')[0].offsetHeight"
                
                self.evaluateJavaScript(js, completionHandler: { result, error in
                    let contentHeight = result as? CGFloat ?? 0.0
                    
                    if contentHeight > self.webViewHeight {
                        print("setting height to \(contentHeight)")
                        self.webViewHeight = contentHeight
                        self.invalidateIntrinsicContentSize()
                    }
                })

            }
            
        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }

    }
}