SwiftUI: Javascript communication with WKWebView on macOS

I want javascript to send a message back to my WKWebView object, but I get nothing in response. I know the script is running, as the color changes, but I am expecting to also see "trigger from JS" printed in the console, which I don't. If I run the html in Chrome, the javascript console says "Cannot read property 'messageHandlers' of undefined". If I build for iOS (using UIViewRepresentable, MakeUIView and UpdateUIView) the result is the same. If anyone can spot what I have missed I would greatly appreciate it.

This is the entirety of the code:

import SwiftUI
import WebKit

class HtmlData {
    let html = """
    <!DOCTYPE html>
    <html>
    <body>

    <button onclick="sendMessage()">Send Message</button>
    <script>
    function sendMessage() {
      document.body.style.backgroundColor = "red";
     window.webkit.messageHandlers.testMessage.postMessage("trigger from JS");
    }
    </script>
    </body>
    </html>
    """
}

struct ContentView: View {
    let htmlData = HtmlData()
    var body: some View {
        JSWebView(html: htmlData.html)
    }
}

struct JSWebView: NSViewRepresentable {
    let html: String
    func makeNSView(context: Context) -> WKWebView {
        let preferences = WKPreferences()
        preferences.javaScriptEnabled = true
        return WKWebView()
    }
    
    func updateNSView(_ view: WKWebView, context: Context) {
        let userContentController = WKUserContentController()
        let handler = ContentController(view)
        let configuration = WKWebViewConfiguration()
        configuration.userContentController = userContentController
        configuration.userContentController.add(handler, name: "testMessage")
        view.loadHTMLString(html, baseURL: Bundle.main.bundleURL)
    }
    
    class ContentController: NSObject, WKScriptMessageHandler {
        var parent: WKWebView?
        init(_ parent: WKWebView?) {
            self.parent = parent
        }
        func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
            print(message.body)
        }
    }
}

Solution

Configuration must be passed into the constructor of WKWebView. It can't be set after initialization.

struct JSWebView: NSViewRepresentable {
    let html: String
    
    func makeNSView(context: Context) -> WKWebView {
        let preferences = WKPreferences()
        preferences.javaScriptEnabled = true
        let handler = MessageHandler()
        let configuration = WKWebViewConfiguration()
        configuration.userContentController.add(handler, name: "testMessage")
        return WKWebView(frame: .zero, configuration: configuration)
    }
    
    func updateNSView(_ view: WKWebView, context: Context) {
        view.loadHTMLString(html, baseURL: Bundle.main.bundleURL)
    }
    
    class MessageHandler: NSObject, WKScriptMessageHandler {
        func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
            print(message.body)
        }
    }
}