How does the iOS 14 API WKScriptMessageHandlerWithReply work for communicating with JavaScript from iOS?

I dug into this a bit and found it uses Javascript Promises to provide a callback mechanism (and the response from the app code back to the javascript must be async).

Here's some sample code to illustrate:

The swift code:

import UIKit
import WebKit
import PureLayout

final class ViewController: UIViewController {

    var webView : WKWebView?
    let JavaScriptAPIObjectName = "namespaceWithinTheInjectedJSCode"
    
    override func viewDidLoad() {
        super.viewDidLoad()
                
        //-------
        
        guard let scriptPath = Bundle.main.path(forResource: "script", ofType: "js"),
              let scriptSource = try? String(contentsOfFile: scriptPath) else { return }

        let userScript = WKUserScript(source: scriptSource, injectionTime: .atDocumentEnd, forMainFrameOnly: true)

        let config = WKWebViewConfiguration()

        let userContentController = WKUserContentController()
        userContentController.addUserScript(userScript)
        
        // REQUIRES IOS14
        if #available(iOS 14, *){
            userContentController.addScriptMessageHandler(self, contentWorld: .page, name: JavaScriptAPIObjectName)
        }

        config.userContentController = userContentController
        
        webView = WKWebView(frame: .zero, configuration: config)
                
        if let webView = webView{
            view.addSubview(webView)
            webView.autoPinEdgesToSuperviewMargins() // using PureLayout for easy AutoLayout syntax

            if let htmlPath = Bundle.main.url(forResource: "page", withExtension: "html"){
                webView.loadFileURL( htmlPath, allowingReadAccessTo: htmlPath);
            }
        }
    }

    // need to deinit and remove webview stuff
    deinit {
        if let webView = webView{
            let ucc = webView.configuration.userContentController
            ucc.removeAllUserScripts()
            ucc.removeScriptMessageHandler(forName:JavaScriptAPIObjectName)
        }
    }
}

extension ViewController: WKScriptMessageHandlerWithReply {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage, replyHandler: @escaping (Any?, String?) -> Void) {
        if message.name == JavaScriptAPIObjectName, let messageBody = message.body as? String {
            print(messageBody)
            replyHandler( 2.2, nil ) // first var is success return val, second is err string if error
        }
    }
}

This is the script.js loaded via that Swift code and injected into the web page:

function sampleMethodTheHTMLCanCall( inputInfo, successFunc, errorFunc ) {
    
    var promise = window.webkit.messageHandlers.namespaceWithinTheInjectedJSCode.postMessage( inputInfo );
    
    promise.then(
      function(result) {
        console.log(result); // "Stuff worked!"
        successFunc( result )
      },
      function(err) {
        console.log(err); // Error: "It broke"
        errorFunc( err )
      });
}

And here is the page.html sample HTML that can call into the app code:

<html>
    <meta name="viewport" content="width=device-width" />
        <script>
            function handleInfoFromApp( fromApp ){
                document.getElementById("valToWrite").innerHTML = fromApp;
            }
            function handleError( err ){
            
            }
        </script>

    <h1 id="valToWrite">Hello</h1>
    <button onclick="sampleMethodTheHTMLCanCall( 'inputInfo', handleInfoFromApp, handleError )">Load Info from App</button>
    
</html>

The HTML above provides functions that will later get called by the app extension code upon success or failure of the javascript-initiated request.

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