In iOS 13 and below, how can WKWebView call an async Javascript function (i.e. one that returns a promise)?

Question

iOS 14 has this API, however how can this be achieved in iOS 13 or below?

For example, suppose the JS has a function defined as:

function upperCaseAsync(text) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(text.toUpperCase());
    }, 300);
  });
}

How to call and handle the resulting promise's result? I tried using the synchronous API, but it throws an error:

Error Domain=WKErrorDomain Code=5 "JavaScript execution returned a result of an unsupported type"

Answer 1

You'd need to register callback handler and change your JS function from returning Promise to post webkit message: webkit.messageHandlers['smth'].postMessage('some info if needed')

let script = WKUserScript(source: <your js code>, injectionTime: .atDocumentStart, forMainFrameOnly: false)

<your web view>.configuration.userContentController.addUserScript(script)
<your web view>.configuration.userContentController.add(<whatever>, name: "smth")

extension <whatever>: WKScriptMessageHandler {
    
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        switch message.name {
            case "smth":
                break // do smth
            default:
                return
        }
    }
    
}