swift - JavaScript synchronous native communication to WKWebView

Question

Is synchronous communication between JavaScript and Swift/Obj-C native code possible using the WKWebView?

These are the approaches I have tried and have failed.

Approach 1: Using script handlers

WKWebView's new way of receiving JS messages is by using the delegate method userContentController:didReceiveScriptMessage: which is invoked from JS by window.webkit.messageHandlers.myMsgHandler.postMessage('What's the meaning of life, native code?') The problem with this approach is that during execution of the native delegate method, JS execution is not blocked, so we can't return a value by immediately invoking webView.evaluateJavaScript("something = 42", completionHandler: nil).

Example (JavaScript)

var something;
function getSomething() {
    window.webkit.messageHandlers.myMsgHandler.postMessage("What's the meaning of life, native code?"); // Execution NOT blocking here :(
    return something;
}
getSomething();    // Returns undefined

Example (Swift)

func userContentController(userContentController: WKUserContentController, didReceiveScriptMessage message: WKScriptMessage) {
    webView.evaluateJavaScript("something = 42", completionHandler: nil)
}

Approach 2: Using a custom URL scheme

In JS, redirecting using window.location = "js://webView?hello=world" invokes the native WKNavigationDelegate methods, where the URL query parameters can be extracted. However, unlike the UIWebView, the delegate method is not blocking the JS execution, so immediately invoking evaluateJavaScript to pass a value back to the JS doesn't work here either.

Example (JavaScript)

var something;
function getSomething() {
    window.location = "js://webView?question=meaning" // Execution NOT blocking here either :(
    return something;
}
getSomething(); // Returns undefined

Example (Swift)

func webView(webView: WKWebView, decidePolicyForNavigationAction navigationAction: WKNavigationAction, decisionHandler decisionHandler: (WKNavigationActionPolicy) -> Void) {
    webView.evaluateJavaScript("something = 42", completionHandler: nil)
    decisionHandler(WKNavigationActionPolicy.Allow)
}

Approach 3: Using a custom URL scheme and an IFRAME

This approach only differs in the way that window.location is assigned. Instead of assigning it directly, the src attribute of an empty iframe is used.

Example (JavaScript)

var something;
function getSomething() {
    var iframe = document.createElement("IFRAME");
    iframe.setAttribute("src", "js://webView?hello=world");
    document.documentElement.appendChild(iframe);  // Execution NOT blocking here either :(
    iframe.parentNode.removeChild(iframe);
    iframe = null;
    return something;
}
getSomething();

This nonetheless, is not a solution either, it invokes the same native method as Approach 2, which is not synchronous.

Appendix: How to achieve this with the old UIWebView

Example (JavaScript)

var something;
function getSomething() {
    // window.location = "js://webView?question=meaning" // Execution is NOT blocking if you use this.

    // Execution IS BLOCKING if you use this.
    var iframe = document.createElement("IFRAME");
    iframe.setAttribute("src", "js://webView?question=meaning");
    document.documentElement.appendChild(iframe);
    iframe.parentNode.removeChild(iframe);
    iframe = null;

    return something;
}
getSomething();   // Returns 42

Example (Swift)

func webView(webView: UIWebView, shouldStartLoadWithRequest request: NSURLRequest, navigationType: UIWebViewNavigationType) -> Bool {
    webView.stringByEvaluatingJavaScriptFromString("something = 42")    
}

Answer 1

No I don't believe it is possible due to the multi-process architecture of WKWebView. WKWebView runs in the same process as your application but it communicates with WebKit which runs in its own process (Introducing the Modern WebKit API). The JavaScript code will be running in the WebKit process. So essentially you are asking to have synchronous communication between two different processes which goes against their design.