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.

Answer 2

I found a hack for doing synchronous communication but haven't tried it yet: https://stackoverflow.com/a/49474323/2870783

Edit: Basically you can use the the JS prompt() to carry your payload from the js side to the native side. In the native WKWebView will have to intercept the prompt call and decide if it is a normal call or if it is a jsbridge call. Then you can return your result as a callback to the prompt call. Because the prompt call is implemented in such a way that it waits for user input your javascript-native communication will be synchronous. The downside is that you can only communicate trough strings.