Wkwebview Evaluate JavaScript Return Value

WKWebView evaluate JavaScript return value

Update: This is not working on iOS 12+ anymore.

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I solved this problem by waiting for result until result value is returned.

I used NSRunLoop for waiting, but I'm not sure it's best way or not...

Here is the category extension source code that I'm using now:

@interface WKWebView(SynchronousEvaluateJavaScript)
- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script;
@end

@implementation WKWebView(SynchronousEvaluateJavaScript)

- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script
{
    __block NSString *resultString = nil;
    __block BOOL finished = NO;

    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    while (!finished)
    {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }

    return resultString;
}
@end

Example code:

NSString *userAgent = [_webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];

NSLog(@"userAgent: %@", userAgent);

How do I get WKWebView.evaluateJavaScript to return data in a function call

you should use a completion handler, something like this:

func checkTitle (forWebView webView: WKWebView, title: String, completion: @escaping (_ titleString: String?) -> Void) {

   webView.evaluateJavaScript("document.title", completionHandler: { (innerHTML, error ) in

        // Logic here
        completion(innerHTML as? String)  
    })
}

WKWebView - evaluate JavaScript without seeing changes

This looks like Apple's defect, because updateUIView is called, but evaluateJavaScript (internal JavaScript) generates exception, because bridge.onMessage is lost and not a function more.

Here is found work-around - reset WebView on scene activation (app comes into foreground), as it recreated with current states the previous values are preserved. Tested with Xcode 13.2 / iOS 15.2

struct DemoView: View {
    @Environment(\.scenePhase) var scenePhase

    @State private var headline: String = "Initial"
    @State private var reset = false

    var body: some View {
        NavigationView {
            VStack {
                Form {
                    TextField("Your headline", text: $headline)
                }
                WebView(headline: $headline).id(reset)     // << here !!
                    .onChange(of: scenePhase) {
                        if case .active = $0 {
                            self.reset.toggle()            // << here !!
                        }
                    }
            }
        }
    }
}

WKWebView Evaluate Javascript without reloading page

[webView evaluateJavaScript:javascriptString completionHandler:nil];

performs the same function as the one you have listed for UIWebView

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