WKWebView evaluate JavaScript return value

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

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);