intercept custom JS handlers and inject custom HTTP headers in a UIWebView

UIWebView-intercept.m

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType
{
    BOOL headersArePresent = [[request allHTTPHeaderFields] objectForKey:@"X-Custom-Timezone"] != nil;
    if (!headersArePresent) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            dispatch_async(dispatch_get_main_queue(), ^{
                NSURL *url = [request URL];
                
                if ([[url scheme] isEqualToString:@"customjs"]) {
                    // parse the rest of the URL object and execute functions
                    if ([[url path] isEqualToString:@"/customNativeCall"]) {
                        [self customNativeCall:nil];
                    }
                } else {
                    NSMutableURLRequest* r = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];
                    
                    // set the new headers
                    [r addValue:[[NSTimeZone systemTimeZone] name] forHTTPHeaderField:@"X-Custom-Timezone"];
                    [r setHTTPMethod:[request HTTPMethod]];
                    [r setHTTPBody:[request HTTPBody]];

                    // reload the request
                    [self.web loadRequest:r];
                }
            });
        });
        
        return NO;
    }
    
    return YES;
}