let conf = WKWebViewConfiguration()
let userScript = WKUserScript(source: "redHeader()", injectionTime: .AtDocumentEnd, forMainFrameOnly: true)
conf.userContentController.addUserScript(userScript)
webView = WKWebView(frame: self.view.frame, configuration: conf)

class ViewController: UIViewController, WKScriptMessageHandler

func userContentController(userContentController: WKUserContentController!, didReceiveScriptMessage message: WKScriptMessage!) {
    if(message.name == "callbackHandler") {
        println("JavaScript is sending a message \(message.body)")
    }
}

contentController.addScriptMessageHandler(
    self,
    name: "callbackHandler" 
)

webkit.messageHandlers.callbackHandler.postMessage("I Love you");