registering callback function for OAuth

I finally succeed, here is my main App:

using Toybox.Application as App;
using Toybox.Background;
using Toybox.System as Sys;
using Toybox.WatchUi as Ui;
using Toybox.Time;
using Toybox.UserProfile;
using Toybox.PersistedContent;
using Toybox.System;
using Toybox.Communications as Comm;


(:background)
class KronosApp extends App.AppBase {
	var inBackground=false;
	var bg=null;
    
    function initialize() {
        AppBase.initialize();
		Sys.println("AppBase initialize");
    	if(Toybox.System has :ServiceDelegate) {
    		Background.registerForTemporalEvent(new Time.Duration(5 * 60));
    	} else {
    		Sys.println("****background not available on this device****");
   		}
   		Background.registerForOAuthResponseEvent();
   		
      }

    // onStart() is called on application start up
    function onStart(state) { 
	    System.println("start");
    	
    }

    // onStop() is called when your application is exiting
    function onStop(state) {
    	if(!inBackground) {
    		Background.deleteTemporalEvent();
    		System.println("stop");
    	}
    	
    }


    //! Return the initial view of your application here
    function getInitialView() {
    	System.println("getInitialView");
    	var Kview = new KronosView();
        return [ Kview];
    }
    
    //store json data in bgdata
    function onBackgroundData(data) {
		System.println("onbg "+ data);
    }    

    function getServiceDelegate(){
    	inBackground=true;	
    	System.println("getServiceDelegate");
        return [new JsonBackground()];
    }


}

As you can see I've add  Background.registerForOAuthResponseEvent();  in initialize

And now I've only a JsonBackground  class and no more LoginTransaction class

using Toybox.Background as Bg;
using Toybox.Application as App;
using Toybox.System as Sys;
using Toybox.Time as Tm;
using Toybox.Communications as Comm;

// The Service Delegate is the main entry point for background processes
// our onTemporalEvent() method will get run each time our periodic event
// is triggered by the system.

(:background)
class JsonBackground extends Toybox.System.ServiceDelegate {
	hidden var _transaction=null;

	(:background_method)
	function initialize() {
		Sys.ServiceDelegate.initialize();
	 	_transaction = new LoginTransaction();
		Comm.registerForOAuthMessages(method(:accessCodeResult));
	}
	(:background_method)	
    function onTemporalEvent() {
    	System.println("onTemporalEvent2");
        go();
    }
    
    (:background_method)
    function go() {
        // Kick of a request for the user's credentials. This will
        // cause a notification from Connect Mobile to file
        Sys.println("go");
        Comm.makeOAuthRequest(
            // URL for the authorization URL
            //"https://app.kronos-sport.com/index.php?tg=addon/kronos/main&idx=login.OAuth&response_type=code&client_id="+$.ClientId+"&redirect_uri="+$.RedirectUri+"&scope=&state=azerty",
            "...",
            // POST parameters
            {
             //.....//	
            },
            // Redirect URL
            $.RedirectUri,
            // Response type
            Comm.OAUTH_RESULT_TYPE_URL,
            // Value to look for
            {"token" => "OAUTH_CODE", "error" => "OAUTH_ERROR"}
            );
         
         
            
    }
    	
    // Handle converting the authorization code to the access token
    // @param value Content of JSON response
    (:background_method)
    function accessCodeResult(response) {
    	Sys.println("accessCodeResult " + response);
        if( response.data != null) {
            //_complete = true;
            // Extract the access code from the JSON response
            Sys.println("accessCodeResult Ok");
            Bg.exit(1);
        }
        else {
            Sys.println("Error in accessCodeResult");
           // _delegate.handleError(value.responseCode);
        }
        
    }

	(:background_method)
    function registerForOAuthResponseEvent(){
		System.print("Registered for an oauth event"); 
		return method(:accessCodeResult);
	}
	
}