//connecting indicator with the scroll-event and altogether with the function indicator_scrolled
g_signal_connect(G_OBJECT(appindicator), "scroll-event", G_CALLBACK(&indicator_scrolled), (gpointer)this);

gboolean indicator_scrolled(GtkStatusIcon  *status_icon, GdkEventScroll *event, gpointer data){
    cout << "The indicator was scrolledn";
    MainWindow* m = (MainWindow*)data;
    m->action_on_scroll();
}

arg0 : The AppIndicator object
arg1 : How many steps the scroll wheel has taken
arg2 : (type Gdk.ScrollDirection) Which direction the wheel went in
user_data : user data set when the signal handler was connected.

static void indicator_scrolled(AppIndicator *indicator, guint steps, GdkScrollDirection direction)
{
    if (direction == GDK_SCROLL_DOWN)
        g_print("DOWNn");
    else if (direction == GDK_SCROLL_UP)
        g_print("UPn");
    else if (direction == GDK_SCROLL_SMOOTH)
       g_print("SMOOTHn");
}