How to handle command line arguments in a gtk application?

Add a call to gtk::Application::add_main_option as documented here - https://gtk-rs.org/docs/gio/trait.ApplicationExt.html#tymethod.add_main_option.

gtk_app.add_main_option(
  "config",
  glib::Char::new("c"),
  glib::OptionFlags::IN_MAIN,
  glib::OptionArg::Filename,
  "some description",
  Some("some other description")
);

then add a call to

gtk_app.connect_handle_local_options( ... );