Macro event_handler 

macro_rules! event_handler {
    (
        $( #[$attr:meta] )*
        $pub:vis
        async
        fn event_handler<$lt:lifetime> ($this:tt : $this_ty:ty , $engine_globals:tt : $engine_globals_ty:ty, $event:tt : $event_ty:ty) -> Swap
        {
            $($body:tt)*
        }
    ) => { ... };
}