Macro event_handler 

macro_rules! event_handler {
    ($(#[$attr:meta])* fn $name:ident($event:ident, $data:ident) $body:block) => { ... };
}

Define an ABI event handler whose body uses an EventView.

The generated trampoline contains the host’s ABI validity assumption and catches panics before they can unwind across extern "C". Payload strings copied by the view may safely be retained after the callback returns.