keyword
cancellable
event cancellable Name { … }
cancellable marks an event as a decision, not just a notification. A plain event tells listeners something happened; a cancellable event is fired before an action so a listener can veto it by calling cancel. The caller reads the verdict back by using emit as an expression: it yields 0 when no listener vetoed and 1 when one did, so the action is applied only on 0. This is the Bukkit / DOM preventDefault shape — observation becomes control — and a foreign mod vetoes the same way by setting the payload's trailing cancelled flag over the ABI.
Example
program CancellableExample {
event cancellable BeforeHurt { amount: int = 0 }
var current_health: int = 100
@On(BeforeHurt) handler AbsorbSmallHits { if amount <= 5 { cancel } }
handler TakeDamage phase Update {
if Input.key() == 'h' {
if emit BeforeHurt(amount: 3) == 0 { current_health = current_health - 3 }
}
}
handler DrawWorld phase Render {
Screen.clear(Color.MidnightBlue)
Screen.draw_number(x: 4, y: 4, value: current_health, color: Color.Crimson, scale: 1)
Screen.show()
}
}