keyword
on enter / on exit
on enter { … } on exit { … }
on introduces a scene's lifecycle hooks: on enter { … } runs once when the scene becomes active, and on exit { … } runs once as it is left. They are hooks, not phases — the place for per-scene setup and teardown that has no equivalent in a mode register. The start scene's on enter fires at boot right after the Start phase; on a become Other the current scene's on exit runs, the active scene switches, then the new scene's on enter runs. Use on enter to spawn the models a scene needs and reset its counters, and on exit to tidy up before leaving.
Example
program SceneHookExample {
property Position { column: int = 0, row: int = 0 }
model Enemy { Position }
var elapsed_frames: int = 0
scene Overworld start {
layer World {
handler EnterBattle phase Update {
if Input.key() == 'b' { become Battle }
}
handler DrawWorld phase Render {
Screen.clear(Color.MidnightBlue)
Screen.show()
}
}
}
scene Battle {
on enter {
elapsed_frames = 0
spawn Enemy { Position { column: 8, row: 4 } }
}
on exit {
for (any_position) in query [Position, {Enemy}] { despawn self() }
}
layer Fight {
handler DrawBattle phase Render {
Screen.clear(Color.Crimson)
for (current_position) in query [Position, {Enemy}] {
Screen.fill_rectangle(x: current_position.column * 16, y: current_position.row * 16, width: 16, height: 16, color: Color.White)
}
Screen.show()
}
}
}
}