54 lines
1.8 KiB
Markdown
54 lines
1.8 KiB
Markdown
---
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id: kw-on
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name: on enter / on exit
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category: scenes
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kind: keyword
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tokens: on
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sig: on enter { … } on exit { … }
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tip: Scene lifecycle hooks that fire as a scene becomes active or is left.
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order: 2
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---
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`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, <b>not phases</b> — 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.
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```ludic
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program SceneHookExample {
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property Position { column: int = 0, row: int = 0 }
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model Enemy { Position }
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state SceneHookExampleState {
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elapsed_frames: int = 0
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}
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scene Overworld start {
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layer World {
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handler EnterBattle phase Update {
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if Input.key() == 'b' { become Battle }
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}
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handler DrawWorld phase Render {
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Screen.clear(Color.MidnightBlue)
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Screen.show()
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}
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}
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}
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scene Battle {
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on enter (scene_hook_example_st: mut SceneHookExampleState) {
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scene_hook_example_st.elapsed_frames = 0
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spawn Enemy { Position { column: 8, row: 4 } }
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}
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on exit {
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for (any_position) in query [Position, {Enemy}] { despawn self() }
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}
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layer Fight {
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handler DrawBattle phase Render {
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Screen.clear(Color.Crimson)
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for (current_position) in query [Position, {Enemy}] {
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Screen.fill_rectangle(x: current_position.column * 16, y: current_position.row * 16, width: 16, height: 16, color: Color.White)
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}
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Screen.show()
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}
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}
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}
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}
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```
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