feat(gift-box): add GiftBox scene and FSM implementation

scripts/objects/gift_box.gd

@@ -0,0 +1,55 @@
+## GiftBox — tap while closed to open: lid rises and fades out, gift inside fades in.
+class_name GiftBox extends Node2D
+
+enum State { CLOSED, OPENING, OPEN }
+
+const LID_OPEN_Y: float = -120.0
+const OPEN_DURATION: float = 0.5
+const GIFT_FADE_DURATION: float = 0.4
+const BUTTON_HALF_WIDTH: float = 40.0
+const BUTTON_HALF_HEIGHT: float = 50.0
+
+var _state: State = State.CLOSED
+
+
+func _ready() -> void:
+	var gift: Node2D = get_node_or_null("Gift") as Node2D
+	if gift != null:
+		gift.modulate.a = 0.0
+
+
+func _input(event: InputEvent) -> void:
+	if _state != State.CLOSED:
+		return
+	if not event is InputEventScreenTouch:
+		return
+	var touch: InputEventScreenTouch = event as InputEventScreenTouch
+	if not touch.pressed:
+		return
+	var local: Vector2 = to_local(touch.position)
+	if abs(local.x) > BUTTON_HALF_WIDTH or abs(local.y) > BUTTON_HALF_HEIGHT:
+		return
+	_start_opening()
+
+
+func _start_opening() -> void:
+	_state = State.OPENING
+	var lid: Node2D = get_node_or_null("Lid") as Node2D
+	if lid == null:
+		_state = State.OPEN
+		return
+	var tween: Tween = create_tween()
+	tween.set_ease(Tween.EASE_OUT)
+	tween.set_trans(Tween.TRANS_BACK)
+	tween.tween_property(lid, "position:y", LID_OPEN_Y, OPEN_DURATION)
+	tween.parallel().tween_property(lid, "modulate:a", 0.0, OPEN_DURATION)
+	tween.finished.connect(_on_lid_opened)
+
+
+func _on_lid_opened() -> void:
+	_state = State.OPEN
+	var gift: Node2D = get_node_or_null("Gift") as Node2D
+	if gift == null:
+		return
+	var tween: Tween = create_tween()
+	tween.tween_property(gift, "modulate:a", 1.0, GIFT_FADE_DURATION)