From c42111ad39d4b1ebef3d02d207e1f6bfa0d4022d Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Wed, 10 Feb 2016 21:58:00 -0500
Subject: game: Add on-start procedure.

* sly/game.scm (on-start): New procedure.
---
 sly/game.scm | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/sly/game.scm b/sly/game.scm
index 615f934..38d230e 100644
--- a/sly/game.scm
+++ b/sly/game.scm
@@ -40,6 +40,7 @@
             draw-hook
             after-game-loop-error-hook
             game-started?
+            on-start
 
             run-game-loop
             stop-game-loop))
@@ -56,6 +57,16 @@
 (define-signal game-started?
   (hook->signal game-start-hook #f (lambda () #t)))
 
+(define-syntax on-start
+  (syntax-rules ()
+    "Create a signal that evaluates EXP once the game loop has started.
+If INIT is specified, the signal uses the result of this expression as
+its initial value, otherwise #f is the initial value."
+    ((_ exp)
+     (signal-if game-started? exp #f))
+    ((_ exp init)
+     (signal-if game-started? exp init))))
+
 
 ;;;
 ;;; Game Loop
-- 
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