game: Add on-start procedure.

* sly/game.scm (on-start): New procedure.

1 files changed, 11 insertions, 0 deletions

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