2 files changed, 37 insertions, 6 deletions

diff --git a/src/stage.c b/src/stage.c
index e596ab0..6420c87 100644
--- a/src/stage.c
+++ b/src/stage.c
@@ -31,6 +31,7 @@ struct stage_t {
 	float			alpha;		/* alpha for the stage */
 	unsigned		active:1;	/* stage is active */
 	unsigned		locked:1;	/* stage is locked */
+	unsigned		dirty:1;	/* stage is dirty (since last render) */
 
 	stage_render_func_t	*render;	/* render object */
 	stage_free_func_t	*free;		/* free object */
@@ -276,15 +277,44 @@ static void _render_stage(const stage_t *stage, float alpha, void *render_ctxt)
 }
 
 
-/* recursively render the supplied stage tree, skipping inactive branches */
-void stage_render(const stage_t *stage, void *render_ctxt)
+/* recursively render the supplied stage tree if dirty, skipping inactive branches */
+/* returns wether the stage was dirty or not, for influence of page flipping etc. */
+int stage_render(stage_t *stage, void *render_ctxt)
 {
 	assert(stage);
+	assert(!stage->parent);	/* XXX: Current dirty implementation is global and only
+				 * maintains the dirty bit in the root node.  So assert
+				 * that render is only called on the root to not break
+				 * dirty detection.
+				 */
 
-	if (!stage->active)
-		return;
+	if (stage->dirty) {
+		if (stage->active)
+			_render_stage(stage, 1.f, render_ctxt);
+
+		stage->dirty = 0;
+
+		return 1;
+	}
+
+	return 0;
+}
+
+
+/* mark a stage as dirty */
+/* as the current stage_render() implementation only checks the dirty flag of
+ * the supplied stage, which is presumed to be the root, this currently
+ * walks up the chain of parents if any to update the root flag.
+ * TODO: perhaps stage_render() should assert that it's called on the root.
+ */
+void stage_dirty(stage_t *stage)
+{
+	assert(stage);
+
+	while (stage->parent)
+		stage = stage->parent;
 
-	_render_stage(stage, 1.f, render_ctxt);
+	stage->dirty = 1;
 }
 
 
diff --git a/src/stage.h b/src/stage.h
index c5d0173..a2a2543 100644
--- a/src/stage.h
+++ b/src/stage.h
@@ -29,7 +29,8 @@ typedef stage_t * (stage_lookup_func_t)(const stage_t *stage, void *object, void
 stage_t * stage_new(stage_t *parent, int layer, const char *name, void *object, stage_render_func_t *render_func, stage_free_func_t *free_func, stage_lookup_func_t *lookup_func);
 void stage_replace(stage_t *stage, const char *name, void *object, stage_render_func_t *render_func, stage_free_func_t *free_func, stage_lookup_func_t *lookup_func);
 stage_t * stage_free(stage_t *stage);
-void stage_render(const stage_t *stage, void *render_ctxt);
+int stage_render(stage_t *stage, void *render_ctxt);
+void stage_dirty(stage_t *stage);
 void stage_clear(stage_t *stage);
 void stage_set_object(stage_t *stage, void *object);
 void * stage_get_object(const stage_t *stage);