gstate: Update cached matrix state after device transform changes on the target

Commit 8d67186cb291cb877e52b987e2ac18c2a1175a57 caches whether the device
transform is identity on context creation. However, the api is quite lax
and allows the user to modify the device transform *after* he has
started to use the surface in a context, as apparently WebKit does.
Since this is not the only instance where we may need to invalidate
caches if the user modifies state, introduce a simple mechanism for
hooking into notifications of property changes.

Fixes test/clip-device-offset.

1 files changed, 24 insertions, 4 deletions

diff --git a/src/cairo-gstate.c b/src/cairo-gstate.c
index 28266dd04..baf614500 100644
--- a/src/cairo-gstate.c
+++ b/src/cairo-gstate.c
@@ -70,6 +70,18 @@ _cairo_gstate_transform_glyphs_to_backend (cairo_gstate_t      *gstate,
 					   int			*num_transformed_glyphs,
 					   cairo_text_cluster_t *transformed_clusters);
 
+static void
+_cairo_gstate_update_device_transform (cairo_observer_t *observer,
+				       void *arg)
+{
+    cairo_gstate_t *gstate = cairo_container_of (observer,
+						 cairo_gstate_t,
+						 device_transform_observer);
+
+    gstate->is_identity = (_cairo_matrix_is_identity (&gstate->ctm) &&
+			   _cairo_matrix_is_identity (&gstate->target->device_transform));
+}
+
 cairo_status_t
 _cairo_gstate_init (cairo_gstate_t  *gstate,
 		    cairo_surface_t *target)
@@ -105,6 +117,10 @@ _cairo_gstate_init (cairo_gstate_t  *gstate,
     gstate->parent_target = NULL;
     gstate->original_target = cairo_surface_reference (target);
 
+    gstate->device_transform_observer.callback = _cairo_gstate_update_device_transform;
+    cairo_list_add (&gstate->device_transform_observer.link,
+		    &gstate->target->device_transform_observers);
+
     gstate->is_identity = _cairo_matrix_is_identity (&gstate->target->device_transform);
     cairo_matrix_init_identity (&gstate->ctm);
     gstate->ctm_inverse = gstate->ctm;
@@ -115,10 +131,6 @@ _cairo_gstate_init (cairo_gstate_t  *gstate,
     /* Now that the gstate is fully initialized and ready for the eventual
      * _cairo_gstate_fini(), we can check for errors (and not worry about
      * the resource deallocation). */
-
-    if (target == NULL)
-	return _cairo_error (CAIRO_STATUS_NULL_POINTER);
-
     status = target->status;
     if (unlikely (status))
 	return status;
@@ -171,6 +183,10 @@ _cairo_gstate_init_copy (cairo_gstate_t *gstate, cairo_gstate_t *other)
     gstate->parent_target = NULL;
     gstate->original_target = cairo_surface_reference (other->original_target);
 
+    gstate->device_transform_observer.callback = _cairo_gstate_update_device_transform;
+    cairo_list_add (&gstate->device_transform_observer.link,
+		    &gstate->target->device_transform_observers);
+
     gstate->is_identity = other->is_identity;
     gstate->ctm = other->ctm;
     gstate->ctm_inverse = other->ctm_inverse;
@@ -199,6 +215,8 @@ _cairo_gstate_fini (cairo_gstate_t *gstate)
 
     _cairo_clip_reset (&gstate->clip);
 
+    cairo_list_del (&gstate->device_transform_observer.link);
+
     cairo_surface_destroy (gstate->target);
     gstate->target = NULL;
 
@@ -309,6 +327,8 @@ _cairo_gstate_redirect_target (cairo_gstate_t *gstate, cairo_surface_t *child)
      * since its ref is now owned by gstate->parent_target */
     gstate->target = cairo_surface_reference (child);
     gstate->is_identity &= _cairo_matrix_is_identity (&child->device_transform);
+    cairo_list_move (&gstate->device_transform_observer.link,
+		     &gstate->target->device_transform_observers);
 
     /* The clip is in surface backend coordinates for the previous target;
      * translate it into the child's backend coordinates. */