node: Make rank mutable.

1 files changed, 12 insertions, 3 deletions

diff --git a/catbird/node.scm b/catbird/node.scm
index c58f2ae..6899d7d 100644
--- a/catbird/node.scm
+++ b/catbird/node.scm
@@ -68,7 +68,7 @@
                 <observer> <asset-container>)
   ;; An integer value that determines priority order for
   ;; updating/rendering.
-  (rank #:getter rank #:init-value 0 #:init-keyword #:rank)
+  (rank #:accessor rank #:init-value 0 #:init-keyword #:rank #:observe? #t)
   ;; List of children, sorted by rank.
   (children #:accessor children #:init-value '())
   ;; Children indexed by name for fast lookup.
@@ -78,9 +78,18 @@
   (next-method)
   (on-boot node))
 
+(define-method (sort-children (node <node>))
+  (set! (children node) (sort-by-rank/ascending (children node))))
+
 (define-method (on-boot (node <node>))
   #t)
 
+(define-method (on-change (node <node>) slot-name old new)
+  (case slot-name
+    ((rank)
+     ;; Re-sort parent when rank of child node changes.
+     (and=> (parent node) sort-children))))
+
 (define-method (reboot (node <node>))
   (for-each-child detach node)
   (with-agenda (agenda node) (reset-agenda))
@@ -147,8 +156,8 @@
             new-children)
   ;; Add the new children and sort them by their rank so that
   ;; updating/rendering happens in the desired order.
-  (set! (children new-parent)
-        (sort-by-rank/ascending (append new-children (children new-parent))))
+  (set! (children new-parent) (append new-children (children new-parent)))
+  (sort-children new-parent)
   ;; Attach children to the parent, triggering initial enter/attach
   ;; hooks.
   (for-each (lambda (child)