Add onBeforeElAttrsUpdated which allows to skip updating an element itself but still process its children

Author: gucki

src/morphdom.js

@@ -39,6 +39,7 @@ export default function morphdomFactory(morphAttrs) {
     var onBeforeNodeAdded = options.onBeforeNodeAdded || noop;
     var onNodeAdded = options.onNodeAdded || noop;
     var onBeforeElUpdated = options.onBeforeElUpdated || noop;
+    var onBeforeElAttrsUpdated = options.onBeforeElAttrsUpdated || noop;
     var onElUpdated = options.onElUpdated || noop;
     var onBeforeNodeDiscarded = options.onBeforeNodeDiscarded || noop;
     var onNodeDiscarded = options.onNodeDiscarded || noop;
@@ -211,8 +212,11 @@ export default function morphdomFactory(morphAttrs) {
           return;
         }
 
-        // update attributes on original DOM element first
-        morphAttrs(fromEl, toEl);
+        if (onBeforeElAttrsUpdated(fromEl, toEl) !== false) {
+          // update attributes on original DOM element first
+          morphAttrs(fromEl, toEl);
+        }
+
         // optional
         onElUpdated(fromEl);