context can now be given as argument in StyleMap.addUniqueValueRules()

this, for example, allows user to access to properties that are not part of the feature attributes.
r=elemoine (closes #1548)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@7216 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Rule.js

@@ -173,6 +173,9 @@ OpenLayers.Rule = OpenLayers.Class({
         if (!context) {
             context = feature.attributes || feature.data;
         }
+        if (typeof this.context == "function") {
+            context = this.context(feature);
+        }
         return context;
     },
         

lib/OpenLayers/StyleMap.js

@@ -130,12 +130,17 @@ OpenLayers.StyleMap = OpenLayers.Class({
      *                rules for
      * symbolizers  - {Object} Hash of symbolizers, keyed by the desired
      *                property values 
+     * context      - {Object} An optional object with properties that
+     *                symbolizers' property values should be evaluated
+     *                against. If no context is specified, feature.attributes
+     *                will be used
      */
-    addUniqueValueRules: function(renderIntent, property, symbolizers) {
+    addUniqueValueRules: function(renderIntent, property, symbolizers, context) {
         var rules = [];
         for (var value in symbolizers) {
             rules.push(new OpenLayers.Rule({
                 symbolizer: symbolizers[value],
+                context: context,
                 filter: new OpenLayers.Filter.Comparison({
                     type: OpenLayers.Filter.Comparison.EQUAL_TO,
                     property: property,