Add utility method for extracting font families from a font spec

src/ol/css.js

@@ -43,3 +43,30 @@ ol.css.CLASS_UNSUPPORTED = 'ol-unsupported';
  * @type {string}
  */
 ol.css.CLASS_CONTROL = 'ol-control';
+
+
+/**
+ * Get the list of font families from a font spec.  Note that this doesn't work
+ * for font families that have commas in them.
+ * @param {string} The CSS font property.
+ * @return {Array.<string>} The font families (or null if the input spec is invalid).
+ */
+ol.css.getFontFamilies = (function() {
+  var style;
+  var cache = {};
+  return function(font) {
+    if (!style) {
+      style = document.createElement('div').style;
+    }
+    if (!(font in cache)) {
+      style.font = font;
+      var family = style.fontFamily;
+      style.font = '';
+      if (!family) {
+        return null;
+      }
+      cache[font] = family.split(/,\s?/);
+    }
+    return cache[font];
+  };
+})();