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];
+  };
+})();

test/spec/ol/css.test.js

@@ -0,0 +1,47 @@
+goog.require('ol.css');
+
+describe('ol.css', function() {
+
+  describe('getFontFamilies()', function() {
+    var cases = [{
+      font: '2em "Open Sans"',
+      families: ['"Open Sans"']
+    }, {
+      font: '2em \'Open Sans\'',
+      families: ['"Open Sans"']
+    }, {
+      font: '2em "Open Sans", sans-serif',
+      families: ['"Open Sans"', 'sans-serif']
+    }, {
+      font: 'italic small-caps bolder 16px/3 cursive',
+      families: ['cursive']
+    }, {
+      font: 'garbage 2px input',
+      families: null
+    }, {
+      font: '100% fantasy',
+      families: ['fantasy']
+    }];
+
+    cases.forEach(function(c, i) {
+      it('works for ' + c.font, function() {
+        var families = ol.css.getFontFamilies(c.font);
+        if (c.families === null) {
+          expect(families).to.be(null);
+          return;
+        }
+        families.forEach(function(family, j) {
+          // Safari uses single quotes for font families, so we have to do extra work
+          if (family.charAt(0) === '\'') {
+            // we wouldn't want to do this in the lib since it doesn't properly escape quotes
+            // but we know that our test cases don't include quotes in font names
+            families[j] = '"' + family.slice(1, -1) + '"';
+          }
+        });
+        expect(families).to.eql(c.families);
+      });
+    });
+
+  });
+
+});