add random switching method

README.md

@@ -98,6 +98,7 @@ reset   | Return to the first switching
 select  | Select the switching at index
 next    | Go to the next switching
 prev    | Go to the previous switching
+random  | Go to the random switching
 destroy | !!Destroy BgSwitcher!!
 
 Example for `select` with button:

jquery.bgswitcher.js

@@ -203,6 +203,19 @@
       this.switching();
     },
 
+    /**
+     * Go to random switching
+     */
+    random: function() {
+      var index;
+      do {
+        index = Math.floor(Math.random() * this.imageList.count())
+      } while (this.index === index);
+
+      this.index = index;
+      this.switching();
+    },
+
     /**
      * Select the switching at index
      *

test/bg_switcher_spec.js

@@ -247,6 +247,22 @@ describe('jQuery.BgSwitcher', function() {
     });
   });
 
+  describe('#random', function() {
+    beforeEach(function() {
+      bs.setConfig({images: ['foo', 'bar', 'baz']});
+    });
+
+    it('go to random switching', function() {
+      bs.select(0);
+      expect(bs.$bg.css('backgroundImage').split('/').pop()).to.have.contain('foo');
+      bs.random();
+      var randomImage = bs.$bg.css('backgroundImage').split('/').pop()
+      expect(randomImage).not.to.have.contain('foo');
+      bs.random();
+      expect(bs.$bg.css('backgroundImage').split('/').pop()).not.to.have.contain(randomImage);
+    });
+  });
+
   describe('#select', function() {
     beforeEach(function() {
       bs.setConfig({images: ['foo', 'bar', 'baz']});