DotsUnited\BundleFu\Filter\CallbackFilter::filter PHP Method

    public function filter($content)
    {
        if (null === $this->callback) {
            return $content;
        }
        return call_user_func($this->callback, $content);
    }

 public function testNullCallbacks()
 {
     $filter = new CallbackFilter();
     $value = "foo";
     $this->assertEquals($value, $filter->filter($value));
     $this->assertEquals($value, $filter->filterFile($value, '/js/js_1.js', new \SplFileInfo(__DIR__ . '/_files/js/js_1.js'), null, null));
 }