title: Compile only needed sass file whit grunt
body: <p>Using contrib-sass and contrib-watching I want to compile only needed files beacause I have a lot of scss file on production server.</p>
  
  <p>I write this Gruntfile, but something not working. Watching works but compiling nothing.</p>
  
  <pre><code>module.exports = function(grunt) {
  
  grunt.initConfig({
      pkg: grunt.file.readJSON('package.json'),
  
      watch: {
       sass: {
           files: [
               'sites/**/scss/*.scss'
           ],
           tasks: ['sass'],
           options: {
               spawn: false
           }
       }
    },
  
    sass: {
      dist:{
        options: {
            style: 'compressed'
        },
        files: objDestSource
      }
    }
  
  });
  
  // On watch events configure jshint:all to only run on changed file
  grunt.event.on('watch', function(action, filepath) {
    var filepath_scss = filepath;
    var filepath_css = filepath.replace("scss", "css");
    filepath_css = filepath_css.replace("scss", "css");
    filepath= '\''+filepath_css+'\' : \''+filepath_scss+'\'';
    console.log(filepath);
    grunt.config('sass.dist.files', filepath);
  });
  
  grunt.loadNpmTasks('grunt-contrib-sass');
  grunt.loadNpmTasks('grunt-contrib-watch');
  grunt.registerTask('default',['watch']);
  </code></pre>
  
  <p>};</p>