watch and run tasks only on changed files

gruntfile.coffee

grunt.event.on 'watch', (action, filepath, target) ->
      if minimatch filepath, 'src/**/*.coffee'
        grunt.config.set 'coffee',
                         dev:
                           expand: true
                           cwd: 'src'
                           src: filepath.slice(4)
                           dest: 'dev'
                           ext: '.js'
      else if minimatch filepath, 'src/views/**/*.jade'
        grunt.config.set 'jade',
                         dev:
                           options: pretty: true
                           expand: true
                           cwd: 'src/views'
                           src: filepath.slice(10)
                           dest: 'dev/public/views'
                           ext: '.html'
      else
        for item in copyDevFiles
           if minimatch filepath, item[0]+'/'+item[2]
             grunt.config.set 'copy',
               dev:
                 expand: true
                 dot: true
                 cwd: item[0]
                 dest: item[1]
                 src: filepath.slice(item[0].length+1)
             break

watch and run tasks only on changed files