Apply tasks only to changed files.

Gruntfile.js

/*global module:false*/
module.exports = function(grunt) {
  // Load tasks
	grunt.loadTasks('util/grunt');

	// Load vendors tasks
	grunt.loadNpmTasks('grunt-contrib-compass');
	grunt.loadNpmTasks('grunt-contrib-uglify');
	grunt.loadNpmTasks('grunt-contrib-jshint');
	grunt.loadNpmTasks('grunt-contrib-watch');

	// Project configuration
	grunt.initConfig({
		compass: {
			dist: {
				options: {
					config: 'config.rb'
				}
			}
		},
		uglify: {
			dist: {
				expand: true,
				cwd: 'app/webroot/js/src/',
				src: ['**/*.js'],
				dest: 'app/webroot/js/build/'
			}
		},
		jshint: {
			dist: {
				src: ['app/webroot/js/src/**/*.js']
			}
		},
		watch: {
			options: {
				nospawn: true
			},
			stylesheets: {
				files: 'app/webroot/css/src/**/*.scss',
				tasks: ['compass']
			},
			scripts: {
				files: 'app/webroot/js/src/**/*.js',
				tasks: ['jshint', 'uglify']
			}
		}
	});

	grunt.event.on('watch', function(action, filepath) {
		if (grunt.file.isMatch(grunt.config('watch.stylesheets.files'), filepath)) {
			grunt.config('compass.dist.options.specify', [filepath]);
		}

		if (grunt.file.isMatch(grunt.config('watch.scripts.files'), filepath)) {
			var uglifySrc = filepath.replace(grunt.config('uglify.dist.cwd'), '');
			grunt.config('jshint.dist.src', [filepath]);
			grunt.config('uglify.dist.src', [uglifySrc]);
		}
	});

	// Default task.
	grunt.registerTask('default', ['compass', 'jshint', 'uglify']);
};

Thank you @crdx. I'm updating the example to reflect your suggestion.

it seems doesn't work

doesn't work either