Watch sass files with awareness of the dependency graph (so that files that don't need to be recompiled don't get recompiled).

sasswatch.js

module.exports = function (grunt) {

    var fs = require('fs');

    grunt.registerTask('sasswatch', 'Watch Sass with awareness of dependency graph', function () {

        var options = this.options({
            sassPath: 'sass',
            logFile: null,
            style: 'nested',
            loadPath: null,
            cacheLocation: null,
            src: null,
            dest: null
        });

        var args = [
            '--scss',
            '--style', options.style,
            '--cache-location', options.cacheLocation,
            '--load-path', options.loadPath,
            '--watch',
            options.src + ':' + options.dest
        ];

        // Add sourcemap option if Sass supports it (check --help for option)
        grunt.util.spawn({
            cmd: options.sassPath,
            args: ['--help']
        }, function (err, res) {
            if (res.toString().indexOf('--sourcemap') !== -1) {
                args.unshift('--sourcemap');
            }

            startDaemon(options, args);
        });

        return true;
    });

    function startDaemon(options, args) {
        var sassDaemon = grunt.util.spawn({
            cmd: options.sassPath,
            args: args,
            // Run from current working dir and inherit stdio from process
            opts: {
                cwd: process.cwd(),
                stdio: options.logFile ? ['ignore', 'pipe', 'ignore'] : 'inherit'
            }
        }, function (err, res /*, code */) {
            if (err) {
                grunt.log.error(res);
            } else {
                grunt.log.writeln('Sass daemon died. I\'m not sure why.');
            }
        });

        if (!sassDaemon) {
            grunt.log.error('Could not start sass.');
            return false;
        } else if (options.logFile) {
            sassDaemon.stdout.pipe( getLogStream(options.logFile) );
        }

        // Listen for terminate signal and clean up sassDaemon
        process.on('SIGTERM', function () {
            if (sassDaemon) {
                sassDaemon.kill();
                sassDaemon = null;
            }
            process.exit();
        });

        process.on('exit', function () {
            if (sassDaemon) {
                sassDaemon.kill();
            }
        });
    }

    function getLogStream(logFilePath) {
        return fs.createWriteStream(logFilePath, {flags: 'a'});
    }
};