Grunt (Less+Autoprefixer+Watch), how to configure watch to process not the entire folder, but only the modified file in it?

Question

Less

So, it has a task configured for grunt with processing all less files in one folder in css with a run through an autoprefixer, watch is hung on changing files in the folder.
How to set up this whole thing so that watch is launched only on the changed file, while the file names do not have to be specified separately in previous tasks?
At the moment the config is:

module.exports = function(grunt) {

    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),

        less: {
            options: {
                paths: ['Less/']
            },
            files: {
                expand: true,
                cwd:    "Less/",
                dest:   "Modules/",
                src:    "*.less",
                ext:    ".css"
            }
        },

        autoprefixer:{
            options: {
                browsers: ['> 1%', 'last 2 versions', 'Firefox ESR', 'Opera 12.1'],
                cascade: false
            },
            multiple_files: {
                expand: true,
                flatten: true,
                src: 'Modules/*.css',
                dest: 'Modules/'
            }
        },

        watch: {
            scripts: {
                files: ['Less/*.less'],
                tasks: ['less', 'autoprefixer'],
                options: {
                    spawn: false
                }
            }
        }

    });

    grunt.loadNpmTasks('grunt-autoprefixer');
    grunt.loadNpmTasks('grunt-contrib-less');
    grunt.loadNpmTasks('grunt-contrib-watch');

    grunt.registerTask('default', ['less', 'autoprefixer', 'watch']);

};

Answer 1

Thanks for the link to an example of using the event, I actually poked around and did it this way so far, a little clumsy - but a working version:

module.exports = function(grunt) {

    var lessFolderPath = 'Less';

    grunt.initConfig({
        pkg: grunt.file.readJSON('package.json'),

        less: {
            options: {
                paths: ['Less/']
            },
            files: {
                expand: true,
                cwd:    lessFolderPath + '/',
                dest:   "Modules/",
                src:    "*.less",
                ext:    ".css"
            }
        },

        autoprefixer:{
            options: {
                browsers: ['> 1%', 'last 2 versions', 'Firefox ESR', 'Opera 12.1'],
                cascade: false
            },
            multiple_files: {
                expand: true,
                flatten: true,
                cwd: 'Modules/',
                src: '*.css',
                dest: 'Modules/'
            }
        },

        watch: {
            less: {
                files: ['Less/*.less'],
                tasks: ['less', 'apr'],
                options: {
                    spawn: false
                }
            }
        }

    });

    grunt.loadNpmTasks('grunt-autoprefixer');
    grunt.loadNpmTasks('grunt-contrib-less');
    grunt.loadNpmTasks('grunt-contrib-watch');

    grunt.registerTask('default', ['watch:less', 'watch:css']);
    grunt.registerTask('apr', ['autoprefixer']);

    grunt.event.on('watch', function(action, filepath, target) {
        var filePathName = filepath.replace(lessFolderPath + '\\','');
        var fileName = filePathName.replace('.less','');

        grunt.log.writeln(target + ': ' + filePathName + ' has ' + action);

        grunt.config('autoprefixer.multiple_files.src', [fileName + '.css']);
        grunt.config('less.files.src', [filePathName]);
    });
};

Answer 2

Documentation?