var gulp = require('gulp');

var rev = require('gulp-rev');

var revReplace = require('gulp-rev-replace');

var useref = require('gulp-useref');

var filter = require('gulp-filter');

var uglify = require('gulp-uglify');

var csso = require('gulp-csso');

gulp.task("default", function() {

var jsFilter = filter("**/*.js", { restore: true });

var cssFilter = filter("**/*.css", { restore: true });

var indexHtmlFilter = filter(['**/*', '!**/index.html'], { restore: true });

return gulp.src("test/index.html")

.pipe(useref())      // Concatenate with gulp-useref

.pipe(jsFilter)

.pipe(uglify())             // Minify any javascript sources

.pipe(jsFilter.restore)

.pipe(cssFilter)

.pipe(csso())               // Minify any CSS sources

.pipe(cssFilter.restore)

.pipe(indexHtmlFilter)

.pipe(rev())                // Rename the concatenated files (but not index.html)

.pipe(indexHtmlFilter.restore)

.pipe(revReplace())         // Substitute in new filenames

.pipe(gulp.dest('dist'));

});

//监听文件的改变，自动执行任务，例如修改了CSS代码或是JS代码，不用手动打包，他会自动去打包或者自动执行一些其他的任务；

var watch = require('gulp-watch');

gulp.task('stream', function () {

  // Endless stream mode

return watch('css/*.css', { ignoreInitial: false })

.pipe(gulp.dest('dist'));

});

gulp.task('callback', function () {

  // Callback mode, useful if any plugin in the pipeline depends on the `end`/`flush` event

return watch('css/*.css', function () {

gulp.src('css/*.css')

.pipe(gulp.dest('dist'));

});

});

1.执行命令：gulp stream callback，显示如下：

PS E:\Program\Nodejs> gulp stream callback

[00:20:43] Using gulpfile E:\Program\Nodejs\gulpfile.js

[00:20:43] Starting 'stream'...

[00:20:43] Starting 'callback'...

2.更改相应目录下的css文件，dist目录没有变化。

问题：请问怎样实现文件修改时，自动自动打包，执行其它任务？