npm install browser-sync--save-dev

var gulp = require('gulp');var sass = require('gulp-sass');var browserSync = require('browser-sync');var uglify = require('gulp-uglify');var imagemin = require('gulp-imagemin');var minifyCSS = require('gulp-minify-css');var cache = require('gulp-cache');var del = require('del');var runSequence = require('run-sequence');var minifyHtml= require("gulp-minify-html");gulp.task('sass', function(){  //打包sass return gulp.src('app/scss/**/*.scss')  .pipe(sass()) // Converts Sass to CSS with gulp-sass  .pipe(gulp.dest('app/css'))  .pipe(browserSync.reload({   stream: true  }))});gulp.task('js',function() {  gulp.src('app/**/*.js')  .pipe(uglify())//壓縮  .pipe(gulp.dest('dist'));});gulp.task('css', function () {  gulp.src('app/css/*.css')  .pipe(minifyCSS())  .pipe(gulp.dest('dist/css'))})gulp.task('minify-html',function() {   gulp.src('app/**/*.html')//要壓縮的html文件   .pipe(gulp.dest('dist'));});gulp.task('images', function(){ return gulp.src('app/images/**/*.+(png|jpg|jpeg|gif|svg)') // Caching images that ran through imagemin .pipe(cache(imagemin({   interlaced: true  }))) .pipe(gulp.dest('dist/images'))});gulp.task('clean', function(callback) { del('dist'); return cache.clearAll(callback);});gulp.task('watch',['browserSync', 'sass'],function(){  //我們可以在watch任務之前告知Gulp，先把browserSync和Sass任務執行了再說。 gulp.watch('app/scss/**/*.scss', ['sass']); gulp.watch('app/*.html', browserSync.reload); gulp.watch('app/js/**/*.js', browserSync.reload); // Other watchers});gulp.task('browserSync', function() { //瀏覽器熱加載 browserSync({  server: {   baseDir: 'app'  }, })});gulp.task('build', function (callback) { runSequence('clean',['minify-html','js','images','css'],callback)});gulp.task('default', function (callback) { runSequence(['sass','browserSync', 'watch'],  callback )});