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())//压缩
  .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')//要压缩的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(){  //我们可以在watch任务之前告知Gulp，先把browserSync和Sass任务执行了再说。
 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() { //浏览器热加载
 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
 )
});