gulp表在第一次通话后没有看

Question

我有 gulp watch 的问题。 在我第一次调用它之后，在我对sass文件进行更改之后，终端没有任何更改。只有当我重新调用gulp-watch时，才会看到样式的变化。 我的gulpfile.js看起来像这样：

'use strict'

var gulp = require('gulp');

var sass = require('gulp-sass'),
    cssnano = require('gulp-cssnano'),
    autoprefixer = require('gulp-autoprefixer'),
    imagemin = require('gulp-imagemin'),
    concat = require('gulp-concat'),
    uglify = require('gulp-uglify'),
    rename = require('gulp-rename'),
    watch = require('gulp-watch');

gulp.task('html', function() {
  return gulp.src('assets/*.html')
    .pipe(gulp.dest('public'));
});

gulp.task('sass', function() {
  return gulp.src('assets/css/main.scss')
    .pipe(sass())
    .pipe(autoprefixer({
            browsers: ['last 2 versions'],
            cascade: false
         }))
    .pipe(cssnano())
    .pipe(gulp.dest("public/css"));
})

gulp.task('scripts', function() {
    return gulp.src('assets/javascript/*.js')
      .pipe(concat('scripts.js'))
      .pipe(uglify())
      .pipe(rename({suffix: '.min'}))
      .pipe(gulp.dest('public/javascript'));
})

gulp.task('imgs', function() {
    return gulp.src("assets/images/*.+(jpg|jpeg|png|gif)")
      .pipe(imagemin({
            progressive: true,
            svgoPlugins: [{ removeViewBox: false }],
            interlaced: true
        }))
      .pipe(gulp.dest("public/images"))
})

gulp.task("watch", function() {
    watch("assets/*.html", ["html"]);
    watch("assets/css/*.scss", ["sass"]);
    watch("assets/javascript/*.js", ["scripts"]);
    watch("assets/images/*.+(jpg|jpeg|png|gif)", ["imgs"]);
});

gulp.task("default", ["html", "sass", "scripts", "imgs", "watch"]);

问题。 为什么gulp watch在调用 gulp watch 或 gulp default 后无法跟踪更改？