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// You can filter out unchanged files between runs of a task using the gulp.src function's since option and gulp.lastRun:const paths = {
  ...
  images: {
    src: 'src/images/**/*.{jpg,jpeg,png}',
    dest: 'build/img/'
  }
}

function images() {
  return gulp.src(paths.images.src, {since: gulp.lastRun('images')})
    .pipe(imagemin({optimizationLevel: 5}))
    .pipe(gulp.dest(paths.images.dest));
}

function watch() {
  gulp.watch(paths.images.src, images);
}

// Task run times are saved in memory and are lost when gulp exits. It will only save time during the watch task when running the images task for a second time.