Docs: Add recipe for static asset revisioning

Author: silvenon

docs/recipes/README.md

@@ -28,3 +28,4 @@
 * [Rollup with rollup-stream](rollup-with-rollup-stream.md)
 * [Run gulp task via cron job](cron-task.md)
 * [Running shell commands](running-shell-commands.md)
+* [Static asset revisioning](static-asset-revisioning.md)

docs/recipes/static-asset-revisioning.md

@@ -0,0 +1,79 @@
+# Sass and Nunjucks with static asset revisioning
+
+Static asset revisioning by appending content hash to filenames. Make sure to set the files to [never expire](http://developer.yahoo.com/performance/rules.html#expires) for this to have an effect.
+
+Sass and Nunjucks are here just to demonstrate how to implement static asset revisioning in a more real-world scenario. The content has will be appended in production, so it's recommended to add the following npm scripts to `package.json`:
+
+```json
+{
+  "scripts": {
+    "dev": "gulp-dev",
+    "build": "NODE_ENV=production gulp build"
+  }
+}
+```
+
+Now to install the required dependencies:
+
+```sh
+npm install --save-dev gulp gulp-rev gulp-rev-rewrite gulp-sass gulp-nunjucks gulp-if del
+```
+
+And, finally, `gulpfile.js`:
+
+```js
+const gulp = require('gulp')
+const sass = require('gulp-sass')
+const nunjucks = require('gulp-nunjucks')
+const rev = require('gulp-rev')
+const revRewrite = require('gulp-rev-rewrite')
+const gulpIf = require('gulp-if')
+const del = require('del')
+
+const isProd = process.env.NODE_ENV === 'production'
+
+function clean() {
+  return del('dist')
+}
+
+function styles() {
+  return gulp
+    .src('styles/style.scss')
+    .pipe(sass().on('error', sass.logError))
+    // appends the content hash
+    .pipe(gulpIf(isProd, rev()))
+    .pipe(gulp.dest('dist'))
+    // output rev-manifest.json
+    .pipe(gulpIf(isProd, rev.manifest()))
+    .pipe(gulp.dest('dist'))
+}
+
+function views() {
+  const manifest = gulp.src(
+    'dist/rev-manifest.json',
+    // in development the manifest doesn't exist
+    { allowEmpty: !isProd }
+   )
+  return gulp
+    .src('views/**/*.njk')
+    .pipe(nunjucks.compile({ title: 'Hello world!' }))
+    // this updates the reference(s) to revisioned assets
+    .pipe(gulpIf(isProd, revRewrite({ manifest })))
+    .pipe(gulp.dest('dist'))
+}
+
+function watch() {
+  gulp.watch('styles/**/*.scss')
+  gulp.watch('views/**/*.njk')
+}
+
+// after everything is done, we no longer need the manifest file
+function deleteRevManifest() {
+  return del('dist/rev-manifest.json')
+}
+
+exports.dev = gulp.series(clean, gulp.parallel(styles, views), watch)
+// in build it's important that "views" runs AFTER "styles"
+// if it runs before, the manifest file won't exist yet
+exports.build = gulp.series(clean, styles, views, deleteRevManifest)
+```