feat: add watcher to Babel transpiling

It is no possible to watch for changes in the source directory and
get the files automatically transpiled. Run AEgir as

    aegir build --no-dist --watch

cmds/build.js

@@ -1,7 +1,12 @@
 'use strict'
 
+const path = require('path')
+
+const chokidar = require('chokidar')
+
 const build = require('../src/build')
 const onError = require('../src/error-handler')
+const utils = require('../src/utils')
 
 module.exports = {
   command: 'build',
@@ -16,6 +21,11 @@ module.exports = {
       alias: 'l',
       describe: 'Transpile src to lib',
       default: true
+    },
+    watch: {
+      alias: 'w',
+      describe: 'Keep watching source files for changes',
+      default: false
     }
   },
   handler (argv) {

package.json

@@ -34,6 +34,7 @@
     "babel-preset-flow-node": "^2.0.1",
     "browserify-zlib": "^0.2.0",
     "chalk": "^2.3.0",
+    "chokidar": "^2.0.3",
     "clean-documentation-theme": "^0.5.2",
     "codecov": "^3.0.0",
     "conventional-changelog": "^1.1.7",

src/build/lib.js

@@ -4,10 +4,13 @@
 const path = require('path')
 
 const babelCore = require('babel-core')
+const chokidar = require('chokidar')
 const fs = require('fs-extra')
 const glob = require('glob')
 const pify = require('pify')
 
+const utils = require('../utils')
+
 const babelConfig = {
   env: {
     development: {
@@ -28,7 +31,17 @@ const babelConfig = {
   }
 }
 
-const transform = (src, dest) => {
+/**
+ * Babel transpiles a file from `src` to `lib`
+ *
+ * @param {string} filename The filename relative to the `src` directory
+ *
+ * @returns {Promise}
+ */
+const transform = (filename) => {
+  const src = path.join('src', filename)
+  const dest = path.join('lib', filename)
+
   // To have the right filename in the source map
   babelConfig.sourceFileName = path.join('..', src)
 
@@ -45,17 +58,37 @@ const transform = (src, dest) => {
   })
 }
 
-const babel = () => {
-  const src = 'src'
-  const dest = 'lib'
+const babel = (ctx) => {
+  const srcDir = path.join(utils.getBasePath(), 'src')
 
-  return pify(glob)('./**/*.js', {
-    cwd: path.join(process.cwd(), src)
-  }).then((filenames) => {
-    return Promise.all(filenames.map((filename) => {
-      return transform(path.join(src, filename), path.join(dest, filename))
-    }))
-  })
+  if (ctx.watch) {
+    // The watcher code is based on the babel-cli code (MIT licensed):
+    // https://github.com/babel/babel/blob/6597a472b30419493f123bff1e9194e4c09e488e/packages/babel-cli/src/babel/dir.js#L164-L188`
+    const watcher = chokidar.watch(srcDir, {
+      persistent: true,
+      ignoreInitial: false,
+      awaitWriteFinish: {
+        stabilityThreshold: 50,
+        pollInterval: 10
+      }
+    })
+
+    ;['add', 'change'].forEach((type) => {
+      watcher.on(type, (filename) => {
+        const relative = path.relative(srcDir, filename)
+        console.log('Transpile file: ' + relative)
+        transform(relative)
+      })
+    })
+  } else {
+    return pify(glob)('./**/*.js', {
+      cwd: srcDir
+    }).then((filenames) => {
+      return Promise.all(filenames.map((filename) => {
+        return transform(filename)
+      }))
+    })
+  }
 }
 
 module.exports = babel