refactor: root mixins injection (#423)

lib/app/util.js

+export function injectMixins (options, mixins) {
+  if (!options.mixins) {
+    options.mixins = []
+  }
+  options.mixins.push(...mixins)
+}
+
 export function pathToComponentName (path) {
   if (path.charAt(path.length - 1) === '/') {
     return `page${path.replace(/\//g, '-') + 'index'}`

lib/code/injectRootMixins.js

-/* eslint-disable */
-import rootMixins from '@app/root-mixins'
-function injectRootMixins (options) {
-  if (!options.mixins) {
-    options.mixins = []
-  }
-  options.mixins.push(...rootMixins)
-}

lib/prepare.js

@@ -342,14 +342,13 @@ async function genRoutesFile ({ siteData: { pages }, sourceDir, pageFiles }) {
     component: ThemeNotFound
   }`
 
-  const injectRootMixins = await fs.readFile(path.resolve(__dirname, 'code/injectRootMixins.js'), 'utf-8')
-
   return (
     `import ThemeLayout from '@themeLayout'\n` +
-    `import ThemeNotFound from '@themeNotFound'\n\n` +
-    `${injectRootMixins}\n` +
-    `injectRootMixins(ThemeLayout)\n` +
-    `injectRootMixins(ThemeNotFound)\n\n` +
+    `import ThemeNotFound from '@themeNotFound'\n` +
+    `import { injectMixins } from '@app/util'\n` +
+    `import rootMixins from '@app/root-mixins'\n\n` +
+    `injectMixins(ThemeLayout, rootMixins)\n` +
+    `injectMixins(ThemeNotFound, rootMixins)\n\n` +
     `export const routes = [${pages.map(genRoute).join(',')}${notFoundRoute}\n]`
   )
 }