Refactor i18n functions a bit

ui/src/i18n/provider.js

@@ -0,0 +1,53 @@
+import polyglotI18nProvider from 'ra-i18n-polyglot'
+import deepmerge from 'deepmerge'
+import dataProvider from '../dataProvider'
+import en from './en.json'
+
+// Only returns current selected locale if its translations are found in localStorage
+const defaultLocale = function () {
+  const locale = localStorage.getItem('locale')
+  const current = JSON.parse(localStorage.getItem('translation'))
+  if (current && current.id === locale) {
+    return locale
+  }
+  return 'en'
+}
+
+const removeEmpty = (obj) => {
+  for (let k in obj) {
+    if (obj.hasOwnProperty(k) && typeof obj[k] === 'object') {
+      removeEmpty(obj[k])
+    } else {
+      if (!obj[k]) {
+        delete obj[k]
+      }
+    }
+  }
+}
+
+const prepareLanguage = (lang) => {
+  removeEmpty(lang)
+  // Make "albumSongs" resource use the same translations as "song"
+  lang.resources.albumSong = lang.resources.song
+  // ra.boolean.null should always be empty
+  lang.ra.boolean.null = ''
+  // Fallback to english translations
+  return deepmerge(en, lang)
+}
+
+export default polyglotI18nProvider((locale) => {
+  // English is bundled
+  if (locale === 'en') {
+    return prepareLanguage(en)
+  }
+  // If the requested locale is in already loaded, return it
+  const current = JSON.parse(localStorage.getItem('translation'))
+  if (current && current.id === locale) {
+    return prepareLanguage(JSON.parse(current.data))
+  }
+  // If not, get it from the server, and store it in localStorage
+  return dataProvider.getOne('translation', { id: locale }).then((res) => {
+    localStorage.setItem('translation', JSON.stringify(res.data))
+    return prepareLanguage(JSON.parse(res.data.data))
+  })
+}, defaultLocale())