[RFR] I18n provider new signature

UPGRADE.md

@@ -1034,6 +1034,56 @@ If you had custom reducer or sagas based on these actions, they will no longer w
 
 **Tip**: If you need to clear the Redux state, you can dispatch the `CLEAR_STATE` action.
 
+## i18nProvider Signature Changed
+
+The i18nProvider, that react-admin uses for translating UI and content, now has a signature similar to the other providers: it accepts a message type (either `I18N_TRANSLATE` or `I18N_CHANGE_LOCALE`) and a params argument.
+
+```jsx
+// react-admin 2.x
+const i18nProvider = (locale) => messages[locale];
+
+// react-admin 3.x
+const i18nProvider = (type, params) => {
+    const polyglot = new Polyglot({ locale: 'en', phrases: messages.en });
+    let translate = polyglot.t.bind(polyglot);
+    if (type === 'I18N_TRANSLATE') {
+        const { key, options } = params;
+        return translate(key, options);
+    }
+    if type === 'I18N_CHANGE_LOCALE') {
+        const newLocale = params;
+        return new Promise((resolve, reject) => {
+            // load new messages and update the translate function
+        })
+    }
+} 
+```
+
+But don't worry: react-admin v3 exports a function called `polyglotI18nProvider`, that you can just wrap around your old `i18nProvider` to make it compatible with the new provider signature:
+
+```diff
+import React from 'react';
+-import { Admin, Resource } from 'react-admin';
++import { Admin, Resource, polyglotI18nProvider } from 'react-admin';
+import englishMessages from 'ra-language-english';
+import frenchMessages from 'ra-language-french';
+
+const messages = {
+    fr: frenchMessages,
+    en: englishMessages,
+};
+const i18nProvider = locale => messages[locale];
+
+const App = () => (
+-   <Admin locale="en" i18nProvider={i18nProvider}>
++   <Admin locale="en" i18nProvider={polyglotI18nProvider(i18nProvider)}>
+        ...
+    </Admin>
+);
+
+export default App;
+```
+
 ## The translation layer no longer uses Redux
 
 React-admin translation (i18n) layer lets developers provide translations for UI and content, based on Airbnb's [Polyglot](https://airbnb.io/polyglot.js/) library. The previous implementation used Redux and redux-saga. In react-admin 3.0, the translation utilities are implemented using a React context and a set of hooks. 

docs/Translation.md

@@ -7,7 +7,7 @@ title: "Translation"
 
 The react-admin user interface uses English as the default language. But you can also display the UI and content in other languages, allow changing language at runtime, even lazy-loading optional languages to avoid increasing the bundle size with all translations. 
 
-The react-admin translation layer is based on [polyglot.js](http://airbnb.io/polyglot.js/), which uses JSON files for translations. You will use translation features mostly via the `i18nProvider`, and a set of hooks (`useTranslate`, `useLocale`, `useSetLocale`).
+You will use translation features mostly via the `i18nProvider`, and a set of hooks (`useTranslate`, `useLocale`, `useSetLocale`).
 
 **Tip**: We'll use a bit of custom vocabulary in this chapter:
 
@@ -19,49 +19,94 @@ The react-admin translation layer is based on [polyglot.js](http://airbnb.io/pol
 Just like for data fetching and authentication, react-admin relies on a simple function for translations. It's called the `i18nProvider`, and here is its signature:
 
 ```jsx
-const i18nProvider = locale => messages;
+const i18nProvider = (type, params) => string | Promise;
 ```
 
-Given a locale, The `i18nProvider` function should return a dictionary of terms. For instance:
+The `i18nProvider` expects two possible `type` arguments: `I18N_TRANSLATE` and `I18N_CHANGE_LOCALE`. Here is the simplest possible implementation for a French and English provider:
 
 ```jsx
-const i18nProvider = locale => {
-    if (locale === 'en') {
-        return {
-            ra: {
-                notification: {
-                    http_error: 'Network error. Please retry',
-                },
-                action: {
-                    save: 'Save',
-                    delete: 'Delete',
-                },
-            },
-        };
+import { I18N_TRANSLATE, I18N_CHANGE_LOCALE } from 'react-admin';
+import lodashGet from 'lodash/get';
+
+const englishMessages = {
+    ra: {
+        notification: {
+            http_error: 'Network error. Please retry',
+        },
+        action: {
+            save: 'Save',
+            delete: 'Delete',
+        },
+    },
+};
+const frenchMessages = {
+    ra: {
+        notification: {
+            http_error: 'Erreur réseau, veuillez réessayer',
+        },
+        action: {
+            save: 'Enregistrer',
+            delete: 'Supprimer',
+        },
+    },
+};
+
+const i18nProvider = (type, params) => {
+    let messages = englishMessages;
+    if (type === I18N_TRANSLATE) {
+        const { key } = params;
+        return lodashGet(messages, key)
     }
-    if (locale === 'fr') {
-        return {
-            ra: {
-                notification: {
-                    http_error: 'Erreur réseau, veuillez réessayer',
-                },
-                action: {
-                    save: 'Enregistrer',
-                    delete: 'Supprimer',
-                },
-            },
-        };
+    if (type === I18N_CHANGE_LOCALE) {
+        const newLocale = params;
+        messages = (newLocale === 'fr') ? frenchMessages : englishMessages;
+        return Promise.resolve();
     }
 };
 ```
 
+But this is too naive: react-admin expects that i18nProviders support string interpolation for translation, and asynchronous message loading for locale change. That's why react-admin bundles an `i18nProvider` *factory* called `polyglotI18nProvider`. This factory relies on [polyglot.js](http://airbnb.io/polyglot.js/), which uses JSON files for translations. It only expects one argument: a function returning a list of messages based on a locale passed as argument. 
+
+So the previous provider can be written as:
+
+```jsx
+import { polyglotI18nProvider } from 'react-admin';
+
+const englishMessages = {
+    ra: {
+        notification: {
+            http_error: 'Network error. Please retry',
+        },
+        action: {
+            save: 'Save',
+            delete: 'Delete',
+        },
+    },
+};
+const frenchMessages = {
+    ra: {
+        notification: {
+            http_error: 'Erreur réseau, veuillez réessayer',
+        },
+        action: {
+            save: 'Enregistrer',
+            delete: 'Supprimer',
+        },
+    },
+};
+
+const i18nProvider = polyglotI18nProvider(locale => 
+    locale === 'fr' ? frenchMessages : englishMessages
+);
+```
+
 If you want to add or update tranlations, you'll have to provide your own `i18nProvider`.
 
 React-admin components use translation keys for their labels, and rely on the `i18nProvider` to translate them. For instance:
 
 ```jsx
 const SaveButton = ({ doSave }) => {
-    const translate = useTranslate();
+    const translate = useTranslate(); // calls the i18nProvider with the I18N_TRANSLATE type
     return (
         <Button onclick={doSave}>
             {translate('ra.action.save')} // will translate to "Save" in English and "Enregistrer" in French
@@ -91,10 +136,10 @@ The default react-admin locale is `en`, for English. If you want to display the
 
 ```jsx
 import React from 'react';
-import { Admin, Resource } from 'react-admin';
+import { Admin, Resource, polyglotI18nProvider } from 'react-admin';
 import frenchMessages from 'ra-language-french';
 
-const i18nProvider = () => frenchMessages;
+const i18nProvider = polyglotI18nProvider(() => frenchMessages);
 
 const App = () => (
     <Admin locale="fr" i18nProvider={i18nProvider}>
@@ -156,15 +201,15 @@ If you want to offer the ability to change locale at runtime, you must provide a
 
 ```jsx
 import React from 'react';
-import { Admin, Resource } from 'react-admin';
+import { Admin, Resource, polyglotI18nProvider } from 'react-admin';
 import englishMessages from 'ra-language-english';
 import frenchMessages from 'ra-language-french';
 
 const messages = {
     fr: frenchMessages,
     en: englishMessages,
 };
-const i18nProvider = locale => messages[locale];
+const i18nProvider = polyglotI18nProvider(locale => messages[locale]);
 
 const App = () => (
     <Admin locale="en" i18nProvider={i18nProvider}>
@@ -232,20 +277,21 @@ export default LocaleSwitcher;
 
 ## Lazy-Loading Locales
 
-Bundling all the possible locales in the `i18nProvider` is a great recipe to increase your bundle size, and slow down the initial application load. Fortunately, the `i18nProvider` may return a *promise* for locale change calls (except the initial call, when the app starts) to load secondary locales on demand. For example:
+Bundling all the possible locales in the `i18nProvider` is a great recipe to increase your bundle size, and slow down the initial application load. Fortunately, the `i18nProvider` returns a *promise* for locale change calls to load secondary locales on demand. And the `polyglotI18nProvider` accepts when its argument function returns a Promise, too. For example:
 
 ```js
+import { polyglotI18nProvider } from 'react-admin';
 import englishMessages from '../en.js';
 
-const i18nProvider = locale => {
+const i18nProvider = polyglotI18nProvider(locale => {
     if (locale === 'en') {
         // initial call, must return synchronously
         return englishMessages;
     }
     if (locale === 'fr') {
         return import('../i18n/fr.js').then(messages => messages.default);
     }
-}
+});
 
 const App = () => (
     <Admin locale="en" i18nProvider={i18nProvider}>
@@ -260,15 +306,20 @@ React-admin provides a helper function named `resolveBrowserLocale()`, which det
 
 ```jsx
 import React from 'react';
-import { Admin, Resource, resolveBrowserLocale } from 'react-admin';
+import { 
+    Admin,
+    Resource,
+    polyglotI18nProvider,
+    resolveBrowserLocale,
+} from 'react-admin';
 import englishMessages from 'ra-language-english';
 import frenchMessages from 'ra-language-french';
 
 const messages = {
     fr: frenchMessages,
     en: englishMessages,
 };
-const i18nProvider = locale => messages[locale] ? messages[locale] : messages.en;
+const i18nProvider = polyglotI18nProvider(locale => messages[locale] ? messages[locale] : messages.en);
 
 const App = () => (
     <Admin locale={resolveBrowserLocale()} i18nProvider={i18nProvider}>
@@ -283,7 +334,7 @@ Beware that users from all around the world may use your application, so make su
 
 ## Translation Messages
 
-The `message` returned by the `i18nProvider` value should be a dictionary where the keys identify interface components, and values are the translated string. This dictionary is a simple JavaScript object looking like the following:
+The `message` returned by the `polyglotI18nProvider` function argument should be a dictionary where the keys identify interface components, and values are the translated string. This dictionary is a simple JavaScript object looking like the following:
 
 ```jsx
 {
@@ -350,6 +401,7 @@ Using `resources` keys is an alternative to using the `label` prop in Field and
 When translating an admin, interface messages (e.g. "List", "Page", etc.) usually come from a third-party package, while your domain messages (e.g. "Shoe", "Date of birth", etc.) come from your own code. That means you need to combine these messages before passing them to `<Admin>`. The recipe for combining messages is to use ES6 destructuring:
 
 ```jsx
+import { Admin, polyglotI18nProvider } from 'react-admin';
 // interface translations
 import englishMessages from 'ra-language-english';
 import frenchMessages from 'ra-language-french';
@@ -361,7 +413,7 @@ const messages = {
     fr: { ...frenchMessages, ...domainMessages.fr },
     en: { ...englishMessages, ...domainMessages.en },
 };
-const i18nProvider = locale => messages[locale];
+const i18nProvider = polyglotI18nProvider(locale => messages[locale]);
 
 const App = () => (
     <Admin i18nProvider={i18nProvider}>

examples/demo/src/App.js

@@ -1,5 +1,5 @@
 import React, { Component } from 'react';
-import { Admin, Resource } from 'react-admin';
+import { Admin, Resource, polyglotI18nProvider } from 'react-admin';
 
 import './App.css';
 
@@ -20,14 +20,14 @@ import reviews from './reviews';
 import dataProviderFactory from './dataProvider';
 import fakeServerFactory from './fakeServer';
 
-const i18nProvider = locale => {
+const i18nProvider = polyglotI18nProvider(locale => {
     if (locale === 'fr') {
         return import('./i18n/fr').then(messages => messages.default);
     }
 
     // Always fallback on english
     return englishMessages;
-};
+});
 
 class App extends Component {
     state = { dataProvider: null };

examples/simple/src/Layout.js

@@ -0,0 +1,41 @@
+import React, { forwardRef } from 'react';
+import { Layout, AppBar, UserMenu, useLocale, useSetLocale } from 'react-admin';
+import { makeStyles, MenuItem, ListItemIcon } from '@material-ui/core';
+import { Language } from '@material-ui/icons';
+
+const useStyles = makeStyles(theme => ({
+    menuItem: {
+        color: theme.palette.text.secondary,
+    },
+}));
+
+const SwitchLanguage = forwardRef((props, ref) => {
+    const locale = useLocale();
+    const setLocale = useSetLocale();
+    const classes = useStyles();
+    return (
+        <MenuItem
+            ref={ref}
+            className={classes.menuItem}
+            onClick={() => {
+                setLocale(locale === 'en' ? 'fr' : 'en');
+                props.onClick();
+            }}
+        >
+            <ListItemIcon>
+                <Language />
+            </ListItemIcon>
+            Switch Language
+        </MenuItem>
+    );
+});
+
+const MyUserMenu = props => (
+    <UserMenu {...props}>
+        <SwitchLanguage />
+    </UserMenu>
+);
+
+const MyAppBar = props => <AppBar {...props} userMenu={<MyUserMenu />} />;
+
+export default props => <Layout {...props} appBar={MyAppBar} />;

examples/simple/src/i18nProvider.js

@@ -1,14 +1,15 @@
+import { polyglotI18nProvider } from 'react-admin';
 import englishMessages from './i18n/en';
 
 const messages = {
     fr: () => import('./i18n/fr.js').then(messages => messages.default),
 };
 
-export default locale => {
+export default polyglotI18nProvider(locale => {
     if (locale === 'fr') {
         return messages[locale]();
     }
 
     // Always fallback on english
     return englishMessages;
-};
+});

examples/simple/src/index.js

@@ -11,6 +11,7 @@ import CustomRouteLayout from './customRouteLayout';
 import CustomRouteNoLayout from './customRouteNoLayout';
 import dataProvider from './dataProvider';
 import i18nProvider from './i18nProvider';
+import Layout from './Layout';
 import posts from './posts';
 import users from './users';
 import tags from './tags';
@@ -22,6 +23,7 @@ render(
         i18nProvider={i18nProvider}
         title="Example Admin"
         locale="en"
+        layout={Layout}
         customReducers={{ tree }}
         customRoutes={[
             <Route