add documentation for migrating from V1

documentation/docs/4_Others/4_1_Migrating from V1.md

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+---
+slug: /Migrating
+title: Migrating from V1 👴
+---
+
+For the V2, our code of conduct is "keep the code simple and concise" 🤓
+
+## Redux
+
+After the redux-toolkit release, and understood that we don't need the **power**, and the **large among of functionalities** that **Redux Saga** provides,
+we conclude that, because we want simple and concise code, we will now use redux-toolkit.
+
+🚨️ **We decided to remove Redux Saga from the boilerplate not because this isn't a good library or a good pattern but for less complexity and use an official and light dependency like redux-toolkit.** 🚨️
+
+|   Libraries           |   V1   | V2     |   Goal                                        |
+| :-------------------- | :----: | :----: | :---------------------------------------------|
+| redux                 |   ✅   |  ✅   | State management                              |
+| redux saga            |   ✅   |  ❌   | Redux middleware                              |
+| redux sauce           |   ✅   |  ❌   | Simplify redux syntax                         |
+| redux-toolkit         |   ❌   |  ✅   | New redux library with some function helpers  |
+| redux-toolkit-wrapper |   ❌   |  ✅   | Easier CRUD redux-toolkit function helpers    |
+
+
+### Migration guide
+
+This is not really a migration guide because there is so much breaking changes between the two versions and mostly because of the update of all dependencies.
+So, in next sections, there is a structure and code comparison.
+
+### Architecture
+
+First, a quick comparison on the tree files. On V1 the state management logic was divide in `Services`, `Sagas` and `Store`.
+In V2 it is divide in `Service` and `Store`. In V2, all directory as an `index.js` file for better imports and a homogenization of the code.
+
+```jsx title="V1"
+Services
+    └- UserService.js
+Sagas
+    ├- UserSaga.js
+    ├- StartupSaga.js
+    └- index.js
+Store
+    ├- Startup
+    │   └- Actions.js
+    ├- Theme...
+    ├- User
+    │   ├- Actions.js
+    │   ├- InitialSate.js
+    │   ├- Reducers.js
+    │   └- Selectors.js
+    ├- CreateStore.js
+    └- index.js
+```
+
+```jsx title="V2"
+Services
+    ├- User
+    │   ├- FetchOne.js
+    │   └- index.js
+    └- index.js
+Store
+    ├- Startup
+    │   ├- index.js
+    │   └- Init.js
+    ├- Theme...
+    ├- User
+    │   ├- FetchOne.js
+    │   └- index.js
+    └- index.js
+```
+
+### Configure store
+
+Thanks to a refactoring and redux-toolkit, the store configuration is now in one file easy to understand and flipper debugging ready.
+
+#### V1
+
+```jsx title="V1  App/Stores/CreateStore.js"
+import { applyMiddleware, compose, createStore } from 'redux'
+import createSagaMiddleware from 'redux-saga'
+import { persistReducer, persistStore } from 'redux-persist'
+import storage from 'redux-persist/lib/storage'
+
+const persistConfig = {
+  key: 'root',
+  storage: storage,
+  blacklist: [
+    // 'auth',
+  ],
+}
+
+export default (rootReducer, rootSaga) => {
+  const middleware = []
+  const enhancers = []
+
+  // Connect the sagas to the redux store
+  const sagaMiddleware = createSagaMiddleware()
+  middleware.push(sagaMiddleware)
+
+  enhancers.push(applyMiddleware(...middleware))
+
+  // Redux persist
+  const persistedReducer = persistReducer(persistConfig, rootReducer)
+
+  const store = createStore(persistedReducer, compose(...enhancers))
+  const persistor = persistStore(store)
+
+  // Kick off the root saga
+  sagaMiddleware.run(rootSaga)
+
+  return { store, persistor }
+}
+```
+```jsx title="V1  App/Stores/index.js"
+import { combineReducers } from 'redux'
+import configureStore from './CreateStore'
+import rootSaga from 'App/Sagas'
+import { reducer as ExampleReducer } from './Example/Reducers'
+
+export default () => {
+  const rootReducer = combineReducers({
+    /**
+     * Register your reducers here.
+     * @see https://redux.js.org/api-reference/combinereducers
+     */
+    example: ExampleReducer,
+  })
+
+  return configureStore(rootReducer, rootSaga)
+}
+```
+
+#### V2
+
+```jsx title="V2 src/Store/index.js"
+import AsyncStorage from '@react-native-async-storage/async-storage'
+import { combineReducers } from 'redux'
+import {
+  persistReducer,
+  persistStore,
+  FLUSH,
+  REHYDRATE,
+  PAUSE,
+  PERSIST,
+  PURGE,
+  REGISTER,
+} from 'redux-persist'
+import { configureStore } from '@reduxjs/toolkit'
+
+import startup from './Startup'
+import user from './User'
+import theme from './Theme'
+
+const reducers = combineReducers({
+  startup,
+  user,
+  theme,
+})
+
+const persistConfig = {
+  key: 'root',
+  storage: AsyncStorage,
+  whitelist: ['theme'],
+}
+
+const persistedReducer = persistReducer(persistConfig, reducers)
+
+const store = configureStore({
+  reducer: persistedReducer,
+  middleware: (getDefaultMiddleware) => {
+    const middlewares = getDefaultMiddleware({
+      serializableCheck: {
+        ignoredActions: [FLUSH, REHYDRATE, PAUSE, PERSIST, PURGE, REGISTER],
+      },
+    })
+
+    if (__DEV__ && !process.env.JEST_WORKER_ID) {
+      const createDebugger = require('redux-flipper').default
+      middlewares.push(createDebugger())
+    }
+
+    return middlewares
+  },
+})
+
+const persistor = persistStore(store)
+
+export { store, persistor }
+```
+
+### Example feature
+
+Now, a comparison with a feature example present in the V1 and in V2
+
+#### V1
+
+##### Store
+In the boilerplate V1, the creation of the Store goes like this :
+
+- init the state
+```jsx title="App/Stores/User/InitialState.js"
+export const INITIAL_STATE = {
+  user: {},
+  userIsLoading: false,
+  userErrorMessage: null,
+}
+```
+
+- creation of actions
+
+```jsx title="App/Stores/User/Actions.js"
+import { createActions } from 'reduxsauce'
+
+const { Types, Creators } = createActions({
+  fetchUser: null,
+  fetchUserLoading: null,
+  fetchUserSuccess: ['user'],
+  fetchUserFailure: ['errorMessage'],
+})
+
+export const ExampleTypes = Types
+export default Creators
+```
+
+- creation of associated reducers
+
+```jsx title="App/Stores/User/Reducers.js"
+import { INITIAL_STATE } from './InitialState'
+import { createReducer } from 'reduxsauce'
+import { ExampleTypes } from './Actions'
+
+export const fetchUserLoading = (state) => ({
+  ...state,
+  userIsLoading: true,
+  userErrorMessage: null,
+})
+
+export const fetchUserSuccess = (state, { user }) => ({
+  ...state,
+  user: user,
+  userIsLoading: false,
+  userErrorMessage: null,
+})
+
+export const fetchUserFailure = (state, { errorMessage }) => ({
+  ...state,
+  user: {},
+  userIsLoading: false,
+  userErrorMessage: errorMessage,
+})
+
+export const reducer = createReducer(INITIAL_STATE, {
+  [ExampleTypes.FETCH_USER_LOADING]: fetchUserLoading,
+  [ExampleTypes.FETCH_USER_SUCCESS]: fetchUserSuccess,
+  [ExampleTypes.FETCH_USER_FAILURE]: fetchUserFailure,
+})
+```
+
+##### Saga
+In the boilerplate V1, the creation of the Saga goes like this :
+
+- creation of the saga
+```jsx title="App/Sagas/UserSaga.js"
+import { put, call } from 'redux-saga/effects'
+import ExampleActions from 'App/Stores/Example/Actions'
+import { userService } from 'App/Services/UserService'
+
+export function* fetchUser() {
+  yield put(ExampleActions.fetchUserLoading())
+
+  const user = yield call(userService.fetchUser)
+  if (user) {
+    yield put(ExampleActions.fetchUserSuccess(user))
+  } else {
+    yield put(
+      ExampleActions.fetchUserFailure('There was an error while fetching user informations.')
+    )
+  }
+}
+```
+
+- listen the saga
+```jsx title="App/Sagas/index.js"
+import { takeLatest, all } from 'redux-saga/effects'
+import { ExampleTypes } from 'App/Stores/Example/Actions'
+import { StartupTypes } from 'App/Stores/Startup/Actions'
+import { fetchUser } from './ExampleSaga'
+import { startup } from './StartupSaga'
+
+export default function* root() {
+  yield all([
+    takeLatest(StartupTypes.STARTUP, startup),
+    takeLatest(ExampleTypes.FETCH_USER, fetchUser), // Add this line
+  ])
+}
+```
+
+
+##### Service
+In the boilerplate V1, the creation of the Service goes like this :
+
+```jsx title="App/Services/UserService.js"
+import axios from 'axios'
+import { Config } from 'App/Config'
+import { is, curryN, gte } from 'ramda'
+
+const isWithin = curryN(3, (min, max, value) => {
+  const isNumber = is(Number)
+  return isNumber(min) && isNumber(max) && isNumber(value) && gte(value, min) && gte(max, value)
+})
+const in200s = isWithin(200, 299)
+
+const userApiClient = axios.create({
+  baseURL: Config.API_URL,
+  headers: {
+    Accept: 'application/json',
+    'Content-Type': 'application/json',
+  },
+  timeout: 3000,
+})
+
+function fetchUser() {
+  const number = Math.floor(Math.random() / 0.1) + 1
+
+  return userApiClient.get(number.toString()).then((response) => {
+    if (in200s(response.status)) {
+      return response.data
+    }
+
+    return null
+  })
+}
+
+export const userService = {
+  fetchUser,
+}
+```
+
+#### V2
+
+##### Store
+In the boilerplate V2 action, initial state and reducers goes like this:
+
+```jsx title="src/Store/User/FetchOne.js"
+import {
+  buildAsyncState,
+  buildAsyncReducers,
+  buildAsyncActions,
+} from '@thecodingmachine/redux-toolkit-wrapper'
+import fetchOneUserService from '@/Services/User/FetchOne'
+
+export default {
+  initialState: buildAsyncState('fetchOne'),
+  action: buildAsyncActions('user/fetchOne', fetchOneUserService),
+  reducers: buildAsyncReducers({
+    errorKey: 'fetchOne.error',
+    loadingKey: 'fetchOne.loading',
+  }),
+}
+```
+
+```jsx title="src/Store/User/index.js"
+import { buildSlice } from '@thecodingmachine/redux-toolkit-wrapper'
+import FetchOne from './FetchOne'
+
+const sliceInitialState = { item: {} }
+
+export default buildSlice('user', [FetchOne], sliceInitialState).reducer
+```
+
+##### Service
+
+In the boilerplate V2, the creation of the Service goes like this :
+
+```javascript
+import api, { handleError } from '@/Services'
+
+export default async (userId) => {
+  if (!userId) {
+    return handleError({ message: 'User ID is required' })
+  }
+  const response = await api.get(`users/${userId}`)
+  return response.data
+}
+```
+
+## I18next
+This is a new feature of the boilerplate V2, now it handles internationalization thanks to [i18next](https://www.i18next.com/).
+See the documentation about it [here](../3_Guides/3_5_AddALangTranslation.md)
+
+## Flipper
+This is a new feature of the boilerplate V2, Flipper is now fully integrate with the redux debugger plugin.
+See the documentation about it [here](../3_Guides/3_6_UsingFlipper.md)