[Examples] Remove getInitialProps from with-rematch (#13834)

Related to [11014](https://github.com/vercel/next.js/issues/11014)

examples/with-rematch/pages/_app.js

-import App from 'next/app'
 import { Provider } from 'react-redux'
+import { useStore } from '../shared/store'
 
-import withRematch from '../shared/withRematch'
+export default function App({ Component, pageProps }) {
+  const store = useStore(pageProps.initialReduxState)
 
-class MyApp extends App {
-  render() {
-    const { Component, pageProps, reduxStore } = this.props
-    return (
-      <Provider store={reduxStore}>
-        <Component {...pageProps} />
-      </Provider>
-    )
-  }
+  return (
+    <Provider store={store}>
+      <Component {...pageProps} />
+    </Provider>
+  )
 }
-
-export default withRematch(MyApp)

examples/with-rematch/shared/store.js

+import { useMemo } from 'react'
 import { init } from '@rematch/core'
 import { counter, github } from './models'
 
+let store
+
 const exampleInitialState = {
   counter: 5,
 }
 
-export const initializeStore = (initialState = exampleInitialState) =>
+export const initStore = (initialState = exampleInitialState) =>
   init({
     models: {
       counter,
@@ -15,3 +18,30 @@ export const initializeStore = (initialState = exampleInitialState) =>
       initialState,
     },
   })
+
+export const initializeStore = (preloadedState) => {
+  let _store = store ?? initStore(preloadedState)
+
+  // After navigating to a page with an initial Redux state, merge that state
+  // with the current state in the store, and create a new store
+  if (preloadedState && store) {
+    _store = initStore({
+      ...store.getState(),
+      ...preloadedState,
+    })
+    // Reset the current store
+    store = undefined
+  }
+
+  // For SSG and SSR always create a new store
+  if (typeof window === 'undefined') return _store
+  // Create the store once in the client
+  if (!store) store = _store
+
+  return _store
+}
+
+export function useStore(initialState) {
+  const store = useMemo(() => initializeStore(initialState), [initialState])
+  return store
+}

examples/with-rematch/shared/withRematch.js

-import React from 'react'
-
-import { checkServer } from './utils'
-import { initializeStore } from './store'
-
-const __NEXT_REDUX_STORE__ = '__NEXT_REDUX_STORE__'
-
-function getOrCreateStore(initialState) {
-  // Always make a new store if server, otherwise state is shared between requests
-  if (checkServer()) {
-    return initializeStore(initialState)
-  }
-
-  // Create store if unavailable on the client and set it on the window object
-  if (!window[__NEXT_REDUX_STORE__]) {
-    window[__NEXT_REDUX_STORE__] = initializeStore(initialState)
-  }
-  return window[__NEXT_REDUX_STORE__]
-}
-
-export default function withRematch(App) {
-  return class AppWithRematch extends React.Component {
-    static async getInitialProps(appContext) {
-      // Get or Create the store with `undefined` as initialState
-      // This allows you to set a custom default initialState
-      const reduxStore = getOrCreateStore()
-
-      // Provide the store to getInitialProps of pages
-      appContext.ctx.reduxStore = reduxStore
-
-      let appProps = {}
-      if (typeof App.getInitialProps === 'function') {
-        appProps = await App.getInitialProps(appContext)
-      }
-
-      return {
-        ...appProps,
-        initialReduxState: reduxStore.getState(),
-      }
-    }
-
-    constructor(props) {
-      super(props)
-      this.reduxStore = getOrCreateStore(props.initialReduxState)
-    }
-
-    render() {
-      return <App {...this.props} reduxStore={this.reduxStore} />
-    }
-  }
-}