useFormState: Reuse state from previous form submission (facebook#27321)

If a Server Action is passed to useFormState, the action may be
submitted before it has hydrated. This will trigger a full page
(MPA-style) navigation. We can transfer the form state to the next page
by comparing the key path of the hook instance.

`ReactServerDOMServer.decodeFormState` is used by the server to extract
the form state from the submitted action. This value can then be passed
as an option when rendering the new page. It must be passed during both
SSR and hydration.

```js
const boundAction = await decodeAction(formData, serverManifest);
const result = await boundAction();
const formState = decodeFormState(result, formData, serverManifest);

// SSR
const response = createFromReadableStream(<App />);
const ssrStream = await renderToReadableStream(response, { formState })

// Hydration
hydrateRoot(container, <App />, { formState });
```

If the `formState` option is omitted, then the state won't be
transferred to the next page. However, it must be passed in both places,
or in neither; misconfiguring will result in a hydration mismatch.

(The `formState` option is currently prefixed with `experimental_`)

Author: acdlite

fixtures/flight/server/global.js

@@ -138,11 +138,15 @@ app.all('/', async function (req, res, next) {
       // For HTML, we're a "client" emulator that runs the client code,
       // so we start by consuming the RSC payload. This needs a module
       // map that reverse engineers the client-side path to the SSR path.
-      const root = await createFromNodeStream(rscResponse, moduleMap);
+      const {root, formState} = await createFromNodeStream(
+        rscResponse,
+        moduleMap
+      );
       // Render it into HTML by resolving the client components
       res.set('Content-type', 'text/html');
       const {pipe} = renderToPipeableStream(root, {
         bootstrapScripts: mainJSChunks,
+        experimental_formState: formState,
       });
       pipe(res);
     } catch (e) {

fixtures/flight/server/region.js

@@ -46,7 +46,7 @@ const {readFile} = require('fs').promises;
 
 const React = require('react');
 
-async function renderApp(res, returnValue) {
+async function renderApp(res, returnValue, formState) {
   const {renderToPipeableStream} = await import(
     'react-server-dom-webpack/server'
   );
@@ -93,13 +93,13 @@ async function renderApp(res, returnValue) {
     React.createElement(App),
   ];
   // For client-invoked server actions we refresh the tree and return a return value.
-  const payload = returnValue ? {returnValue, root} : root;
+  const payload = {root, returnValue, formState};
   const {pipe} = renderToPipeableStream(payload, moduleMap);
   pipe(res);
 }
 
 app.get('/', async function (req, res) {
-  await renderApp(res, null);
+  await renderApp(res, null, null);
 });
 
 app.post('/', bodyParser.text(), async function (req, res) {
@@ -108,6 +108,7 @@ app.post('/', bodyParser.text(), async function (req, res) {
     decodeReply,
     decodeReplyFromBusboy,
     decodeAction,
+    decodeFormState,
   } = await import('react-server-dom-webpack/server');
   const serverReference = req.get('rsc-action');
   if (serverReference) {
@@ -139,7 +140,7 @@ app.post('/', bodyParser.text(), async function (req, res) {
       // We handle the error on the client
     }
     // Refresh the client and return the value
-    renderApp(res, result);
+    renderApp(res, result, null);
   } else {
     // This is the progressive enhancement case
     const UndiciRequest = require('undici').Request;
@@ -153,12 +154,14 @@ app.post('/', bodyParser.text(), async function (req, res) {
     const action = await decodeAction(formData);
     try {
       // Wait for any mutations
-      await action();
+      const result = await action();
+      const formState = decodeFormState(result, formData);
+      renderApp(res, null, formState);
     } catch (x) {
       const {setServerState} = await import('../src/ServerState.js');
       setServerState('Error: ' + x.message);
+      renderApp(res, null, null);
     }
-    renderApp(res, null);
   }
 });
 

fixtures/flight/src/App.js

@@ -15,7 +15,7 @@ import {Client} from './Client.js';
 
 import {Note} from './cjs/Note.js';
 
-import {like, greet} from './actions.js';
+import {like, greet, increment} from './actions.js';
 
 import {getServerState} from './ServerState.js';
 
@@ -32,9 +32,9 @@ export default async function App() {
       <body>
         <Container>
           <h1>{getServerState()}</h1>
-          <Counter />
-          <Counter2 />
-          <Counter3 />
+          <Counter incrementAction={increment} />
+          <Counter2 incrementAction={increment} />
+          <Counter3 incrementAction={increment} />
           <ul>
             {todos.map(todo => (
               <li key={todo.id}>{todo.text}</li>

fixtures/flight/src/Counter.js

@@ -1,14 +1,17 @@
 'use client';
 
 import * as React from 'react';
+import {experimental_useFormState as useFormState} from 'react-dom';
 
 import Container from './Container.js';
 
-export function Counter() {
-  const [count, setCount] = React.useState(0);
+export function Counter({incrementAction}) {
+  const [count, incrementFormAction] = useFormState(incrementAction, 0);
   return (
     <Container>
-      <button onClick={() => setCount(c => c + 1)}>Count: {count}</button>
+      <form>
+        <button formAction={incrementFormAction}>Count: {count}</button>
+      </form>
     </Container>
   );
 }

fixtures/flight/src/actions.js

@@ -18,3 +18,7 @@ export async function greet(formData) {
   }
   return 'Hi ' + name + '!';
 }
+
+export async function increment(n) {
+  return n + 1;
+}

fixtures/flight/src/index.js

@@ -24,21 +24,33 @@ async function callServer(id, args) {
   return returnValue;
 }
 
-let data = createFromFetch(
-  fetch('/', {
-    headers: {
-      Accept: 'text/x-component',
-    },
-  }),
-  {
-    callServer,
-  }
-);
-
 function Shell({data}) {
-  const [root, setRoot] = useState(use(data));
+  const [root, setRoot] = useState(data);
   updateRoot = setRoot;
   return root;
 }
 
-ReactDOM.hydrateRoot(document, <Shell data={data} />);
+async function hydrateApp() {
+  const {root, returnValue, formState} = await createFromFetch(
+    fetch('/', {
+      headers: {
+        Accept: 'text/x-component',
+      },
+    }),
+    {
+      callServer,
+    }
+  );
+
+  ReactDOM.hydrateRoot(document, <Shell data={root} />, {
+    // TODO: This part doesn't actually work because the server only returns
+    // form state during the request that submitted the form. Which means it
+    // the state needs to be transported as part of the HTML stream. We intend
+    // to add a feature to Fizz for this, but for now it's up to the
+    // metaframework to implement correctly.
+    experimental_formState: formState,
+  });
+}
+
+// Remove this line to simulate MPA behavior
+hydrateApp();

packages/react-client/src/ReactFlightReplyClient.js

@@ -7,7 +7,12 @@
  * @flow
  */
 
-import type {Thenable, ReactCustomFormAction} from 'shared/ReactTypes';
+import type {
+  Thenable,
+  FulfilledThenable,
+  RejectedThenable,
+  ReactCustomFormAction,
+} from 'shared/ReactTypes';
 
 import {
   REACT_ELEMENT_TYPE,
@@ -23,10 +28,6 @@ import {
 } from 'shared/ReactSerializationErrors';
 
 import isArray from 'shared/isArray';
-import type {
-  FulfilledThenable,
-  RejectedThenable,
-} from '../../shared/ReactTypes';
 
 import {usedWithSSR} from './ReactFlightClientConfig';
 

packages/react-dom/src/client/ReactDOMLegacy.js

@@ -142,6 +142,7 @@ function legacyCreateRootFromDOMContainer(
       noopOnRecoverableError,
       // TODO(luna) Support hydration later
       null,
+      null,
     );
     container._reactRootContainer = root;
     markContainerAsRoot(root.current, container);

packages/react-dom/src/client/ReactDOMRoot.js

@@ -7,7 +7,7 @@
  * @flow
  */
 
-import type {ReactNodeList} from 'shared/ReactTypes';
+import type {ReactNodeList, ReactFormState} from 'shared/ReactTypes';
 import type {
   FiberRoot,
   TransitionTracingCallbacks,
@@ -21,6 +21,8 @@ import {
   enableHostSingletons,
   allowConcurrentByDefault,
   disableCommentsAsDOMContainers,
+  enableAsyncActions,
+  enableFormActions,
 } from 'shared/ReactFeatureFlags';
 
 import ReactDOMSharedInternals from '../ReactDOMSharedInternals';
@@ -55,6 +57,7 @@ export type HydrateRootOptions = {
   unstable_transitionCallbacks?: TransitionTracingCallbacks,
   identifierPrefix?: string,
   onRecoverableError?: (error: mixed) => void,
+  experimental_formState?: ReactFormState<any> | null,
   ...
 };
 
@@ -302,6 +305,7 @@ export function hydrateRoot(
   let identifierPrefix = '';
   let onRecoverableError = defaultOnRecoverableError;
   let transitionCallbacks = null;
+  let formState = null;
   if (options !== null && options !== undefined) {
     if (options.unstable_strictMode === true) {
       isStrictMode = true;
@@ -321,6 +325,11 @@ export function hydrateRoot(
     if (options.unstable_transitionCallbacks !== undefined) {
       transitionCallbacks = options.unstable_transitionCallbacks;
     }
+    if (enableAsyncActions && enableFormActions) {
+      if (options.experimental_formState !== undefined) {
+        formState = options.experimental_formState;
+      }
+    }
   }
 
   const root = createHydrationContainer(
@@ -334,6 +343,7 @@ export function hydrateRoot(
     identifierPrefix,
     onRecoverableError,
     transitionCallbacks,
+    formState,
   );
   markContainerAsRoot(root.current, container);
   Dispatcher.current = ReactDOMClientDispatcher;

packages/react-dom/src/server/ReactDOMFizzServerBrowser.js

@@ -8,7 +8,7 @@
  */
 
 import type {PostponedState} from 'react-server/src/ReactFizzServer';
-import type {ReactNodeList} from 'shared/ReactTypes';
+import type {ReactNodeList, ReactFormState} from 'shared/ReactTypes';
 import type {BootstrapScriptDescriptor} from 'react-dom-bindings/src/server/ReactFizzConfigDOM';
 import type {ImportMap} from '../shared/ReactDOMTypes';
 
@@ -41,6 +41,7 @@ type Options = {
   onPostpone?: (reason: string) => void,
   unstable_externalRuntimeSrc?: string | BootstrapScriptDescriptor,
   importMap?: ImportMap,
+  experimental_formState?: ReactFormState<any> | null,
 };
 
 type ResumeOptions = {
@@ -117,6 +118,7 @@ function renderToReadableStream(
       onShellError,
       onFatalError,
       options ? options.onPostpone : undefined,
+      options ? options.experimental_formState : undefined,
     );
     if (options && options.signal) {
       const signal = options.signal;