useFormStatus in progressively-enhanced forms

Before this change, `useFormStatus` is only activated if a form is
submitted by an action function (either <form action={actionFn}> or
<button formAction={actionFn}>).

After this change, `useFormStatus` will also be activated if you call
`startTransition(actionFn)` inside a submit event handler that is
`preventDefault`-ed.

This is the last missing piece for implementing a custom `action` prop
that is progressively enhanced using `onSubmit` while maintaining the
same behavior as built-in form actions.

Here's the basic recipe for implementing a progressively-enhanced
form action:

```js
import {requestFormReset} from 'react-dom';

// To implement progressive enhancement, pass both a form action *and* a
// submit event handler. The action is used for submissions that happen
// before hydration, and the submit handler is used for submissions that
// happen after.
<form
  action={action}
  onSubmit={(event) => {
    // After hydration, we upgrade the form with additional client-
    // only behavior.
    event.preventDefault();

    // Manually dispatch the action.
    startTransition(async () => {
      // (Optional) Reset any uncontrolled inputs once the action is
      // complete, like built-in form actions do.
      requestFormReset();

      // ...Do extra action-y stuff in here, like setting a custom
      // optimistic state...

      // Call the user-provided action
      const formData = new FormData(event.target);
      await action(formData);
    });
  }}
/>
```

packages/react-dom-bindings/src/events/plugins/FormActionEventPlugin.js

@@ -17,6 +17,7 @@ import type {FormStatus} from 'react-dom-bindings/src/shared/ReactDOMFormActions
 import {enableTrustedTypesIntegration} from 'shared/ReactFeatureFlags';
 import {getFiberCurrentPropsFromNode} from '../../client/ReactDOMComponentTree';
 import {startHostTransition} from 'react-reconciler/src/ReactFiberReconciler';
+import {didCurrentEventScheduleTransition} from 'react-reconciler/src/ReactFiberRootScheduler';
 import sanitizeURL from 'react-dom-bindings/src/shared/sanitizeURL';
 import {checkAttributeStringCoercion} from 'shared/CheckStringCoercion';
 
@@ -44,6 +45,30 @@ function coerceFormActionProp(
   }
 }
 
+function createFormDataWithSubmitter(
+  form: HTMLFormElement,
+  submitter: HTMLInputElement | HTMLButtonElement,
+) {
+  // The submitter's value should be included in the FormData.
+  // It should be in the document order in the form.
+  // Since the FormData constructor invokes the formdata event it also
+  // needs to be available before that happens so after construction it's too
+  // late. We use a temporary fake node for the duration of this event.
+  // TODO: FormData takes a second argument that it's the submitter but this
+  // is fairly new so not all browsers support it yet. Switch to that technique
+  // when available.
+  const temp = submitter.ownerDocument.createElement('input');
+  temp.name = submitter.name;
+  temp.value = submitter.value;
+  if (form.id) {
+    temp.setAttribute('form', form.id);
+  }
+  (submitter.parentNode: any).insertBefore(temp, submitter);
+  const formData = new FormData(form);
+  (temp.parentNode: any).removeChild(temp);
+  return formData;
+}
+
 /**
  * This plugin invokes action functions on forms, inputs and buttons if
  * the form doesn't prevent default.
@@ -67,8 +92,10 @@ function extractEvents(
   }
   const formInst = maybeTargetInst;
   const form: HTMLFormElement = (nativeEventTarget: any);
-  let action = (getFiberCurrentPropsFromNode(form): any).action;
-  let submitter: null | HTMLInputElement | HTMLButtonElement =
+  let action = coerceFormActionProp(
+    (getFiberCurrentPropsFromNode(form): any).action,
+  );
+  let submitter: null | void | HTMLInputElement | HTMLButtonElement =
     (nativeEvent: any).submitter;
   let submitterAction;
   if (submitter) {
@@ -87,10 +114,6 @@ function extractEvents(
     }
   }
 
-  if (typeof action !== 'function') {
-    return;
-  }
-
   const event = new SyntheticEvent(
     'action',
     'action',
@@ -101,44 +124,60 @@ function extractEvents(
 
   function submitForm() {
     if (nativeEvent.defaultPrevented) {
-      // We let earlier events to prevent the action from submitting.
-      return;
-    }
-    // Prevent native navigation.
-    event.preventDefault();
-    let formData;
-    if (submitter) {
-      // The submitter's value should be included in the FormData.
-      // It should be in the document order in the form.
-      // Since the FormData constructor invokes the formdata event it also
-      // needs to be available before that happens so after construction it's too
-      // late. We use a temporary fake node for the duration of this event.
-      // TODO: FormData takes a second argument that it's the submitter but this
-      // is fairly new so not all browsers support it yet. Switch to that technique
-      // when available.
-      const temp = submitter.ownerDocument.createElement('input');
-      temp.name = submitter.name;
-      temp.value = submitter.value;
-      if (form.id) {
-        temp.setAttribute('form', form.id);
+      // An earlier event prevented form submission. If a transition update was
+      // also scheduled, we should trigger a pending form status — even if
+      // no action function was provided.
+      if (didCurrentEventScheduleTransition()) {
+        // We're going to set the pending form status, but because the submission
+        // was prevented, we should not fire the action function.
+        const formData = submitter
+          ? createFormDataWithSubmitter(form, submitter)
+          : new FormData(form);
+        const pendingState: FormStatus = {
+          pending: true,
+          data: formData,
+          method: form.method,
+          action: action,
+        };
+        if (__DEV__) {
+          Object.freeze(pendingState);
+        }
+        startHostTransition(
+          formInst,
+          pendingState,
+          // Pass `null` as the action
+          // TODO: Consider splitting up startHostTransition into two separate
+          // functions, one that sets the form status and one that invokes
+          // the action.
+          null,
+          formData,
+        );
+      } else {
+        // No earlier event scheduled a transition. Exit without setting a
+        // pending form status.
       }
-      (submitter.parentNode: any).insertBefore(temp, submitter);
-      formData = new FormData(form);
-      (temp.parentNode: any).removeChild(temp);
-    } else {
-      formData = new FormData(form);
-    }
+    } else if (typeof action === 'function') {
+      // A form action was provided. Prevent native navigation.
+      event.preventDefault();
 
-    const pendingState: FormStatus = {
-      pending: true,
-      data: formData,
-      method: form.method,
-      action: action,
-    };
-    if (__DEV__) {
-      Object.freeze(pendingState);
+      // Dispatch the action and set a pending form status.
+      const formData = submitter
+        ? createFormDataWithSubmitter(form, submitter)
+        : new FormData(form);
+      const pendingState: FormStatus = {
+        pending: true,
+        data: formData,
+        method: form.method,
+        action: action,
+      };
+      if (__DEV__) {
+        Object.freeze(pendingState);
+      }
+      startHostTransition(formInst, pendingState, action, formData);
+    } else {
+      // No earlier event prevented the default submission, and no action was
+      // provided. Exit without setting a pending form status.
     }
-    startHostTransition(formInst, pendingState, action, formData);
   }
 
   dispatchQueue.push({

packages/react-dom-bindings/src/shared/ReactDOMFormActions.js

@@ -25,7 +25,7 @@ type FormStatusPending = {|
   pending: true,
   data: FormData,
   method: string,
-  action: string | (FormData => void | Promise<void>),
+  action: string | (FormData => void | Promise<void>) | null,
 |};
 
 export type FormStatus = FormStatusPending | FormStatusNotPending;