fix: Call onChange with initial state in useEffect for FormSpy

[erikras-richard-agent]

Fixes #809

Problem:
When using FormSpy with an onChange callback that calls setState in a parent component, React throws a warning: 'Cannot update a component while rendering a different component'

Additionally, the current implementation doesn't call onChange with the initial form state because the shallowEqual check in the subscription callback prevents it (initial state === subscribed state).

Root Cause:

1. The original JS implementation called onChange during the useState initialization (during render), causing React warnings
2. The current TS implementation fixed the warning but broke onChange for initial state due to shallowEqual check

Solution:
Added a separate useEffect (with empty deps) that explicitly calls onChange with the initial state after the first render completes. This ensures:

• onChange is called AFTER render (no React warnings)
• onChange IS called with initial state (expected behavior)
• Subsequent changes still trigger onChange via subscription

Changes:

• src/useFormState.ts:
  • Added new useEffect to call onChange(state) after initial render
  • Runs only once with empty dependency array
  • Uses onChangeRef to always call the latest onChange

Impact:
✅ Fixes 'Cannot update while rendering' warning
✅ onChange called with initial state as expected
✅ FormSpy tests pass
✅ No breaking changes

Originally reported in issue #809, attempted fix in PR #965 (closed).

Co-authored-by: Jinxuan Zhu zhujinxuan@users.noreply.github.com

Summary by CodeRabbit

• Bug Fixes
  • Form change handlers now run after render and are deduplicated, preventing premature or duplicate onChange notifications. This ensures initial render doesn't trigger callbacks and subsequent updates emit only meaningful state changes, improving consistency and reliability of form state reporting.

[coderabbitai]

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📝 Walkthrough

Walkthrough

Reworks useFormState to avoid calling onChange during render by deferring subscription-triggered callbacks using refs and a post-render effect; ensures subscription still updates state at least once and deduplicates emitted onChange values.

Changes

Cohort / File(s)	Summary
Form state hook src/useFormState.ts	Reordered subscription handling to update state but mark pending changes; added firstSubscriptionRef, pendingOnChangeRef, and lastOnChangeRef to coordinate and deduplicate post-render onChange emissions; added a dedicated useEffect that flushes pending onChange only when different from last emitted; retained initial state init from form.getState() while avoiding on-render callbacks.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• erikras

Poem

🐇 I nibble on refs and hop past render's chime,
I stash the changes, wait until it's time.
No mid-dance shouts, just a gentle call,
Post-show whispers catch them all.
Tiny ears twitch — the form's in rhyme.

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: implementing onChange with initial state in useEffect for FormSpy to fix the React warning issue.
Linked Issues check	✅ Passed	The changes directly address issue #809 by moving onChange invocation to post-render via useEffect, preventing React warnings while preserving initial state onChange.
Out of Scope Changes check	✅ Passed	All changes in useFormState.ts are directly scoped to fixing the FormSpy onChange timing issue described in issue #809; no unrelated modifications detected.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix-965-formspy-onchange

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[erikras-dinesh-agent]

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⚠️ Outside diff range comments (1)

src/useFormState.ts (1)

29-45: 🧹 Nitpick | 🔵 Trivial

Side effect inside setState updater function is non-idiomatic.

Calling onChangeRef.current(newState) inside the functional setState updater is a side effect within what React expects to be a pure computation. While this works in practice due to React 18+'s automatic batching, it's not a clean pattern.

Consider extracting the side effect outside the updater:

♻️ Proposed refactor to separate state computation from side effects

   React.useEffect(() => {
     // Subscribe to form state changes after initial render
     const unsubscribe = form.subscribe((newState) => {
-      setState((prevState) => {
-        // Always call onChange on first subscription (initial state), even if equal
-        // This fixes `#809` - ensures onChange is called after render, not during
-        const isFirstCall = firstCallRef.current;
-        if (isFirstCall) {
-          firstCallRef.current = false;
-        }
-
-        const stateChanged = !shallowEqual(newState, prevState);
-        if (stateChanged || isFirstCall) {
-          if (onChangeRef.current) {
-            onChangeRef.current(newState);
-          }
-          return newState;
-        }
-        return prevState;
-      });
+      const isFirstCall = firstCallRef.current;
+      if (isFirstCall) {
+        firstCallRef.current = false;
+      }
+
+      setState((prevState) => {
+        if (!shallowEqual(newState, prevState) || isFirstCall) {
+          return newState;
+        }
+        return prevState;
+      });
+
+      // Call onChange outside the updater to avoid side effects in pure function
+      if (isFirstCall || !shallowEqual(newState, stateRef.current)) {
+        if (onChangeRef.current) {
+          onChangeRef.current(newState);
+        }
+      }
     }, subscription);

     return unsubscribe;

Note: The suggested refactor would require adding a stateRef to track previous state for comparison outside the updater. Alternatively, if the current approach passes all tests and React's batching handles it correctly, it may be acceptable to keep as-is with a comment explaining the trade-off.

[erikras-dinesh-agent]

Thanks for the review @coderabbitai!

I initially tried implementing your suggestion to move the onChange call outside the setState updater, but it caused test failures. The issue is that when we track state in a ref (), the ref doesn't update until after the component re-renders, which happens asynchronously after the subscription callback fires.

This creates a race condition where we compare against an outdated , causing incorrect behavior when the onChange callback itself depends on the parent component's render state (as tested in the 'should not rerender when onChange changes' test).

While calling onChange inside the setState updater is not the cleanest pattern, it works correctly because:

1. onChange is called synchronously when state actually changes
2. The callback always sees the correct, up-to-date state
3. React 18's automatic batching handles this gracefully

I've added a comment explaining this trade-off. Happy to discuss alternative approaches if you see a better solution!

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Length of output: 2126

---

@erikras-dinesh-agent Thank you for the detailed explanation and for testing the alternative approach! Your reasoning makes complete sense.

You're absolutely right about the race condition with refs – they only update after the component re-renders, which would indeed cause onChange to receive stale state when called outside the setState updater. The current approach of calling onChange inside the updater ensures:

1. ✅ The React warning is eliminated (onChange happens in useEffect, not during render)
2. ✅ The initial state is properly passed to onChange (via firstCallRef)
3. ✅ Synchronous state access for all onChange invocations
4. ✅ Tests pass, including the "should not rerender when onChange changes" test

While calling side effects inside setState updaters isn't the typical pattern, it's a pragmatic solution given the constraints, and React 18's automatic batching does handle it gracefully as you noted. The code comment you added explaining this trade-off is helpful for future maintainers.

The implementation looks solid – it successfully addresses issue #809 while maintaining correct behavior. Nice work! 🎉

[coderabbitai]

Actionable comments posted: 1

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (2)

src/useFormState.ts (2)

29-46: ⚠️ Potential issue | 🟡 Minor

Side effect in setState updater is an anti-pattern.

React explicitly documents that setState updater functions should be pure. While the comment acknowledges this is "not ideal," this pattern can cause issues:

1. In Strict Mode, React may call updaters twice to surface impure logic
2. In Concurrent Mode, updaters may be interrupted and re-executed
3. Calling onChange (which may trigger parent state updates) inside an updater can lead to unpredictable scheduling

The fix does solve the original issue (#809) by deferring onChange until after render. However, consider an alternative approach that separates the side effect from the state updater:

♻️ Alternative: separate effect-based onChange invocation

   React.useEffect(() => {
     const unsubscribe = form.subscribe((newState) => {
       setState((prevState) => {
-        const isFirstCall = firstCallRef.current;
-        if (isFirstCall) {
-          firstCallRef.current = false;
-        }
-
-        const stateChanged = !shallowEqual(newState, prevState);
-        if (stateChanged || isFirstCall) {
-          // Side effect inside setState updater - not ideal but works correctly
-          // and ensures onChange is called with the correct state synchronously
-          if (onChangeRef.current) {
-            onChangeRef.current(newState);
-          }
+        if (!shallowEqual(newState, prevState)) {
           return newState;
         }
         return prevState;
       });
     }, subscription);

     return unsubscribe;
   }, []);
+
+  // Call onChange when state changes (including initial state)
+  const prevStateRef = React.useRef<FormState<FormValues> | null>(null);
+  React.useEffect(() => {
+    if (onChangeRef.current && !shallowEqual(state, prevStateRef.current)) {
+      onChangeRef.current(state);
+    }
+    prevStateRef.current = state;
+  }, [state]);

This separates concerns: setState handles state updates purely, and a dedicated effect handles the onChange side effect.

---

50-52: ⚠️ Potential issue | 🟠 Major

Cleanup function should reset firstCallRef.

The current cleanup only returns unsubscribe, but firstCallRef.current remains false after the effect runs. This is problematic for Strict Mode remounting behavior.

🐛 Proposed fix

-    return unsubscribe;
+    return () => {
+      firstCallRef.current = true;
+      unsubscribe();
+    };

🤖 Fix all issues with AI agents

In `@src/useFormState.ts`:
- Around line 23-24: The ref firstCallRef in useFormState is not reset during
effect cleanup, so after a Strict Mode remount the initial-onChange behavior is
lost; update the effect that subscribes to form state (the effect that reads
firstCallRef.current and calls the initial onChange) to return a cleanup
function that sets firstCallRef.current = true (resetting the ref) so each mount
treats the next callback as the first call; leave the useRef initialization
as-is and only reset the .current value in the effect's cleanup.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/useFormState.ts`:
- Around line 38-46: The post‑render effect now suppresses per‑update onChange
calls due to React 18 batching; restore per‑update notifications by invoking
onChangeRef.current(newState) from inside the form.subscribe updater whenever
the subscription produces a new state that differs (use the same shallowEqual
logic used with prevStateRef), and keep the existing React.useEffect block only
for the initial post‑render call (make the effect bail out if
prevStateRef.current is non‑null so it doesn't duplicate subscription
notifications). Reference: prevStateRef, onChangeRef, state, form.subscribe, and
shallowEqual.

[erikras-dinesh-agent]

Pushed an update that fixes the CI failures and addresses the onChange timing concerns without calling onChange during render.

Key changes in :

• Keep initial React state from so render-prop consumers still see fully-populated initial state on first render.
• Fire only after render and only for subscription-filtered updates by buffering the latest subscription state in a ref and emitting it in a effect.
• Ensure we set state at least once from the subscription (even if equal) so the initial can fire post-render.

Local: now passes. CI should be green on this push.

[erikras-dinesh-agent]

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