# `react-doctor/rn-animation-reaction-as-derived`

Use a derived value only when the destination shared value has one writer and no identity-sensitive consumers

- **Status:** Active
- **Category:** React Native
- **Assessment:** Evidence-required risk
- **Required evidence:** source code
- **Default configuration:** Enabled
- **Default severity:** warn
- **Scope:** React Native
- **Active when:** framework=react-native and capabilities=react-native
- **Requirements:** react-native
- **Tags:** react-native, test-noise
- **Priority:** 14 (P3)
- **Source:** oxlint-plugin-react-doctor
- **Rule set:** oxlint-plugin-react-doctor 0.9.3 (prompt schema 2)

## Validation prompt

Confirm the detector match and collect the required evidence before deciding whether an edit is warranted.

Verify the reaction body is exactly one assignment to a shared value, then inspect the destination's complete ownership. Confirm it has no other writers, is not passed by identity to imperative APIs or external consumers, and can be replaced at every read site. Multiple statements, runOnJS, conditions, logging, another writer, or an identity-sensitive consumer make useAnimatedReaction intentional or require a broader design change; do not apply the mechanical rewrite.

## Evidence boundary

The diagnostic proves only that the detector's modeled source pattern matched. It does not prove runtime impact, product intent, rendered failure, or that one remediation is correct.

Establish the environment, explicit repository policy, relevant exceptions, and required rendered or runtime evidence before deciding the occurrence. This page’s Assessment and Required evidence fields define the review contract.

Default severity is registry metadata. Use the occurrence's JSON severity after repository configuration when ordering real findings.

Record one outcome:

- **Confirmed failure:** The required evidence establishes the violation.
- **Rejected:** A documented exception or false-positive predicate applies.
- **Needs evidence:** Named evidence can still be collected.
- **Unavailable:** Required evidence cannot be collected in this run.
- **Waived with evidence:** An authorized, scoped exception applies to an established failure.
- **Observation:** The review records an optional tradeoff without claiming a defect.

A waiver records its scope, authority, evidence, and review or expiry condition. It is not a pass or false positive.

## Fix prompt

Apply this candidate correction only after the required evidence confirms the risk.

Only after proving exclusive ownership, replace the destination shared-value declaration with useDerivedValue(() => prepareExpression) and update every consumer to read that derived value; remove the reaction and the superseded declaration together. Preserve dependency-array behavior required by the installed Reanimated version. If the destination must keep its identity or has another writer, keep useAnimatedReaction. See https://docs.swmansion.com/react-native-reanimated/docs/core/useDerivedValue

## Repository-wide copy prompt

Use this only for a repository-wide pass after validating each occurrence. For one occurrence, use the occurrence-level guidance above.

````text
Fix every confirmed `react-doctor/rn-animation-reaction-as-derived` diagnostic in the current repository.

Required change:
- Only after proving exclusive ownership, replace the destination shared-value declaration with useDerivedValue(() => prepareExpression) and update every consumer to read that derived value; remove the reaction and the superseded declaration together. Preserve dependency-array behavior required by the installed Reanimated version. If the destination must keep its identity or has another writer, keep useAnimatedReaction. See https://docs.swmansion.com/react-native-reanimated/docs/core/useDerivedValue.

Validation before editing:
Verify the reaction body is exactly one assignment to a shared value, then inspect the destination's complete ownership. Confirm it has no other writers, is not passed by identity to imperative APIs or external consumers, and can be replaced at every read site. Multiple statements, runOnJS, conditions, logging, another writer, or an identity-sensitive consumer make useAnimatedReaction intentional or require a broader design change; do not apply the mechanical rewrite.

Constraints:
- Make the smallest change that fixes the root cause.
- Preserve behavior and interfaces unrelated to this diagnostic.
- Reuse existing project components, utilities, and conventions.
- Do not introduce render-phase side effects, render-phase state updates, or Hooks rule violations.
- Before editing, inspect the manifest, lockfile, native app configuration, React Native and Expo versions, and New Architecture status.
- Also inspect the Reanimated major, installed list-library majors, and every affected platform-specific file.
- Do not add or upgrade a native dependency, change linking, or alter Pods/Gradle setup unless explicitly authorized.
- Do not enable the New Architecture or change Expo/native configuration unless explicitly authorized.
- Preserve navigation, gesture, keyboard, safe-area, accessibility, lifecycle, and platform semantics; do not treat them as file-local import substitutions.
- Verify behavior in a release build on each affected device platform in scope. Name any unavailable platform instead of claiming it passed.
- Adapt identifiers and framework details instead of copying blindly.
- Do not disable the rule or suppress matching code.
- Confirm this rule is enabled for the project: `framework=react-native and capabilities=react-native`.

Assessment:
- Record detector evidence, applicability facts, assumptions, missing evidence, and the rule class for this occurrence.
- Return one outcome: Confirmed failure, Rejected, Needs evidence, Unavailable, Waived with evidence, or Observation.
- A waiver records the established failure, scope, authority, evidence, and review or expiry condition. It is not a pass or false positive.

Verification:
- Run focused tests for the changed behavior.
- Run React Doctor and confirm this diagnostic no longer appears from changed code.
- Run an unfiltered scan of the affected scope before claiming no cross-category regression.
- Report the files changed and any checks you could not run.
````