Fix fatal crash Cannot read property 'translationX' of undefined when a touch event is serialized without allTouches

[huextrat]

Note

This PR was written with AI assistance (Claude), based on a production crash investigated from Sentry.

Description

Fixes a fatal, unhandled production crash in apps using the v3 API (usePanGesture + GestureDetector) with Reanimated on Android (Fabric / New Architecture):

TypeError: Cannot read property 'translationX' of undefined
    at diffCalculator
    at getChangeEventCalculator
    at handleUpdateEvent
    at eventHandler

Since the error is thrown inside a worklet running on the UI thread, it results in a native CppException caught by the UncaughtExceptionHandler — a hard crash, not a JS redbox.

Observed trigger: very fast repeated taps on a screen with a GestureDetector + pan gesture (reproduced in production on a low-end Android 15 device, but the underlying race is not device-specific).

Root cause

The failure is a chain across the Android event serialization and the v3 event classification:

1. Kotlin — GestureHandler.kt: dispatchTouchEvent() only guards on changedTouchesPayload != null. In dispatchTouchUpEvent(), extractAllPointersData() runs before the changed pointer is re-added to trackedPointers. If the tracked pointers were already cleared at that point (e.g. cancelPointers() fired by a rapid succession of taps racing with the touch-up dispatch), allTouchesPayload is null while changedTouchesPayload is still populated — so the event is dispatched anyway.

2. Kotlin — RNGestureHandlerTouchEvent.kt: the serializer omitted the key entirely when the payload was null:

   handler.consumeAllTouchesPayload()?.let { putArray("allTouches", it) }

   → a touch event can reach JS without the allTouches key. Note that iOS always serializes allTouches/changedTouches as (possibly empty) arrays (RNGestureHandlerManager.mm initializes .allTouches = {}), so this was also a platform inconsistency.

3. TS — src/v3/hooks/utils/eventUtils.ts: touch events are discriminated by key presence:

   export function isTouchEvent(...) { 'worklet'; return 'allTouches' in event; }

   Key absent → the touch event is misclassified as an update event.

4. TS — src/v3/hooks/callbacks/eventHandler.ts: the event is routed to handleUpdateEvent() → getChangeEventCalculator() reads current.handlerData (undefined for a touch event) and passes it to the gesture's diffCalculator, which reads current.translationX → TypeError in a UI-thread worklet → fatal crash.

Fix

JS (defensive, worklet-safe, no behavior change for valid events):

• eventHandler() now drops events that are neither state-change events, nor touch events, nor carry handlerData, instead of treating them as update events. (handlerData is present on all well-formed update events on every platform: Android createNativeEventData, web GestureHandler.ts, and jestUtils.)
• getChangeEventCalculator() returns the event unchanged when handlerData is undefined instead of calling the diff calculator with undefined data.

Android (root cause):

• RNGestureHandlerTouchEvent.createEventData() always serializes allTouches and changedTouches, falling back to empty arrays instead of omitting the keys — matching the iOS implementation.

Test plan

• New regression test in src/__tests__/api_v3.test.tsx: fires a malformed touch event (no allTouches, no oldState, no handlerData) through a pan gesture's jsEventHandler. Without the JS fix it reproduces the exact production error (Cannot read properties of undefined (reading 'translationX') through diffCalculator); with the fix the event is dropped without invoking any callback, and subsequent valid update events still compute changeX/changeY correctly.
• New unit tests for getChangeEventCalculator in src/__tests__/utils.test.tsx: change payload computed for well-formed events; event returned untouched when handlerData is missing.
• Full Jest suite passes (80/80), yarn ts-check clean, yarn lint:js clean.
• Android: spotlessCheck passes, library compiles via apps/basic-example (:react-native-gesture-handler:compileDebugKotlin — BUILD SUCCESSFUL).

[Copilot]

Pull request overview

This PR fixes a hard crash in the v3 gesture event pipeline caused by a malformed Android touch event being serialized without the allTouches key, which led to misclassification as an update event and a UI-thread worklet exception.

Changes:

• Make the v3 JS event router defensively drop events that are neither state-change nor touch events and that also lack handlerData, preventing the diff calculator from being called with undefined.
• Make the v3 change calculator return the event unchanged when handlerData is missing (worklet-safe guard).
• Fix Android touch event serialization to always include changedTouches and allTouches keys (empty arrays when payloads are unavailable), matching iOS behavior; add Jest regression/unit tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react-native-gesture-handler/src/v3/hooks/utils/eventUtils.ts	Adds a worklet-safe guard in getChangeEventCalculator and relies on allTouches key presence for touch-event discrimination.
packages/react-native-gesture-handler/src/v3/hooks/callbacks/eventHandler.ts	Drops malformed non-touch/non-state-change events with missing handlerData instead of routing them to update handling.
packages/react-native-gesture-handler/src/tests/utils.test.tsx	Adds unit tests covering normal change-payload computation and the missing-handlerData regression case.
packages/react-native-gesture-handler/src/tests/api_v3.test.tsx	Adds an integration-style regression test ensuring malformed touch events don’t crash and don’t break subsequent valid updates.
packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/react/events/RNGestureHandlerTouchEvent.kt	Ensures changedTouches and allTouches are always serialized (empty arrays as fallback) to avoid JS-side misclassification.

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