[hryhoriiK97]

Summary:

On Android, every touch event goes through TouchesHelper.sendTouchEvent, which builds the payload JS receives as nested WritableNativeMaps.

Current behavior. For each pointer of the MotionEvent, createPointersArray creates a native map and puts eight values into it, one JNI call each. Then, for each changed pointer, the payload is assembled from copies of those maps: the event itself is a copy of the changed pointer, and changedTouches and touches are arrays of further copies (copy(), pushMap(), putArray(), each one or more JNI calls). With one finger moving, that is about 20–25 JNI calls and 8 native-backed objects for every ACTION_MOVE.

With this change. TouchesHelper collects the pointer values once into a DoubleArray. For each event it creates an empty WritableNativeMap and passes it with the pointer data to a new native method, TouchesHelper.nativePutTouchPayload, which fills it in C++ through the map's existing putDouble, putInt and putNativeArray. That is two JNI calls per event (creating the map and filling it) instead of 20–25. TouchesHelper still decides which pointers go into changedTouches and touches, now as bit masks of pointer indices (start: the new pointer / all; end: the lifted pointer / all but it; move: all / all; cancel: all / none). The keys, values and value types of the payload are the same as before, and the events are dispatched exactly as before.

WritableNativeMap is unchanged. The native method is registered like TransformHelper's, from TouchesHelper.cpp in reactnativejni_common, and TouchesHelper is @DoNotStrip because the class is looked up by name when the library loads. TouchesHelper is internal, so this adds no public Kotlin API; the C++ API snapshots get the new TouchesHelper class. The layout of the pointer data (eight values per pointer) is documented on nativePutTouchPayload, and the C++ side and the test shadow point to it.

Changelog:

[ANDROID] [CHANGED] - Build touch event payloads in native code, making each touch event cheaper on the main thread

Test Plan:

• TouchEventDispatchTest passes. It checks the payloads of the start/move/end, start/move/cancel and multi-pointer start/pointer-up/cancel sequences, now through ShadowTouchesHelper, a Kotlin version of nativePutTouchPayload, and compares the map contents. With the end case deliberately broken (lifted pointer kept in touches), 2 of its 3 tests fail.
• ktfmt and the repository's clang-format report no changes for the touched files. The C++ API snapshots were regenerated with scripts/cxx-api.
• ./gradlew :packages:rn-tester:android:app:assembleRelease -PreactNativeArchitectures=arm64-v8a succeeds, from main at 01d5527.
• Same payloads. A local self-test (not part of this PR) built synthetic MotionEvents for 1 to 5 pointers, a pointer going down and up at every index, a move and a cancel, ran them through the previous implementation and through the first version of this PR, and compared the payloads with toHashMap(): 40 events, 60 payloads, no differences, on a moto g57 power. The current version builds the payload with the same C++ code; only the way the result is put into the map changed.
• Release builds of this PR and of main. Two separate RNTester release APKs (this PR, and main at 01d5527), RNTester Playground screen with a PanResponder drag area, 36 drags of 1.8 s per launch driven by adb shell input swipe (about 4,200 touch events per launch), installs alternated, moto g57 power (Android 16). On-CPU time of the main and JS threads during the drags from /proc, trace sections from Perfetto:

	main	this PR	change
Main thread, 36 drags	9.07 / 8.75 / 8.99 s	8.22 / 8.12 / 8.15 s	−8.7%
JS thread, 36 drags	5.01 / 4.88 / 5.05 s	4.97 / 4.92 / 5.00 s	no change
TouchesHelper.sentTouchEventModern(topTouchMove) trace section, ~700 events	449 µs	309 µs	−31%
Native heap after the drags (dumpsys meminfo)	44.3 / 42.5 / 42.4 MB	33.2 / 33.3 / 33.1 MB	−10 MB

The trace-section times include the tracing overhead. FabricEventEmitter.receiveEvent('topTouchMove') is about 11–17 µs slower with this PR (95 → 111 µs), which is small next to the saving in sentTouchEventModern. In every run JS received all touch moves, 10 out of 10 taps on Pressable rows were handled, and there were no errors in logcat.

• Not run: a C++ unit test of nativePutTouchPayload (JVM tests can't load the native library; it is covered on the device by the runs above and, through the shadow, by TouchEventDispatchTest).

🤖 Generated with Claude Code

[javache]

WritableNativeMap is a generic data structure, it should not be aware of touches.

Instead can we add a native method to TouchesHelper which creates the NativeMap specifically for that use-case?

[hryhoriiK97]

WritableNativeMap is a generic data structure, it should not be aware of touches.

Instead can we add a native method to TouchesHelper which creates the NativeMap specifically for that use-case?

@javache I've moved it. WritableNativeMap is unchanged now. TouchesHelper has its own native method, registered like TransformHelper, which fills an empty WritableNativeMap through its existing putDouble/putInt/putNativeArray.

I've re-measured on the same device: main thread −8.7% over 36 drags, topTouchMove 449 → 309 µs, same as before.

[javache]

Should be ReactNativeJNISoLoader

[hryhoriiK97]

Done, it now loads ReactNativeJNISoLoader and is registered in OnLoad.cpp next to TransformHelper - 279fc96

[javache]

This has been deprecated for long enough - let's remove it.

[hryhoriiK97]

Removed 👍

[javache]

Create the WritableNativeMap directly from C++

Suggested change

	val eventData = WritableNativeMap()
	nativePutTouchPayload(eventData, pointerData, pointer, changedPointers, touchPointers)
	val eventData = nativeCreateTouchPayload(pointerData, pointer, changedPointers, touchPointers)

[hryhoriiK97]

Done - 279fc96 via newObjectJavaArgs() + cthis(). newObjectCxxArgs aborts here ("Attempt to set C++ native pointer twice"), since the Kotlin constructor already calls initHybrid. Same perf as before.

[meta-codesync]

@javache has imported this pull request. If you are a Meta employee, you can view this in D128055442.