multithreading
Software Mansion's best practices for multithreading in React Native apps using react-native-worklets. Use when running JavaScript on multiple threads, offloading heavy computation from the JS thread, communicating between runtimes, or sharing data across threads. Trigger on: 'worklet', 'worklets', 'react-native-worklets', 'runOnUI', 'runOnJS', 'scheduleOnUI', 'scheduleOnRN', 'scheduleOnRuntime', 'createWorkletRuntime', 'background thread', 'UI thread', 'worker runtime', 'Serializable', 'Synchronizable', 'multithreading', 'parallel execution', 'offload computation', 'background processing', 'Bundle Mode', or any request to move work off the JS thread in a React Native app.
What this skill does
# React Native Multithreading with Worklets
Software Mansion's production multithreading patterns for React Native using `react-native-worklets`.
React Native Worklets lets you run JavaScript code in parallel across multiple threads and runtimes. It powers Reanimated, Gesture Handler, and Skia under the hood.
## Version Check
Before answering any multithreading question, check that `react-native-worklets` is up to date:
1. Read the user's `package.json` to find the installed `react-native-worklets` version.
2. Run `npm view react-native-worklets version` to get the latest published version.
3. If the installed version is older than the latest, inform the user and recommend upgrading before proceeding with implementation advice.
## Runtime Model
React Native apps have three kinds of runtimes. Picking the right target is the first decision:
```
What does the work need?
├── Respond to native events or drive animations on the same frame?
│ └── UI Runtime (main thread, one per app)
├── Heavy computation, data processing, or background tasks?
│ └── Worker Runtime (custom thread, many per app)
└── Access React state, navigation, or RN APIs?
└── RN Runtime (JS thread, one per app)
```
Runtimes do not share memory. Data crosses runtime boundaries through serialization (immutable copies) or Synchronizable (shared mutable state).
## API Decision Tree
```
Need to run code on a different runtime?
├── Fire-and-forget (no return value needed)?
│ ├── Target is UI Runtime → scheduleOnUI(fn, ...args)
│ ├── Target is RN Runtime → scheduleOnRN(fn, ...args)
│ └── Target is Worker → scheduleOnRuntime(runtime, fn, ...args)
├── Need the return value asynchronously (Promise)?
│ ├── Target is UI Runtime → await runOnUIAsync(fn, ...args)
│ └── Target is Worker → await runOnRuntimeAsync(runtime, fn, ...args)
└── Need the return value synchronously (blocks caller)?
├── Target is UI Runtime → runOnUISync(fn, ...args)
└── Target is Worker → runOnRuntimeSync(runtime, fn, ...args)
```
## Critical Rules
**The `'worklet'` directive**: functions that run on Worklet Runtimes must be workletized. Add `'worklet';` as the first statement in the function body. Callbacks passed to `scheduleOnUI`, `scheduleOnRuntime`, and similar APIs are autoworkletized by the Babel plugin.
```tsx
function computeOnUI() {
'worklet';
return 2 + 2;
}
```
**Don't call scheduling APIs from the wrong runtime**: `scheduleOnUI`, `runOnUISync`, `runOnUIAsync`, `runOnRuntimeSync`, `runOnRuntimeAsync`, `scheduleOnRuntime` can only be called from the RN Runtime (unless Bundle Mode is enabled). Calling them from a Worklet Runtime throws an error.
**Closures are copied, not shared**: when a worklet runs on a different runtime, its closure variables are serialized at invocation time. Mutating the original variable after scheduling has no effect on the worklet's copy.
**Deprecated APIs**: `runOnUI` is replaced by `scheduleOnUI`. `runOnJS` is replaced by `scheduleOnRN`. `runOnRuntime` is replaced by `scheduleOnRuntime`. The new APIs pass arguments directly instead of returning a curried function.
## References
Load at most one reference file per question.
| File | Load when question is about |
|------|------------------------------|
| `threading-api.md` | Scheduling work across runtimes, creating Worker Runtimes, sync vs async execution, migrating from deprecated APIs |
| `shared-memory.md` | Passing data between runtimes, closures in worklets, Serializable, Synchronizable, shared mutable state |
| `setup-and-advanced.md` | Installing worklets, Babel plugin config, Bundle Mode, testing with Jest, feature flags, troubleshooting |
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