expo-module — Claude Code skill: build Expo native modules

---
name: expo-module
description: Guide for creating and writing Expo native modules and views using the Expo Modules API (Swift, Kotlin, TypeScript). Covers module definition DSL, native views, shared objects, config plugins, lifecycle hooks, autolinking, and type system. Use when building or modifying native modules for Expo. Not for migrating an existing Swift module from the definition DSL to the Expo Modules API 2.0 macros; use expo-migrate-module (from the expo-experiments plugin) for that.
version: 1.0.0
license: MIT
---

# Writing Expo Modules

Complete reference for building native modules and views using the Expo Modules API. Covers Swift (iOS), Kotlin (Android), and TypeScript.

## When to Use

- Creating a new Expo native module or native view
- Adding native functionality (camera, sensors, system APIs) to an Expo app
- Wrapping platform SDKs for React Native consumption
- Building config plugins that modify native project files
- Adding Android, Apple, or web support to an existing Expo module
- Editing `expo-module.config.json`, config plugins, or lifecycle hooks

To migrate an existing Swift module from the definition DSL to the Expo Modules API 2.0 macros (`@ExpoModule`, `@JS`, `@Event`), use the `expo-migrate-module` skill (from the `expo-experiments` plugin) instead.

## References

Consult these resources as needed:

```
references/
  create-expo-module.md      Scaffolding and add-platform-support workflow, defaults, and quirks
  native-module.md           Module definition DSL: Name, Function, AsyncFunction, Property, Constant, Events, type system, shared objects
  native-view.md             Native view components: View, Prop, EventDispatcher, view lifecycle, ref-based functions
  lifecycle.md               Lifecycle hooks: module, iOS app/AppDelegate, Android activity/application listeners
  config-plugin.md           Config plugins: modifying Info.plist, AndroidManifest.xml, reading values in native code
  module-config.md           expo-module.config.json fields, file placement, and autolinking behavior
```

## Quick Start

Prefer `create-expo-module` over manually creating native module files and directories. In practice, the best path is usually to create the scaffold first and then build on top of it. The scaffold sets up the expected layout, `expo-module.config.json`, podspec or Gradle files, TypeScript bindings, and the standalone example app flow.

If an existing Expo module only needs another platform, use `create-expo-module add-platform-support` instead of manually copying native directories.

See [references/create-expo-module.md](references/create-expo-module.md) before scaffolding or extending a module. It covers:

- local vs standalone modules
- `--platform`, `--features`, `--barrel`, `--package-manager`, and non-interactive mode
- `expo.autolinking.nativeModulesDir`
- `add-platform-support` behavior and quirks

## Recommended Workflow

1. Choose the scaffold type first:
   - **Local module** for one app
   - **Standalone module** for reuse, monorepos, or publishing
2. Determine native `expo-module` features that you will need.
   - Based on the user's instructions determine which feature scaffolding will be useful.
   - Available features: `Constant`, `Function`, `AsyncFunction`, `Event`, `View`, `ViewEvent`, `SharedObject`
3. Scaffold deliberately:
   - pass an explicit slug or path
   - choose `--platform` intentionally instead of relying on defaults
   - use `--features` to choose code samples which you will modify in the next step to match the real implementation.
4. Replace generated example code with the real implementation.
5. If you add a new platform later, prefer `add-platform-support` over manual file copying.

## Practical Scaffolding Rules

- Feature examples are **opt-in**. A newly scaffolded module may be minimal if no features were selected.
- `ViewEvent` implies `View`.
- Local modules do **not** generate an `index.ts` barrel by default. Use `--barrel` only if you want one.
- In non-interactive local scaffolding, pass the positional slug or path explicitly. `--name` changes the native class name, not the folder name.
- Local modules live in `expo.autolinking.nativeModulesDir` when configured, otherwise in `modules/`.
- Standalone modules have their own package metadata, scripts, and usually an example app. Local modules use the host app's tooling instead.

## Core File Shapes

The Swift and Kotlin DSL share the same structure. Swift is usually the clearest primary example; consult the references for feature-specific details.

## Module Structure Reference

The Swift and Kotlin DSL share the same structure. Both platforms are shown here for reference — in other reference files, Swift is shown as the primary language unless the Kotlin pattern meaningfully differs.

**Swift (iOS):**

```swift
import ExpoModulesCore

public class MyModule: Module {
  public func definition() -> ModuleDefinition {
    Name("MyModule")

    Function("hello") { (name: String) -> String in
      return "Hello \(name)!"
    }
  }
}
```

**Kotlin (Android):**

```kotlin
package expo.modules.mymodule

import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition

class MyModule : Module() {
  override fun definition() = ModuleDefinition {
    Name("MyModule")

    Function("hello") { name: String ->
      "Hello $name!"
    }
  }
}
```

**TypeScript:**

```typescript
import { requireNativeModule } from "expo";

const MyModule = requireNativeModule("MyModule");

export function hello(name: string): string {
  return MyModule.hello(name);
}
```

### expo-module.config.json

```json
{
  "platforms": ["android", "apple"],
  "apple": {
    "modules": ["MyModule"]
  },
  "android": {
    "modules": ["expo.modules.mymodule.MyModule"]
  }
}
```

Note: iOS uses just the class name; Android uses the fully-qualified class name (package + class). See `references/module-config.md` for all fields.

## Submitting Feedback
If you encounter errors, misleading or outdated information in this skill, report it so Expo can improve:
```bash
npx --yes @expo/agent-cli@latest feedback --category skills --subject "expo-module" "<actionable feedback>"
```
Only submit when you have something specific and actionable to report. Include as much relevant context as possible.
If an agent repeatedly fails at an Expo task or the user has to step in, submit a brief report of the task, expected result, and what went wrong using `--category evals` and the relevant Expo package or command as `--subject`.