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Tutorial: Creating a native module

A tutorial on creating a native module with Expo modules API.

In this tutorial, we are going to build a module that stores the user's preferred app theme - either dark, light, or system. We'll use UserDefaults on iOS and SharedPreferences on Android. It is possible to implement web support using localStorage, but we'll leave that as an exercise for the reader.

1. Initialize a new module

First, we'll create a new module. On this page we will use the name expo-settings/ExpoSettings. You can name it whatever you like, just adjust the instructions accordingly:

Terminal
- npx create-expo-module expo-settings

Tip: Since you aren't going to actually ship this library, you can hit return for all of the prompts to accept the default values.

2. Set up our workspace

Now let's clean up the default module a little bit so we have more of a clean slate and delete the view module that we won't use in this guide.

Terminal
- cd expo-settings
- rm ios/ExpoSettingsView.swift
- rm android/src/main/java/expo/modules/settings/ExpoSettingsView.kt
- rm src/ExpoSettingsView.tsx src/ExpoSettings.types.ts
- rm src/ExpoSettingsView.web.tsx src/ExpoSettingsModule.web.ts

Find the following files and replace them with the provided minimal boilerplate:

ios/ExpoSettingsModule.swift
import ExpoModulesCore

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

    Function("getTheme") { () -> String in
      "system"
    }
  }
}
android/src/main/java/expo/modules/settings/ExpoSettingsModule.kt
package expo.modules.settings

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

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

    Function("getTheme") {
      return@Function "system"
    }
  }
}
src/index.ts
import ExpoSettingsModule from './ExpoSettingsModule';

export function getTheme(): string {
  return ExpoSettingsModule.getTheme();
}
example/App.tsx
import * as Settings from 'expo-settings';
import { Text, View } from 'react-native';

export default function App() {
  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>Theme: {Settings.getTheme()}</Text>
    </View>
  );
}

3. Run the example project

Now let's run the example project to make sure everything is working. We'll need to start the TypeScript compiler to watch for changes and rebuild the module JavaScript, and separately in another terminal window we'll compile and run the example app.

Terminal
# Run this in the root of the project to start the TypeScript compiler
- npm run build
Terminal
- cd example
# Run the example app on iOS
- npx expo run:ios
# Run the example app on Android
- npx expo run:android

We should now see the text "Theme: system" in the center of the screen when we launch the example app. The value "system" is the result of synchronously calling the getTheme() function in the native module. We'll change this value in the next step.

4. Get, set, and persist the theme preference value

iOS native module

To read the value on iOS, we can look for a UserDefaults string under the key "theme", and fall back to "system" if there isn't any.

To set the value, we can use UserDefaults's set(_:forKey:) method. We'll make our setTheme function accept a value of type String.

ios/ExpoSettingsModule.swift
import ExpoModulesCore

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

    Function("setTheme") { (theme: String) -> Void in
      UserDefaults.standard.set(theme, forKey:"theme")
    }

    Function("getTheme") { () -> String in
      UserDefaults.standard.string(forKey: "theme") ?? "system"
    }
  }
}

Android native module

To read the value, we can look for a SharedPreferences string under the key "theme", and fall back to "system" if there isn't any. We can get the SharedPreferences instance from the reactContext (a React Native ContextWrapper) using getSharedPreferences().

To set the value, we can use SharedPreferences's edit() method to get an Editor instance, and then use putString() to set the value. We'll make our setTheme function accept a value of type String.

android/src/main/java/expo/modules/settings/ExpoSettingsModule.kt
package expo.modules.settings

import android.content.Context
import android.content.SharedPreferences
import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition

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

    Function("setTheme") { theme: String ->
      getPreferences().edit().putString("theme", theme).commit()
    }

    Function("getTheme") {
      return@Function getPreferences().getString("theme", "system")
    }
  }

  private val context
  get() = requireNotNull(appContext.reactContext)

  private fun getPreferences(): SharedPreferences {
    return context.getSharedPreferences(context.packageName + ".settings", Context.MODE_PRIVATE)
  }
}

TypeScript module

Now we can call our native modules from TypeScript.

src/index.ts
import ExpoSettingsModule from './ExpoSettingsModule';

export function getTheme(): string {
  return ExpoSettingsModule.getTheme();
}

export function setTheme(theme: string): void {
  return ExpoSettingsModule.setTheme(theme);
}

Example app

We can now use the Settings API in our example app.

example/App.tsx
import * as Settings from 'expo-settings';
import { Button, Text, View } from 'react-native';

export default function App() {
  const theme = Settings.getTheme();
  // Toggle between dark and light theme
  const nextTheme = theme === 'dark' ? 'light' : 'dark';

  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>Theme: {Settings.getTheme()}</Text>
      <Button title={`Set theme to ${nextTheme}`} onPress={() => Settings.setTheme(nextTheme)} />
    </View>
  );
}

When we re-build and run the app, we'll see the "system" theme is still set. When we press the button, nothing happens. When you reload the app, you'll see the theme has changed. This is because we're never fetching the new theme value and re-rendering the app. We'll fix this in the next step.

5. Emit change events for the theme value

We can ensure that developers using our API can react to changes in the theme value by emitting a change event when the value changes. We'll use the Events definition component to describe events that our module can emit, sendEvent to emit the event from native, and the EventEmitter API to subscribe to events in JavaScript. Our event payload will be { theme: string }.

iOS native module

ios/ExpoSettingsModule.swift
import ExpoModulesCore

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

    Events("onChangeTheme")

    Function("setTheme") { (theme: String) -> Void in
      UserDefaults.standard.set(theme, forKey:"theme")
      sendEvent("onChangeTheme", [
        "theme": theme
      ])
    }

    Function("getTheme") { () -> String in
      UserDefaults.standard.string(forKey: "theme") ?? "system"
    }
  }
}

Android native module

Events payloads are represented as Bundle instances on Android, which we can create using the bundleOf function.

android/src/main/java/expo/modules/settings/ExpoSettingsModule.kt
package expo.modules.settings

import android.content.Context
import android.content.SharedPreferences
import androidx.core.os.bundleOf
import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition

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

    Events("onChangeTheme")

    Function("setTheme") { theme: String ->
      getPreferences().edit().putString("theme", theme).commit()
      this@ExpoSettingsModule.sendEvent("onChangeTheme", bundleOf("theme" to theme))
    }

    Function("getTheme") {
      return@Function getPreferences().getString("theme", "system")
    }
  }

  private val context
  get() = requireNotNull(appContext.reactContext)

  private fun getPreferences(): SharedPreferences {
    return context.getSharedPreferences(context.packageName + ".settings", Context.MODE_PRIVATE)
  }
}

TypeScript module

src/index.ts
import { EventEmitter, Subscription } from 'expo-modules-core';
import ExpoSettingsModule from './ExpoSettingsModule';

const emitter = new EventEmitter(ExpoSettingsModule);

export type ThemeChangeEvent = {
  theme: string;
};

export function addThemeListener(listener: (event: ThemeChangeEvent) => void): Subscription {
  return emitter.addListener<ThemeChangeEvent>('onChangeTheme', listener);
}

export function getTheme(): string {
  return ExpoSettingsModule.getTheme();
}

export function setTheme(theme: string): void {
  return ExpoSettingsModule.setTheme(theme);
}

Example app

example/App.tsx
import * as Settings from 'expo-settings';
import * as React from 'react';
import { Button, Text, View } from 'react-native';

export default function App() {
  const [theme, setTheme] = React.useState<string>(Settings.getTheme());

  React.useEffect(() => {
    const subscription = Settings.addThemeListener(({ theme: newTheme }) => {
      setTheme(newTheme);
    });

    return () => subscription.remove();
  }, [setTheme]);

  // Toggle between dark and light theme
  const nextTheme = theme === 'dark' ? 'light' : 'dark';

  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>Theme: {Settings.getTheme()}</Text>
      <Button title={`Set theme to ${nextTheme}`} onPress={() => Settings.setTheme(nextTheme)} />
    </View>
  );
}

6. Improve type safety with Enums

It's easy for us to make a mistake when using the Settings.setTheme() API in its current form, because we can set the theme to any string value. We can improve the type safety of this API by using an enum to restrict the possible values to system, light, and dark.

iOS native module

ios/ExpoSettingsModule.swift
import ExpoModulesCore

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

    Events("onChangeTheme")

    Function("setTheme") { (theme: Theme) -> Void in
      UserDefaults.standard.set(theme.rawValue, forKey:"theme")
      sendEvent("onChangeTheme", [
        "theme": theme.rawValue
      ])
    }

    Function("getTheme") { () -> String in
      UserDefaults.standard.string(forKey: "theme") ?? Theme.system.rawValue
    }
  }

  enum Theme: String, Enumerable {
    case light
    case dark
    case system
  }
}

Android native module

android/src/main/java/expo/modules/settings/ExpoSettingsModule.kt
package expo.modules.settings

import android.content.Context
import android.content.SharedPreferences
import androidx.core.os.bundleOf
import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition
import expo.modules.kotlin.types.Enumerable

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

    Events("onChangeTheme")

    Function("setTheme") { theme: Theme ->
      getPreferences().edit().putString("theme", theme.value).commit()
      this@ExpoSettingsModule.sendEvent("onChangeTheme", bundleOf("theme" to theme.value))
    }

    Function("getTheme") {
      return@Function getPreferences().getString("theme", Theme.SYSTEM.value)
    }
  }

  private val context
  get() = requireNotNull(appContext.reactContext)

  private fun getPreferences(): SharedPreferences {
    return context.getSharedPreferences(context.packageName + ".settings", Context.MODE_PRIVATE)
  }
}

enum class Theme(val value: String) : Enumerable {
  LIGHT("light"),
  DARK("dark"),
  SYSTEM("system")
}

TypeScript module

src/index.ts
import { EventEmitter, Subscription } from 'expo-modules-core';

import ExpoSettingsModule from './ExpoSettingsModule';

const emitter = new EventEmitter(ExpoSettingsModule);

export type Theme = 'light' | 'dark' | 'system';

export type ThemeChangeEvent = {
  theme: Theme;
};

export function addThemeListener(listener: (event: ThemeChangeEvent) => void): Subscription {
  return emitter.addListener<ThemeChangeEvent>('onChangeTheme', listener);
}

export function getTheme(): Theme {
  return ExpoSettingsModule.getTheme();
}

export function setTheme(theme: Theme): void {
  return ExpoSettingsModule.setTheme(theme);
}

Example app

If we change Settings.setTheme(nextTheme) to Settings.setTheme("not-a-real-theme"), TypeScript will complain, and if we ignore that and go ahead and press the button anyways, we'll see the following error:

 ERROR  Error: FunctionCallException: Calling the 'setTheme' function has failed (at ExpoModulesCore/SyncFunctionComponent.swift:76)
→ Caused by: ArgumentCastException: Argument at index '0' couldn't be cast to type Enum (at ExpoModulesCore/JavaScriptUtils.swift:41)
→ Caused by: EnumNoSuchValueException: 'not-a-real-theme' is not present in Theme enum, it must be one of: 'light', 'dark', 'system' (at ExpoModulesCore/Enumerable.swift:37)

We can see from the last line of the error message that not-a-real-theme is not a valid value for the Theme enum, and that light, dark, and system are the only valid values.

Next steps

Congratulations! You have created your first simple yet non-trivial Expo module for Android and iOS. You can continue to the next tutorial to learn how to create a native view.