Tutorial: Create a native module - Expo

import { Grid01Icon } from '@expo/styleguide-icons/outline/Grid01Icon';

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In this tutorial, you build a module that stores the user's preferred app theme: dark, light, or system. On Android, use SharedPreferences, and on iOS, use UserDefaults. You can implement web support with localStorage, but this tutorial does not cover that.

Initialize a new module

First, create a new module. For this tutorial, the module is named expo-settings or ExpoSettings. You can choose a different name, but adjust the instructions to match your choice.

info Since you aren't going to actually ship this library, you can hit <kbd>return</kbd> for all the prompts to accept the default values.

</Step> <Step label="2">

Set up workspace

Clean up the default module to start with a clean slate. Delete the view module since this guide does not use it.

Locate the following files and replace their contents with the provided minimal boilerplate:

kotlin

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"
    }
  }
}

swift

import ExpoModulesCore

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

    Function("getTheme") { () -> String in
      "system"
    }
  }
}

export type ExpoSettingsModuleEvents = {};

import { NativeModule, requireNativeModule } from 'expo';

import { ExpoSettingsModuleEvents } from './ExpoSettings.types';

declare class ExpoSettingsModule extends NativeModule<ExpoSettingsModuleEvents> {
  getTheme: () => string;
}

// This call loads the native module object from the JSI.
export default requireNativeModule<ExpoSettingsModule>('ExpoSettings');

import ExpoSettingsModule from './ExpoSettingsModule';

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

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>
  );
}

</Step> <Step label="3">

Run the example project

Start the TypeScript compiler to watch for changes.

In a separate terminal window, run the example app.

You should see the text "Theme: system" in the center of the screen when you launch the example app. The value "system" comes from synchronously calling the getTheme() function in the native module. You will change this value in the next step.

</Step> <Step label="4">

Get, set, and persist the theme preference value

Android native module

To read the value, look for a SharedPreferences string under the key "theme". If the key does not exist, default to "system". Use the reactContext (a React Native ContextWrapper) to access the SharedPreferences instance with getSharedPreferences().

To set the value, use the edit() method of SharedPreferences to get an Editor instance. Then, use putString() to set the value. Ensure the setTheme function accepts a value of type String.

kotlin

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)
  }
}

iOS native module

To read the value on iOS, look for a UserDefaults string under the key "theme". If the key does not exist, default to "system".

To set the value, use the set(_:forKey:) method of UserDefaults. Ensure the setTheme function accepts a value of type String.

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"
    }
  }
}

TypeScript module

Update the ExpoSettingsModule.ts to add a TypeScript interface for the ExpoSettingsModule native module to update the theme.

import { NativeModule, requireNativeModule } from 'expo';

import { ExpoSettingsModuleEvents } from './ExpoSettings.types';

declare class ExpoSettingsModule extends NativeModule<ExpoSettingsModuleEvents> {
  setTheme: (theme: string) => void;
  getTheme: () => string;
}

// This call loads the native module object from the JSI.
export default requireNativeModule<ExpoSettingsModule>('ExpoSettings');

Now, call your native modules from TypeScript.

import ExpoSettingsModule from './ExpoSettingsModule';

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

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

Example app

You can now use the Settings API in your 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 you rebuild and run the app, the "system" theme is still set. Pressing the button does nothing, but when you reload the app, the theme changes. This happens because the app does not fetch the new theme value or re-render. You will fix this in the next step.

</Step> <Step label="5">

Emit change events for the theme value

Ensure developers using your API can react to theme value changes by emitting a change event whenever the value updates. Use the Events definition component to describe the events your module emits, sendEvent to emit the event from native code, and the EventEmitter API to subscribe to events in JavaScript. The event payload is { theme: string }.

Android native module

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

kotlin

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)
  }
}

iOS native module

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"
    }
  }
}

TypeScript module

export type ThemeChangeEvent = {
  theme: string;
};

export type ExpoSettingsModuleEvents = {
  onChangeTheme: (params: ThemeChangeEvent) => void;
};

import { EventSubscription } from 'expo-modules-core';
import ExpoSettingsModule from './ExpoSettingsModule';
import { ThemeChangeEvent } from './ExpoSettings.types';

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

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

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

Example app

tsx

import * as Settings from 'expo-settings';
import { useEffect, useState } from 'react';
import { Button, Text, View } from 'react-native';

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

  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>
  );
}

</Step> <Step label="6">

Improve type safety with Enums

It's easy to make mistakes when using the Settings.setTheme() API in its current form, as it allows any string value. Improve the type safety of this API by using an enum to restrict the possible values to system, light, and dark.

Android native module

kotlin

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")
}

iOS native module

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
  }
}

TypeScript module

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

export type ThemeChangeEvent = {
  theme: Theme;
};

export type ExpoSettingsModuleEvents = {
  onChangeTheme: (params: ThemeChangeEvent) => void;
};

import { NativeModule, requireNativeModule } from 'expo';

import { ExpoSettingsModuleEvents, Theme } from './ExpoSettings.types';

declare class ExpoSettingsModule extends NativeModule<ExpoSettingsModuleEvents> {
  setTheme: (theme: Theme) => void;
  getTheme: () => Theme;
}

// This call loads the native module object from the JSI.
export default requireNativeModule<ExpoSettingsModule>('ExpoSettings');

import { EventSubscription } from 'expo-modules-core';

import ExpoSettingsModule from './ExpoSettingsModule';

import { Theme, ThemeChangeEvent } from './ExpoSettings.types';

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

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

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

Example app

If you change Settings.setTheme(nextTheme) to Settings.setTheme("not-a-real-theme"), TypeScript will raise an error. If you ignore the error and press the button, you will see the following runtime error:

text

 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<Theme> (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)

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

</Step>

Congratulations! You have created your first Expo Module for Android and iOS.

Next steps