Tutorial: Create a module with a config plugin

Initialize a module

Start by initializing a new Expo module project with create-expo-module. This sets up scaffolding for Android, iOS, and TypeScript and includes an example project to test the module within an app. Run the following command to get started:

This guide uses the name expo-native-configuration/ExpoNativeConfiguration for the project. However, you can choose any name you prefer.

Set up workspace

In this example, you don't need the view module included by create-expo-module. Clean up the default module with the following command:

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

• android/src/main/java/expo/modules/nativeconfiguration/ExpoNativeConfigurationModule.kt
• ios/ExpoNativeConfigurationModule.swift
• src/ExpoNativeConfigurationModule.ts
• src/index.ts
• example/App.tsx

package expo.modules.nativeconfiguration

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

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

    Function("getApiKey") {
      return@Function "api-key"
    }
  }
}

import ExpoModulesCore

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

    Function("getApiKey") { () -> String in
      "api-key"
    }
  }
}

import { NativeModule, requireNativeModule } from 'expo';

declare class ExpoNativeConfigurationModule extends NativeModule {
  getApiKey(): string;
}

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

import ExpoNativeConfigurationModule from './ExpoNativeConfigurationModule';

export function getApiKey(): string {
  return ExpoNativeConfigurationModule.getApiKey();
}

import * as ExpoNativeConfiguration from 'expo-native-configuration';
import { Text, View } from 'react-native';

export default function App() {
  return (
    <View style={{ flex: 1, alignItems: 'center', justifyContent: 'center' }}>
      <Text>API key: {ExpoNativeConfiguration.getApiKey()}</Text>
    </View>
  );
}

Run the example project

In the root of your project, run the TypeScript compiler to watch for changes and rebuild the module's JavaScript:

In another terminal window, compile and run the example app:

You should see a screen with the text "API key: api-key".

Create a new config plugin

Plugins are synchronous functions that accept an ExpoConfig and return a modified ExpoConfig. By convention, these functions are prefixed with the word with. Name your plugin withMyApiKey or use a different name, as long as it follows this convention.

Here is an example of a basic config plugin function:

const withMyApiKey = config => {
  return config;
};

You can also use mods, which are async functions that modify files in native projects, such as source code or configuration files (plist, xml). The mods object is different from the rest of the app config because it doesn't serialize after the initial reading. This allows you to perform actions during code generation.

When writing config plugins, follow these considerations:

• Plugins must be synchronous, and their return value must be serializable, except for any mods that are added.
• plugins are invoked whenever the getConfig method from expo/config reads the configuration. In contrast, mods are invoked only during the "syncing" phase of npx expo prebuild.

Although optional, use expo-module-scripts to simplify plugin development. It provides a recommended default configuration for TypeScript and Jest. For more information, see the config plugins guide.

Start creating your plugin with this minimal boilerplate. Create a plugin directory for writing the plugin in TypeScript and add an app.plugin.js file in the project root, which will be the plugin's entry point.

Create a plugin/tsconfig.json file

{
  "extends": "expo-module-scripts/tsconfig.plugin",
  "compilerOptions": {
    "outDir": "build",
    "rootDir": "src"
  },
  "include": ["./src"],
  "exclude": ["**/__mocks__/*", "**/__tests__/*"]
}

Create a plugin/src/index.ts file for your plugin

import { ConfigPlugin } from 'expo/config-plugins';

const withMyApiKey: ConfigPlugin = config => {
  console.log('my custom plugin');
  return config;
};

export default withMyApiKey;

Create an app.plugin.js file in the root directory

// This file configures the entry file for your plugin.
module.exports = require('./plugin/build');

At the root of your project, run npm run build plugin to start the TypeScript compiler in watch mode. Next, configure your example project to use your plugin by adding the following line to the example/app.json file:

{
  "expo": {
    ...
    "plugins": ["../app.plugin.js"]
  }
}

When you run the npx expo prebuild command inside your example directory, the terminal logs "my custom plugin" through a console statement.

To inject your custom API keys into AndroidManifest.xml and Info.plist, use helper mods provided by expo/config-plugins. These make it easy to modify native files. For this example, use withAndroidManifest and withInfoPlist.

As the name suggests, withAndroidManifest allows you to read and modify the AndroidManifest.xml file. Use AndroidConfig helpers to add a metadata item to the main application, as shown below:

const withMyApiKey: ConfigPlugin<{ apiKey: string }> = (config, { apiKey }) => {
  config = withAndroidManifest(config, config => {
    const mainApplication = AndroidConfig.Manifest.getMainApplicationOrThrow(config.modResults);

    AndroidConfig.Manifest.addMetaDataItemToMainApplication(
      mainApplication,
      'MY_CUSTOM_API_KEY',
      apiKey
    );
    return config;
  });

  return config;
};

Similarly, you can use withInfoPlist to modify the Info.plist values. Using the modResults property, you can add custom values as shown in the code snippet below:

const withMyApiKey: ConfigPlugin<{ apiKey: string }> = (config, { apiKey }) => {
  config = withInfoPlist(config, config => {
    config.modResults['MY_CUSTOM_API_KEY'] = apiKey;
    return config;
  });

  return config;
};

You can create a custom plugin by merging everything into a single function:

import {
  withInfoPlist,
  withAndroidManifest,
  AndroidConfig,
  ConfigPlugin,
} from 'expo/config-plugins';

const withMyApiKey: ConfigPlugin<{ apiKey: string }> = (config, { apiKey }) => {
  config = withInfoPlist(config, config => {
    config.modResults['MY_CUSTOM_API_KEY'] = apiKey;
    return config;
  });

  config = withAndroidManifest(config, config => {
    const mainApplication = AndroidConfig.Manifest.getMainApplicationOrThrow(config.modResults);

    AndroidConfig.Manifest.addMetaDataItemToMainApplication(
      mainApplication,
      'MY_CUSTOM_API_KEY',
      apiKey
    );
    return config;
  });

  return config;
};

export default withMyApiKey;

With the plugin ready to use, update the example app to pass your API key to the plugin as a configuration option. Modify the plugins field in example/app.json as shown below:

{
  "expo": {
    ...
    "plugins": [["../app.plugin.js", { "apiKey": "custom_secret_api" }]]
  }
}

After making this change, test that the plugin works correctly by running npx expo prebuild --clean inside the example directory. This command executes your plugin and updates native files, injecting "MY_CUSTOM_API_KEY" into AndroidManifest.xml and Info.plist. You can verify this by checking the contents of example/android/app/src/main/AndroidManifest.xml.

Read native values from the module

Now, make your native module read the fields added to AndroidManifest.xml and Info.plist by using platform-specific methods to access their contents.

On Android, access metadata information from the AndroidManifest.xml file using the packageManager class. To read the "MY_CUSTOM_API_KEY" value, update the android/src/main/java/expo/modules/nativeconfiguration/ExpoNativeConfigurationModule.kt file:

package expo.modules.nativeconfiguration

import expo.modules.kotlin.modules.Module
import expo.modules.kotlin.modules.ModuleDefinition
import android.content.pm.PackageManager

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

    Function("getApiKey") {
      val applicationInfo = appContext?.reactContext?.packageManager?.getApplicationInfo(appContext?.reactContext?.packageName.toString(), PackageManager.GET_META_DATA)

      return@Function applicationInfo?.metaData?.getString("MY_CUSTOM_API_KEY")
    }
  }
}

On iOS, you can read the content of an Info.plist property using the Bundle.main.object(forInfoDictionaryKey: "") method. To access the "MY_CUSTOM_API_KEY" value added earlier, update the ios/ExpoNativeConfigurationModule.swift file as shown:

import ExpoModulesCore

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

    Function("getApiKey") {
     return Bundle.main.object(forInfoDictionaryKey: "MY_CUSTOM_API_KEY") as? String
    }
  }
}

Run your module

With your native modules reading the fields added to the native files, you can now run the example app and access your custom API key using the ExamplePlugin.getApiKey() function.