Building React Native apps in the cloud

EAS Build is a cloud-based build service for React Native applications. Instead of managing builds locally, you can leverage Expo's cloud infrastructure for this purpose.

One significant advantage of EAS Build is its capacity to streamline the build and deployment process in the React Native workflow. It eliminates the need for multiple tools like Fastlane, XCode, Android Studio, and others for building React Native applications.

However, a potential drawback is the cost associated with running builds on a cloud infrastructure. Despite this, the simplicity and convenience offered by EAS Build justify its use for most applications, except perhaps for very large applications that might benefit more from a customized build setup.

How it works

EAS Build initiates each build in a newly created Docker container or VM. It uses either a MacOS runner (ARM-based) for iOS or a Linux machine for Android builds.

The process involves installing NPM packages, executing the prebuild command for managed projects (standard Expo project), generating iOS and Android folders, signing the app, and then running the build commands.

This description has been simplified; the actual process includes additional tasks such as caching, installing iOS pods, and executing build hooks.

Logs

Build logs can be accessed via the Expo Dashboard. This feature is invaluable for troubleshooting failed builds and obtaining detailed environment and runtime information for debugging purposes.

Builds

A previous blog post detailed our experience creating a development client build using EAS Build, specifically when incorporating external packages in React Native.

Environment variables

Local variables

As of Expo version 49, standard .env files can be used to declare environment variables.

Variables prefixed with EXPO_PUBLIC_ are publicly exposed within the application code. These public variables typically include information safe for user visibility, such as API URLs, public keys, and identifiers.

Variables on EAS

The same public environment variables can be specified in the eas.json file. This is useful if you have multiple environment (called profiles in EAS), where you have different value per environment.

{
  "build": {
    "development": {
      "env": {
        "EXPO_PUBLIC_URL": "testUrlDev"
      }
    },
    "production": {
      "env": {
        "EXPO_PUBLIC_URL": "testUrlProd"
      }
    }
  }
}

Secrets

Certain private environment variables, like the Sentry Auth token (SENTRY_AUTH_TOKEN) used for uploading source maps, are necessary during the build process.

These can be uploaded directly to the EAS Dashboard (EAS Secrets) and are automatically injected during the build.

Running the EAS builds locally

If your organization restricts the use of external build services or to mitigate costs, EAS builds can also be executed locally.

There are two methods for local compilation:

• The first involves running the prebuild command followed by expo run:ios or expo run:android. This process generates the required Android and iOS folders, simulating the "plain React Native" approach.

• The second method uses the development client but builds it locally instead of using the EAS cloud service. Using the --local flag with the eas build command replicates the cloud build workflow locally. The full command are eas build -p ios --local or eas build -p android --local.

Resources

eas.json reference: https://docs.expo.dev/eas/json/#eas-build

EAS pricing: https://expo.dev/pricing

Environment variables: https://docs.expo.dev/guides/environment-variables/