This guide is intended to give developers who have already used React Native a quick outline on some of the key concepts, resources, and differences they will encounter when using Expo.
What is Expo?
If you've ever upgraded React Native or a native module you'll appreciate Expo's ability to seamlessly do this for you by only changing the version number.
Expo extends the React Native platform by offering additional, battle-tested modules that are maintained by the team. This means you're spending less time configuring and more time building.
If you've ever had to go through the trouble of upgrading a module or installing something like react-native-maps, you'll appreciate when things just work.
Expo also offers OTA (Over The Air) updates and a push notification service.
If you've ever been in a situation where you find a spelling mistake in your app and have to wait for Apple to approve a change, you'll appreciate OTA updates - these changes will appear as soon as you run exp publish! You aren't limited to text either, this applies to assets like images and configuration updates too!
There's no need re-build or redeploy your app to the App and Play store. It's like Code Push if you've used that before. There are a few limitations, however. Read about those here.
Expo offers a shared configuration file we call a manifest. Typically you'd update your Xcode plist or Android Studio xml files to handle changes. For example, if you want to lock screen orientation, change your icon, customize your splash screen or add/remove permissions you'd do this within app.json once and it would apply to both.
Configuration that you would typically do inside of your Xcode / plist files or Android studio / xml files is handled through app.json. For example, if you want to lock the orientation, change your icon, customize your splash screen, add/remove permissions and entitlements (in standalone apps), configure keys for Google Maps and other services, you set this in app.json. See the guide here.
With Expo, you can share your app with anyone, anywhere in the world while you're working through the Expo client (available on the App / Play Store). Scan a QR code, or enter in a phone number and we'll send you a link that will instantly load your app on your device.
Instead of having to sign up several external testers through iTunes connect, you can easily have them download the Expo client app and immediately have a working version on their phone.
We talk about permissions we set within app.json, but there's also the Permissions Api. Permissions inside app.json are meant to be used by Android standalone apps for things like camera access, geolocation, fingerprint, etc. The Permissions Api on the other hand, is used to request and verify access at runtime. It offers an easy API for asking your users for push notifications, location, camera, audio recording and contacts.
How does Expo work?
Developing in Expo
Apps are served from our exp/XDE through a tunnel service by default (we currently use ngrok for this) -- this means that you don't have to have your device connected to your computer, or to even be in the same room or country (or planet? I guess it should work from space) as the development machine and you can still live reload, use hot module reloading, enable remote JS debugging, and all of those things you can do normally with React Native. One caveat to this is that using a tunnel is a bit slower than using your LAN address or localhost, so if you can, you should use LAN or localhost. See how to configure this in exp/XDE.
Expo streams your device logs to exp and XDE so you don't need to run adb logcat or the iOS equivalent -- the console.log / warn /error messages from any device that is connected to your app will show up automatically in your exp/XDE logs window.
What Expo Can't Do
Sometimes Expo doesn't provide access to a native module or API you might need. In this case, you can detach and use ExpoKit instead. This allows you to continue using Expo's foundation but allowing you to create and use any custom native modules.
When you're ready, you can run exp build:ios or exp build:android and Expo will build your app and output a link to the binary required for you to submit. Then you can use something like Application Loader for iOS, or directly upload an APK for Android.
The best way to test and share Expo features directly from your browser. Point your phone at the QR code and you have a sandbox environment you can build in the browser and test directly on your device.