Skip to content
Permalink
master
Switch branches/tags
Go to file
 
 
Cannot retrieve contributors at this time

Prebuilding

In Expo, prebuilding is the process of generating the native runtime code for your React project. Prebuild can be used to automatically link and configure complex native modules that have implemented CocoaPods, autolinking, and config plugins.

How it works

Think of prebuild like a bundler for native code, and the app.json like the bundler config file (webpack.config.js or metro.config.js).

When you run a bundler, it'll generate an output folder (like dist, or build) which is ready to deploy and run, this folder can be modified by hand but it's strongly discouraged because rerunning the bundler would overwrite any manual changes you made.

In expo prebuild the generated folders are /ios and /android, when you run prebuild it'll generate iOS and Android projects which are ready to deploy and run. If you want to upgrade a library or change a native value, simply update your app.json and rerun expo prebuild (sometimes expo prebuild --clean to nuke the changes). This custom managed workflow provides most of the benefits of bare, and managed workflow at the same time.

After expo prebuild is complete, the native ios and android folders should always be in a runnable state, if not then the project failed to prebuild.

New libraries can be added by installing them with npm or yarn, then adding them to the plugins array of your Expo config (app.json or app.config.js), running expo prebuild will ensure

However you might need to use a library that doesn't have a config plugin yet, or maybe you need to write some custom native code yourself. For these cases you'll have to manually modify the /ios and /android folders, doing this means you'll no longer be able to safely rerun expo prebuild. Any new library you want to install will have to be setup manually, and all of your packages will have to be upgraded by hand (basically just standard react native development).

You can technically still rerun expo prebuild in bare workflow but it's strongly discouraged since config plugins may not be aware of the state of your project. For example, if you installed expo-camera the plugin would add a line to your Android app's build.gradle, if you then uninstalled expo-camera the only way prebuild would know to remove that line would be if the build.gradle was reset (by regenerating the entire project).

Closer look

Running expo prebuild will do the following:

  1. Prebuild will clone a blank native template and copy the /ios, and /android folders into the project if they don't exist.
  2. Ensure the project is no longer using the Expo fork of react-native which has sandboxed AsyncStorage and disables the DevMenu for use in the Expo Go app.
  3. Then the app.json or app.config.js config files are read, and a list of all the config plugins is collected.
  4. CocoaPods are installed before any native changes are applied to ensure that all the proper native code is linked on iOS.
  5. Finally, the config mods are evaluated, this applies changes to the native files. i.e. name -> rename ios name and android name. icon -> generate icons. splash -> generate splash screens.

And that's it! Most of the work is done at the last step inside the config plugin system.

Difference between eject and prebuild

expo prebuild is very similar to expo eject, the core difference being that eject is intended to be run once, and prebuild can be used multiple times. The eject command assumes that your ios and android folders are modified by hand (bare workflow) and will warn you if they might be overwritten, whereas the prebuild command should only be used when you're ios and android folders are completely generated and can be regenerated any time (kinda like the node_modules folder).

Can my project use prebuild?

Any managed Expo project can use prebuild.

If your project has native folder /ios or /android, and those projects have manual native changes (bare workflow), then you cannot use prebuild safely. You'll have to migrate your project to the custom managed workflow before you can use prebuild.

Migrating

Project migration depends on the packages your project is using. All Expo packages support prebuild as of SDK 41, switching to Expo packages can make migration much smoother.

The first step is to ensure your iOS and Android projects build when correctly when running yarn ios or yarn android. Ensure your app is in a working state before continuing.

Next, setup version control for your project, we recommend using git. This will help you revert changes if anything goes wrong.

Create an app.config.js or app.json in your project root folder. Manually define all of the value in your app.json that align with your native app. This is the hardest step. You can test your changes by running expo prebuild and seeing if anything critical in your ios or android folders is changing (especially in the Info.plist or AndroidManifest.xml).

Now open your package.json and take note of any package in your dependencies object that has native code. You can check reactnative.directory and see if they say "Expo Go", if they don't then they might need extra setup.

After you have your list of plugins, check each plugin's docs to see if they mention a config plugin, if so add them to your app.json plugins array.

For example:

module.exports = {
  plugins: ["expo-camera", "expo-facebook"],
};

Run expo config --type prebuild to ensure the plugins actually exists and no errors occur.

Finally, you can clear your /ios and /android folders, then regenerating them by running: expo prebuild --clean

Running yarn ios and yarn android should build successfully build the native projects. You can keep modifying the app.json and config plugins, then running expo prebuild or expo prebuild --clean to sync up changes until it works.

If you have an issue, you can always revert changes in git to undo.

Your project is fully migrated when you can safely delete and regenerate working native code by running expo prebuild --clean.

Anti Pitch

Prebuild was created to give users the ease of creating native apps with just React and TypeScript while still allowing them to add custom native code. But this may not be right for all users. Here are some current issues with expo prebuild:

Not all packages have config plugins yet

Since prebuild is new, not enough projects have a config plugin yet. Adding one is pretty straightforward so this may not be an issue for very long, but it does take a bit of time for new technology to catch up with an ecosystem.

If you find a library that requires extra setup and doesn't have a config plugin, you should open a PR or an issue so that the maintainer is aware of the feature request.

Some packages like react-native-blurhash and react-native-mmkv don't require any additional native setup so they don't even need a config plugin!

Packages like react-native-ble-plx, or react-native-firebase do require additional setup and therefore require a config plugin to be used with expo prebuild and managed EAS build.

React Native version

expo prebuild generates native code based on the version of expo you're project has installed, so a project with SDK 41 (expo@41.0.0) would generate a react-native 62 app.

Expo SDK lags behind the react-native releases because it can only upgrade when all of the modules in the Expo ecosystem are fully compatible with a version of react-native, this often means having to upstream bug fixes to ensure stability for our users.

Expo platform compatibility

Prebuild can only be used for native platforms that are supported by the Expo SDK. This means iOS and Android for the time being. In time we plan to support many more platforms, but team size and limited resources prevent us from being able to maintain them at the moment. The stability of out-of-tree platforms like react-native-macos and react-native-windows are also considered.

You can still use prebuild for your ios, and android projects but any extra platforms will have to be configured manually for the time being.

With the exception of web, which doesn't require expo prebuild since it uses the browser instead of a custom native runtime.

Adding simple native code changes is slower

All native changes must be added via node modules, CocoaPods, autolinking, and config plugins.

This means if you want to add some quick native file to your project your best bet is to do it in bare workflow then later add it to a module via a monorepo (we recommend expo-yarn-workspaces).

If you want to do something like modifying the gradle properties file, you'll have to write a plugin for that example. Of course this could be easily automated with helper plugin libraries, but it is a bit slower if you need to do it often. If your native project requires many custom changes, then your best bet might be to drop down to bare, and take another look at custom managed workflow in a couple months.