> ## Documentation Index > Fetch the complete documentation index at: https://docs.mobster.dev/llms.txt > Use this file to discover all available pages before exploring further. # Expo and React Native > Open your Expo app on a simulator or your iPhone from Metro, check a screen, reload after a change, and build it for checks and CI. `mobster expo` puts your Expo or React Native app on a simulator or your iPhone and checks it. One command opens the JavaScript you're editing on the phone and says whether a screen shows what it should. The same app builds for `mobster test` and CI, with the JavaScript inside, so a check needs no Metro. Everything runs on your Mac: Metro, `npx expo prebuild`, `xcodebuild`, `npx expo run:ios` and `eas build --local`. Mobster never builds on Expo's servers. ## Check it on your iPhone From your app's folder: ```sh theme={"theme":"css-variables"} mobster expo preview --device "Test iPhone" --expect "Welcome back" ``` ```text theme={"theme":"css-variables"} Starting Metro on port 8081: npx expo start (log: .mobster/expo/metro.log) Opened Habits on Test iPhone from Metro on port 8081 (your dev client). ✓ text "Welcome back": found in text "Welcome back" Screenshot .mobster/expo/preview.png Metro keeps running in the background: `mobster expo reload` after a change, `mobster expo stop` when you're done. ``` 1. Mobster reads `package.json` and the Expo config (`app.json`, or `app.config.js` through `npx expo config`). 2. When nothing answers on port 8081, it starts `npx expo start --lan` in the background. 3. It opens the app on the device: your dev client when the app has `expo-dev-client` (`exp+://expo-development-client/?url=…`), else Expo Go (`exp://…`). A phone reaches Metro at this Mac's address on your Wi-Fi, so both need the same network. A simulator uses `127.0.0.1`. 4. With `--expect`, it reads the screen's accessibility tree until every text is there, up to `--wait` seconds (45 by default, for the first bundle), and saves a screenshot. It exits 1 when one isn't there. `--path settings/profile` opens a screen by the app's own deep link (its `scheme`) once the app is up. Without `--device`, Mobster uses your iPhone, else the only device it sees. The dev client has to be on the device first. Build and install it once: ```sh theme={"theme":"css-variables"} mobster expo build --device "Test iPhone" ``` That runs `npx expo run:ios --device --configuration Debug --no-bundler`, which signs it with the team in your Xcode project and installs it. Expo Go comes from the App Store. ## After a change ```sh theme={"theme":"css-variables"} mobster expo reload ``` Every app connected to Metro reloads its JavaScript, as pressing `r` in `npx expo start` does. Then check again with `mobster expo preview --expect …`, or let your coding agent do the loop through `mobster mcp`: `expo_preview` opens the app on a device and `expo_reload` reloads it, and each returns the screen. A simulator needs no plan; a real iPhone uses the same plan check as the other phone tools. See [the MCP server](/mcp-server). `mobster expo stop` stops the Metro that `preview` or `dev` started. ## The dev loop ```sh theme={"theme":"css-variables"} mobster expo dev --device "Test iPhone" ``` ```text theme={"theme":"css-variables"} Metro on port 8081 (started; log: .mobster/expo/metro.log). Opened Habits on Test iPhone. Watching for saves: ctrl+c stops. Reloaded. mobster expo dev: Metro can't bundle the app: Unable to resolve module ./Missing from App.tsx (App.tsx:3) Fix it and save: the next save tries again. Reloaded. ``` 1. It starts this project's Metro, or reuses the one it started before. A Metro on port 8081 that serves another project is left alone and the next free port is used. 2. When the device has no dev client, it builds one (`npx expo run:ios` for an iPhone, a Debug build for a simulator) and installs it. `--no-install` stops there with the command to run instead. 3. It opens the app, then polls your source files (`.js`, `.jsx`, `.ts`, `.tsx`, `.json`, outside `node_modules`, `ios` and `android`) and reloads the app on each save. 4. `--check .mobster/checks` runs `mobster test --device NAME` on those checks once the app is up. `--once` stops after that. 5. ctrl+c stops the Metro this command started. A Metro that was already running stays running. Metro's error is a state, not a timeout. When Metro can't bundle the app, `mobster expo dev` exits 1 with Metro's own text, the file and the line (cut to 600 characters). After a save, it prints the same text and keeps watching, so the next save tries again. `mobster expo preview --expect` ends its wait the same way, and also when the dev client shows its red error screen. `--json` prints one JSON line per event (`metro`, `opened`, `reloaded`, `failed`, `checks`). Mobster doesn't replace Metro's fast refresh: it reloads the whole app on a save, which also picks up changes fast refresh skips. ## Over the API The Mac app starts and watches the dev server through the local API: | Route | Does | | - | - | | `GET /api/tests/expo?project=/abs/path` | the app and its server: `state` is `stopped`, `starting`, `running` or `failed`, with `error.message`, `error.file` and `error.line` when it failed. `&bundle=1` asks Metro to bundle the app first | | `POST /api/tests/expo/dev` | `{project, port?}` starts or reuses the server and says whether it started it | | `POST /api/tests/expo/reload` | `{project}` reloads every app connected to it; 409 `not_running` when nothing is running | | `POST /api/tests/expo/stop` | `{project}` stops the Metro Mobster started | A Metro that a route started is stopped when Mobster quits. ## Builds for checks and CI A Debug React Native build loads its JavaScript from Metro, and a check doesn't start Metro. So Mobster builds Release for the simulator, with the bundle inside the app: ```sh theme={"theme":"css-variables"} mobster expo build ``` ```text theme={"theme":"css-variables"} Making the ios folder: npx expo prebuild --platform ios Building Habits for the simulator Built Habits: .mobster/build/Build/Products/Release-iphonesimulator/Habits.app Release, with the JavaScript inside: it runs with no Metro. Point a check's app.path at it. ``` * When the app has no `ios` folder, `npx expo prebuild --platform ios` makes it first. CocoaPods has to be installed. * `--install --device "iPhone 17 Pro"` installs it on a Mobster simulator. * `--debug` builds Debug instead, for a simulator dev client that loads from Metro. * `--eas development` builds that EAS profile on this Mac with `eas build --local`. A simulator profile's `.app` is unpacked into `.mobster/expo/eas/`, and `--install` installs it. [`mobster test pr`](/testing#test-a-pull-request) and [`mobster ci`](/ci) do the same on their own: in an Expo or React Native project they build Release, after `npx expo prebuild` when there's no `ios` folder. A check names the build the usual way: ```yaml .mobster/checks/home.yaml theme={"theme":"css-variables"} version: 1 name: Home greets a returning user app: bundle: com.example.habits path: .mobster/build/Build/Products/Release-iphonesimulator/Habits.app launch: url: habits://home expect: - text: Welcome back ``` ## What Mobster found ```sh theme={"theme":"css-variables"} mobster expo doctor ``` ```text theme={"theme":"css-variables"} Habits: Expo app (~54.0.0) Bundle ID com.example.habits Schemes habits Opens in your dev client (expo-dev-client) ios folder no: `mobster expo build` runs npx expo prebuild first Metro running on port 8081 Checks and CI build Release for the simulator, with the JavaScript inside: mobster expo build ``` It reads only. A React Native app without Expo builds and runs in checks from its `ios` folder. `preview` is for Expo apps. Mobster reuses Metro only when its status names this project. Before stopping it, Mobster checks the process group, command and project folder; it leaves an unverified or replaced process alone. `--check` runs after the first open and after every successful reload. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.