Self hosting mobile applications

Jung uses a fictional game, Buggy Valley, to illustrate loading different JavaScript bundles in a React Native app. He explains that the native UI thread loads a bundle into memory through the bridge, where the JavaScript thread can run it.

Jung turns the simple bundle-loading example into a service that downloads a new JavaScript bundle while keeping native modules unchanged. To avoid repeatedly downloading the same bundle, the app first checks a small `index.json` file, whose bundle URL uses an MD5-based name; it downloads the bundle only when that URL is not cached.

Expo users could publish through the DevTools GUI or by running `publish` in their project directory. Jung says the default was to check for updates at app startup, and that configurable update policies let teams change when to poll or opt out of updates.

Jung describes self-hosting as an option for teams facing high latency to Expo’s CDNs. Running `expo export` writes platform-specific `index.json` files, JavaScript bundles, and assets to disk; teams can host these files on GitHub or another server, then build a standalone iOS binary with a URL to the hosted index. For assets, the app checks local storage first, fetches missing files from the server, and caches them by hash.

Jung closes by announcing that Snack, a live coding environment for React Native similar to JSFiddle, is open source. He describes changing an app’s background color in the editor and seeing the change load on a phone.