The New React Native: Bringing the Fabric renderer to the “Facebook” app

Gross says React Native had separate implementations for iOS, Android, Windows, and other platforms, which could behave differently. The team began a cross-platform rewrite in 2018 to support future features such as concurrent rendering while bringing the change to existing apps, starting with Facebook.

The Facebook app had more than 1,000 React Native surfaces, from smaller pages to complex products such as Marketplace, Dating, and Jobs. Because changes from React Native’s main branch reached users in weekly releases, the team had to watch stability, performance, and crashes across many devices and usage conditions. The migration took about two years to enable Fabric on the first surface, then another nine months to roll it out across the rest.

The work uncovered undocumented features, optimizations, and differences between the Android and iOS implementations. The team documented and fixed many discrepancies, removed platform-specific code, and improved areas such as Android ScrollView and layout animations. Most surfaces worked with both renderers without code changes, but some deprecated APIs needed migration paths.

Gross recounts a crash that appeared only on certain Android devices: some manufacturers crashed when code removed a child from a parent that had no children. Fabric was made resilient to that operation and logs a console message when it occurs. In another case, a highly optimized screen stalled because its startup logic assumed a particular order of asynchronous lifecycle events; making the product code tolerate different orders improved both Fabric and the older renderer.

By the time of the talk, the Facebook app was fully running on the new architecture, but Gross said broader adoption still needed tutorials, guides, and compatibility work on popular open-source packages. The team was working with Expo, Callstack, Software Mansion, and package maintainers, without committing to a release date.