How do React Native's precompiled iOS binaries speed up builds since 0.84, and when would you set RCT_USE_PREBUILT_RNCORE=0?

## What used to happen on every clean iOS build A React Native iOS app depends on a large amount of native code: React Native core (Fabric, Turbo Modules, the runtime, the Hermes integration) plus third-party C++ libraries such as **Folly** and **glog**. CocoaPods used to install all of it **as source**, so every clean build — a new machine, a CI runner, a deleted DerivedData folder — compiled it from scratch. On many projects that was most of the build. ## How precompiled binaries arrived | Release | Change | How to enable | |---|---|---| | 0.80 | third-party dependencies prebuilt | `RCT_USE_RN_DEP=1` at `pod install` (experimental) | | 0.81 | React Native core prebuilt too | `RCT_USE_RN_DEP=1 RCT_USE_PREBUILT_RNCORE=1` (experimental) | | 0.84 | both on by default | nothing; set to `0` to opt out | In 0.87 the pods script sets both variables to `1` unless you explicitly pass `0`. During `pod install` it downloads **`.xcframework`** tarballs of React Native core and its dependencies — published on Maven with each release — and installs them as pods in place of the sources. The 0.81 release post described compile times cut **by up to 10x** in projects where React Native is the main dependency; the 0.80 post measured dependencies alone at roughly 12% faster. ## What stays source-built - **Your own code** — Swift and Objective-C in your app target. - **Third-party native modules** from npm, which still compile from their pods. - **Anything you opted out of**, below. So the gain is largest for apps where React Native itself dominates the native code, and smaller for apps with many heavy native libraries. ## How the prebuilt pods are wired In prebuilt mode the pods scripts still declare React Native's usual core pods, but as **facades** without sources or headers, so they cannot shadow the binary; the code and headers come from a `React-Core-prebuilt` pod that wraps the downloaded xcframework. Two practical consequences: - **You can tell which mode you are in** from the `pod install` output, which logs that it is using the prebuilt React Native Core and Dependencies, and from the installed pods, which include `React-Core-prebuilt`. - **Flags are read at `pod install` time.** Changing `RCT_USE_PREBUILT_RNCORE` or `RCT_USE_RN_DEP` does nothing until you run `pod install` again; a team can pin the choice for everyone with an `ENV['RCT_USE_PREBUILT_RNCORE'] = '0'` line near the top of the Podfile, the pattern the 0.80 release post showed for `RCT_USE_RN_DEP`. ## When to set `RCT_USE_PREBUILT_RNCORE=0` Build core from source when the prebuilt binary cannot be what you need: 1. **Opting out of Hermes V1** — the 0.84 notes pair `RCT_HERMES_V1_ENABLED=0` with `RCT_USE_PREBUILT_RNCORE=0`. 2. **Re-enabling legacy-architecture code** on iOS — `RCT_USE_PREBUILT_RNCORE=0 RCT_REMOVE_LEGACY_ARCH=0 bundle exec pod install`. 3. **Patching React Native's native sources** — a patch to files that are no longer compiled has no effect. 4. **Stepping into React Native internals** in the debugger — prebuilt binaries let you debug your own native code, not React Native's. `RCT_USE_RN_DEP=0` similarly forces the third-party dependencies back to source. ## Trade-offs to keep in mind - **You depend on published artifacts.** A clean `pod install` needs the matching tarballs to be downloadable, from Maven Central or your mirror. - **Everyone should use the same mode.** A developer on a source build and CI on binaries are debugging different code. - **Debugging stops at React Native's boundary.** Your own native code is debuggable; React Native's internals are not, unless you switch to a source build. ## Working with a mirror The download URL defaults to Maven Central. Setting the **`ENTERPRISE_REPOSITORY`** environment variable points the pods scripts at your organisation's mirror instead, as long as it mirrors Maven's layout — the iOS counterpart of Android's `exclusiveEnterpriseRepository` Gradle property. ## Common mistakes - **A leftover `RCT_USE_PREBUILT_RNCORE=0`** in a CI script or the Podfile from an old workaround, silently compiling core on every build. - **Patching React Native's iOS sources with prebuilt binaries on** and wondering why the patch does nothing. - **Expecting prebuilt binaries to speed up your own Swift code** or third-party native modules. - **Mixing flags across a team**, so developers debug different binaries than CI ships. Precompiled core also changes what `ccache` can save on iOS: with less React Native C++ compiled locally, a compiler cache has less to reuse, so measure before adding one.