How do React Native's interop layers let a library written for the old architecture run on the New Architecture, and what can they not provide?

## Why interop exists When the New Architecture became React Native's default in 0.76, thousands of libraries were written against the old one: **native modules** built on the bridge and **native components** built as view managers for the old renderer, Paper. Requiring all of them to migrate first would have blocked every app. So React Native shipped **interop layers**: adapters that let old-style code run inside the new runtime without changes. Since 0.82 the New Architecture is mandatory, so for an unmigrated library the interop layer is the only way it runs at all. ## The module side A legacy module is a class written the old way, for example extending `ReactContextBaseJavaModule` with `@ReactMethod` methods on Android, or using `RCT_EXPORT_MODULE` on iOS, with no **codegen spec** describing its API in TypeScript. - The New Architecture's module system, **Turbo Native Modules**, normally loads modules generated from a spec. Its interop path also accepts these legacy classes; on Android it finds the `@ReactMethod`-annotated methods by reflection. - JavaScript reaches such a module through **`NativeModules`**. In the bridgeless runtime, `NativeModules` is backed by a native proxy object rather than a bridge message queue. - Migrated modules are instead loaded with `TurboModuleRegistry.getEnforcing` against a typed spec, which is how new modules should be written. ## The view side Fabric looks a component name up in a fixed order. On iOS the order is: 1. a component view class registered for that name in Fabric; 2. a class supplied by the app's third-party Fabric components provider; 3. the **legacy view manager interop**: the old view manager wrapped in `RCTLegacyViewManagerInteropComponentView`, which forwards props and commands to it; 4. otherwise, an **unimplemented view** that renders the text `Unimplemented component: <Name>`. On Android the equivalent Fabric interop is enabled by default. The placeholder is a useful clue: it means the name matched nothing at all, typically because the native code is not linked or the view manager is not registered, not merely that the library is old. ## What interop cannot give you | Missing capability | Why it matters | |---|---| | Access to **custom shadow nodes** | Components that compute their own layout natively cannot do it through interop | | **Concurrent features** on that surface | A legacy component cannot take part in transitions and other concurrent rendering | | **Synchronous, typed module calls** | Legacy modules miss codegen type safety, synchronous methods and shared C++ | | **Paper-only APIs** | Calls such as `UIManager.createView`, `updateView`, `manageChildren` or `setJSResponder` log "is not available in the new React Native architecture" and do nothing | | **Full bridge access** | On iOS a legacy module's `bridge` is a proxy; methods such as `delegate` return nil and log that they are unsupported | Those limits come from React Native's own guidance: without migrating native modules you do not get shared C++, synchronous calls or codegen type safety, and without migrating native components you cannot use concurrent features. ## How long interop will last - The 0.76 announcement says a future release will remove the interop layer and modules will then need to support the New Architecture. - The 0.82 post says the interop layers stay in the codebase **for the foreseeable future**. - The 0.84 post, which compiled legacy iOS code out, states that the interop code needed for compatibility remains. So interop is a bridge across a migration, not a destination. A library that only works through it is a library with a known expiry. ## Practical reading - **Works through interop and has no custom layout**: usually fine today; track the library's migration. - **Renders `Unimplemented component`**: check installation and linking first. - **Logs Paper-only API errors**: the library calls APIs interop does not cover; it needs an update, a replacement or a port. - **A legacy module is missing from `NativeModules`**: interop only adapts modules that are present in the native build, so check installation and autolinking before suspecting the architecture. - **Works, but the team wants synchronous calls or concurrent rendering on that screen**: that is the signal to migrate the module or component rather than to tune the interop path. ## Summary Interop adapts legacy modules through `NativeModules` and legacy views through a Fabric wrapper, so most old libraries keep running. It does not add the New Architecture's advantages to them, it does not cover every old API, and React Native intends to remove it one day.