Hera

Components

This page summarizes the main moving pieces inside rn_host and how they fit together. For deep dives, see react-native/rn_host/docs/ARCHITECTURE.md.

Core Runtime Pieces

  • Entrypoint (react-native/rn_host/src/main.cpp): parses CLI flags, connects to Hera, creates a surface, and starts MainLoop.
  • MainLoop (react-native/rn_host/src/platform/core/main_loop.*): owns the Hera event loop, mount dispatcher, and surface controller.
  • SurfaceSession: per-surface bundle of ReactHost, mount queue, and scroll manager, tracked by SessionRegistry.
  • HeraClient (react-native/rn_host/src/platform/client/hera_client.*): C++ wrapper around the Rust FFI client for IPC with the compositor.

React Native Runtime

  • ReactHost (react-native/rn_host/src/runtime/instance/react_host.*): boots Hermes, installs Bridgeless runtime bindings, and hosts Fabric.
  • FabricRuntime (react-native/rn_host/src/fabric/runtime/runtime.*): owns Scheduler, SurfaceHandler, EventDispatcher, and PointerEventsProcessor.
  • JsThread / HermesEngine (react-native/rn_host/src/runtime/jsi/*): dedicated JS thread and Hermes runtime lifecycle.

Mount Pipeline (Fabric -> Hera)

  1. FabricMountObserver receives a MountingTransaction.
  2. adapter::translate_mutations maps RN mutations into rn_host::Mutation entries, including text shaping and view props.
  3. MountTransactionSink converts mutations into HeraOp commands and sends them via HeraClient::commit.
  4. MainLoop drains the mount queue and ensures scroll updates are applied before commits.

Input And Scrolling

  • Input events from Hera are translated into input::InputEvent and dispatched to Fabric via FabricRuntime::handle_input.
  • The scroll manager (react-native/rn_host/src/platform/scroll/*) maintains a scroll tree and emits offset mutations plus scroll events back to JS.

Text Measurement And Shaping

HeraTextLayoutManager implements Fabric text measurement using hera_text_layout (FFI). The adapter converts text props into glyph runs so the server can rasterize and cache glyphs.

Threading And Scheduling

  • Main thread: event loop, Hera polling, mount draining.
  • JS thread: Hermes runtime and JS task queue.
  • Timer worker: host timers post tasks back onto the JS thread.

Frame scheduling from Hera drives Fabric's event beat so animation timing stays aligned with the compositor frame clock.

Key File Map

  • react-native/rn_host/src/platform/core/*: MainLoop and event loop wiring.
  • react-native/rn_host/src/platform/session/*: surface sessions and routing.
  • react-native/rn_host/src/fabric/*: Fabric runtime, adapter, mount observer.
  • react-native/rn_host/src/fabric/mounting/*: mount sink and transaction flow.
  • react-native/rn_host/src/platform/scroll/*: scroll state and input control.
  • react-native/rn_host/src/runtime/*: Hermes runtime and Bridgeless install.