Alex Hunt
0d66c524cf
Summary: Pull Request resolved: https://github.com/facebook/react-native/pull/49353 This change adds an opt-in to restore JavaScript log streaming via the Metro dev server, [removed from React Native core in 0.77](https://reactnative.dev/blog/2025/01/21/version-0.77#removal-of-consolelog-streaming-in-metro). Users can opt into this legacy behaviour by adding the `--client-logs` flag to `npx react-native-community/cli start`. - The default experience remains without streamed JS logs. - The existing "JavaScript logs have moved! ..." notice is printed in all cases, and we do not advertise the new flag for new users. - Under non-Community CLI dev servers (i.e. Expo), log streaming is restored implicitly. We will clean up this functionality again when we eventually remove JS log streaming over `HMRClient`, tasked in T214991636. **Implementation notes** - Logs are always sent over `HMRClient` (previous status quo), even with log streaming off in the dev server. This is a necessary evil to be able to flag this functionality in a user-accessible place, and to move fast for 0.78. - Necessarily, emitting `fusebox_console_notice` moves to the dev server itself, on first device (Fusebox) connection. Changelog: [General][Added] - Add opt in for legacy Metro log streaming via `--client-logs` flag Reviewed By: robhogan Differential Revision: D69469039 fbshipit-source-id: be99d02a3b1c977a59bf7d2726f0e6cf2e60b28a
@react-native/dev-middleware
Dev server middleware supporting core React Native development features. This package is preconfigured in all React Native projects.
Usage
Middleware can be attached to a dev server (e.g. Metro) using the createDevMiddleware API.
import { createDevMiddleware } from '@react-native/dev-middleware';
function myDevServerImpl(args) {
...
const {middleware, websocketEndpoints} = createDevMiddleware({
projectRoot: metroConfig.projectRoot,
serverBaseUrl: `http://${args.host}:${args.port}`,
logger,
});
await Metro.runServer(metroConfig, {
host: args.host,
...,
unstable_extraMiddleware: [
middleware,
// Optionally extend with additional HTTP middleware
],
websocketEndpoints: {
...websocketEndpoints,
// Optionally extend with additional WebSocket endpoints
},
});
}
Included middleware
@react-native/dev-middleware is designed for integrators such as @expo/dev-server and @react-native/community-cli-plugin. It provides a common default implementation for core React Native dev server responsibilities.
We intend to keep this to a narrow set of functionality, based around:
- Debugging — The Chrome DevTools protocol (CDP) endpoints supported by React Native, including the Inspector Proxy, which facilitates connections with multiple devices.
- Dev actions — Endpoints implementing core Dev Menu actions, e.g. reloading the app, opening the debugger frontend.
HTTP endpoints
DevMiddlewareAPI.middleware
These are exposed as a connect middleware handler, assignable to Metro.runServer or other compatible HTTP servers.
GET /json/list, /json (CDP)
Returns the list of available WebSocket targets for all connected React Native app sessions.
GET /json/version (CDP)
Returns version metadata used by Chrome DevTools.
GET /debugger-frontend
Subpaths of this endpoint are reserved to serve the JavaScript debugger frontend.
POST /open-debugger
Open the JavaScript debugger for a given CDP target. Must be provided with one of the following query params:
device — An ID unique to a combination of device and app, stable across installs. Implemented bygetInspectorDeviceIdon each native platform.target— The target page ID as returned by/json/listfor the current dev server session.appId(deprecated, legacy only) — The application bundle identifier to match (non-unique across multiple connected devices). This param will only match legacy Hermes debugger targets.
Example
curl -X POST 'http://localhost:8081/open-debugger?target=<targetId>'
WebSocket endpoints
DevMiddlewareAPI.websocketEndpoints
/inspector/device
WebSocket handler for registering device connections.
/inspector/debug
WebSocket handler that proxies CDP messages to/from the corresponding device.
Contributing
Changes to this package can be made locally and tested against the rn-tester app, per the Contributing guide. During development, this package is automatically run from source with no build step.