@rn-iso/metro

The two things rn-iso wires into Metro: one transform cache shared by every worktree on the machine, and a reporter that writes the dev server's events as NDJSON.

npm i -D @rn-iso/metro

The shared transform cache

Metro's default cache lives under the project, so a second worktree starts cold and re-transforms the whole module graph — thousands of modules, every time. Pointing every checkout at one store means only the first one pays.

// metro.config.js
const { sharedCacheStores } = require('@rn-iso/metro');

const config = getDefaultConfig(__dirname);
config.cacheStores = sharedCacheStores('myapp');
module.exports = config;

The FileStore itself is six lines; what this packages is the housekeeping. Metro's cache has no eviction logic whatsoever, so left alone it grows until the disk does. Registering it with rn-iso makes it visible:

npx rn-iso gc                            # what it has grown to (reported on every run)
npx rn-iso gc --delete --older-than 30   # drop entries unused for 30 days

Entries are trimmed individually — one file per cache key — so trimming costs only the entries nothing has touched, not the whole cache.

rn-iso is an optional peer. Without it the cache works exactly the same; it is just invisible to housekeeping.

The location can be overridden by RN_ISO_METRO_CACHE, or machine-wide by caches.metroCache in ~/.rn-iso/config.json (an absolute path; the env var wins). The CLI and this package resolve both identically, so they always share one store.

The NDJSON log reporter

ndjsonReporter({ dir }) is a Metro Reporter that writes every event the dev server emits as one JSON object per line: bundler events and transform failures into <dir>/metro.ndjson, forwarded in-app console logs and redboxes into <dir>/client.ndjson.

const { ndjsonReporter } = require('@rn-iso/metro');

config.reporter = ndjsonReporter({ dir: '.rn-iso/logs' });
await Metro.runServer(config, { host, port });

It only survives when you host Metro yourself. Both the Expo CLI and the React Native CLI overwrite config.reporter after loading metro.config.js, so a reporter set there is discarded without a word. Setting it on a config you pass to Metro.runServer is the path that works, and it is how rn-iso start captures a bare React Native project's logs.

Each record is { ts, src, level, msg } plus, when they apply, event (the Metro event name), stack (passed through as Metro gave it) and marker: true (written on a finished bundle build, which is what rn-iso logs --errors counts errors from). dir defaults to .rn-iso/logs under the working directory.

A logging failure is never a server failure: an unwritable directory or an event shape from a Metro version this package has never seen is counted on reporter.drops and swallowed, not thrown into Metro's build pipeline.