@facilitronworks/react-native-windows-sync-kv

Synchronous, persistent key/value storage for React Native Windows on the new architecture (Fabric / Composition) — the storage primitive RNW does not have.

Status: pre-release (0.1.0-pre.1). The C++ implementation is extracted from a shipping production Windows app (RNW 0.83.2, Windows-on-ARM and x64), where it is the storage engine behind four separate JS packages' Windows shims.

What is verified here: see Verification status. Read it before depending on this.

Why this exists

React Native Windows ships no synchronous, JS-accessible persistent store at all. Not a slow one — none. Every option is either async or absent:

That gap bites hardest in offline-first apps. The MMKV programming model — const token = storage.getString('token') returning a value at module-load time, not a promise — is what lets a screen render populated on first paint instead of flashing empty and filling in a tick later. Ported to Windows, that code does not merely run slower; it does not run.

This module supplies the missing primitive: a REACT_SYNC_METHOD-based store that blocks the JS thread for microseconds and hands back the value.

In the app it was extracted from, this single native module backs Windows shims for all four packages above — each one namespaces its keys into the same store.

How it works

• Hydrates one JSON blob from ApplicationData.LocalFolder\SyncKV.json into an in-memory std::unordered_map once, lazily, on first access (mirroring MMKV's mmap-hydrate-once model), via synchronous std::ifstream.
• Serves reads and writes from that map with zero async hops.
• Debounces writes (300 ms quiet period) onto a background thread, then persists by writing a sibling .tmp and MoveFileExW(..., MOVEFILE_REPLACE_EXISTING) — so a kill mid-write can only corrupt the temp file, never the real one.
• clearAll() flushes synchronously, so a logout/purge is durable immediately.

The lifetime trap this encodes

This is the part worth reading even if you write your own.

React module instances are created and destroyed per React instance — every Metro reload tears yours down. The obvious design (map, mutex, condition variable as instance members; a detached worker thread capturing this) produces a use-after-free the moment you reload: the orphaned worker keeps reading and writing the freed instance. It presented as random Hermes access violations and fail-fasts, in three distinct crash signatures, and got worse the more data was stored — i.e. it looked like anything except a lifetime bug.

The fix, which this module ships: all mutable state lives in a deliberately leaked process-lifetime singleton, and exactly one worker thread exists per process no matter how many times the React instance reloads. A leaked singleton cannot dangle; the OS reclaims it at process death.

Other constraints encoded here

• RNW sync methods cannot return void — ModuleSyncMethodInfo requires a TResult. That is why set / remove / clearAll return bool.
• Every method is noexcept per the sync-method contract, so every allocating body is try-wrapped. A bad_alloc escaping a noexcept function fast-fails the process, and multi-MB values make that reachable.
• Writes are kept synchronous (not REACT_METHOD) specifically so a set() immediately followed by a getString() observes the new value.

Installation

npm install @facilitronworks/react-native-windows-sync-kv
# or: yarn add @facilitronworks/react-native-windows-sync-kv

Then autolink and rebuild:

npx react-native autolink-windows
npx react-native run-windows

Autolinking derives #include <winrt/RNWSyncKv.h> and packageProviders.Append(winrt::RNWSyncKv::ReactPackageProvider()) from the <RootNamespace> in the .vcxproj. A Metro reload will not pick up a new native module — you must rebuild.

No WinMain wiring is required: this module has no host-window dependency.

Usage

import {
  getString, set, remove, contains, getAllKeys, clearAll, createStore,
} from '@facilitronworks/react-native-windows-sync-kv';

set('token', 'abc');
getString('token');   // 'abc'  — a value, not a promise
contains('token');    // true
getAllKeys();         // ['token']
remove('token');
clearAll();           // wipes everything, flushes synchronously

Namespaced stores, when several logical stores share the one native blob:

const prefs = createStore('prefs');
prefs.set('darkMode', true);
prefs.getBoolean('darkMode');  // true
prefs.getString('missing');    // undefined  (not '')
prefs.clearAll();              // only clears the 'prefs.' namespace

Note the deliberate difference: the flat getString() returns '' for a missing key (the raw native contract), while createStore(...).getString() returns undefined, matching the react-native-mmkv surface.

Shimming another package

The typical use is a .windows.ts platform extension that re-exports another package's API on top of this store, e.g. for react-native-mmkv:

// mmkvStorage.windows.ts
import { createStore } from '@facilitronworks/react-native-windows-sync-kv';
export const storage = createStore('mmkv');

Traps you will hit

• Values are strings. Serialize anything else yourself (createStore coerces booleans and numbers via String() and parses them back). The on-disk format is a flat JSON object of string→string; a non-string JSON value in a hand-edited file is skipped at hydrate.
• getString() cannot distinguish "missing" from ''. Use contains() — or createStore, which does this for you.
• This blocks the JS thread. Storing megabytes per key and reading them in a render path will show up as jank. It is a preferences/session store, not a database.
• Not encrypted. LocalFolder\SyncKV.json is plain JSON on disk. Shimming expo-secure-store or react-native-encrypted-storage onto it gives you their API, not their security guarantee. Do not put secrets here without adding encryption (e.g. DPAPI) yourself.
• The store is process-wide and unpartitioned. Two libraries choosing the same key collide. Namespace via createStore.
• Building from source requires a NuGet restore first (msbuild -restore). Without it the CppWinRT targets never load, .idl files fall through to classic midl.exe instead of midlrt, and every namespace declaration fails with MIDL2025.

Verification status

Verified, on this exact packaged source:

• The packaged form compiles. Autolinked via a portal: dependency into a consuming RNW 0.83.2 new-arch app and built clean for ARM64 Debug, producing RNWSyncKv.dll, RNWSyncKv.winmd and RNWSyncKv.lib.
• autolink-windows resolves the package and emits both the include and the ReactPackageProvider registration.

NOT verified — do not assume any of these:

• Runtime behaviour of the DLL built from this project. The source is production-proven inside its original app, but the binary produced by this repo has not been exercised on-device: no read/write/persistence/reload test has been run against it.
• x64, Win32, and Release configurations. Only ARM64 Debug was built.
• The src/index.ts JS layer is new to this package and was not extracted from the production app (which called the native module through its own per-package shims). It typechecks; it has not been run.
• No example app, no automated tests, no CI.
• Behaviour under multiple processes/instances of the same app sharing one LocalFolder is undefined and untested — last writer wins, at best.

Changelog

• 0.1.0-pre.1 — backing store file is now SyncKV.json (was RNWSyncKv.json), so it adopts an existing SyncKV.json store in place; drop-in for consumers migrating off the in-tree module.
• 0.1.0-pre.0 — initial pre-release.

License

MIT