Notifications

A banner outside the app's own windows — "export finished", "update ready" — on whatever the machine has.

import { useNotifier } from 'react-x11';

function Exporter() {
  const notifier = useNotifier();

  const run = async () => {
    const file = await exportReport();
    if (!notifier.available) return;
    await notifier.notify({
      summary: 'Export finished',
      body: file.name,
      actions: [{ key: 'open', label: 'Open' }],
      onAction: (key) => key === 'open' && reveal(file),
    });
  };
  return <Button label="Export" onPress={run} />;
}

An in-app toast is deliberately not this. That is layout, and it belongs to whoever wants it; this page is about the desktop's own banners.

The ladder

The file dialog's shape again, four rungs:

1	the app's notification centre	The cocoa backend: UNUserNotificationCenter, the system's own banners, list and action buttons. Delivers only for a code-signed app bundle with a bundle id — see below.
2	org.freedesktop.Notifications	The desktop's daemon over D-Bus: updating in place, the daemon's capabilities, and the two signals that say what the user did. What a Linux desktop should get.
3	osascript	display notification, on a Mac with neither of the above — the X11 backend under XQuartz, or an unbundled cocoa app. Posted under Script Editor's identity; no actions, no update, no report back.
4	notify-send	libnotify's CLI, for a Linux box where the bus transport is missing (Node 20) but a daemon runs. The same limits, though a new enough one prints an id, which gives update() back.

notificationBackend() reports which one this machine lands on without posting anything. Where none answers, notify() rejects with a typed NoNotificationServiceError — the signal to fall back to your own UI, not a crash — and useNotifier().available is that branch as render state.

The top rung is never chosen by naming a backend: notify() asks the connection the tree renders through whether it has a centre, and whether that centre can deliver.

notify() and the handle

const banner = await notify({
  summary,            // required — the one line every daemon shows
  body?, icon?, urgency?: 'low' | 'normal' | 'critical', timeout?,
  actions?: [{ key, label }], onAction?(key), onClose?(reason),
  subtitle?, silent?, category?, resident?, appName?, appId?, userInfo?,
  backend?, app?,
});

banner.id;                        // the daemon's id / the centre's identifier / null
banner.backend;                   // which rung
await banner.update({ body });    // in place, where the rung can
await banner.close();             // take it down, where the rung can

The daemon's vocabulary is the API's. urgency is low | normal | critical, an action is { key, label }, a click on the banner itself is the action keyed 'default', and a banner that went away reports 'expired' | 'dismissed' | 'closed' | 'unknown' — the freedesktop words, because it is a published protocol with many daemons and this is one of many clients. The cocoa rung translates into them.

Attribution is filled in for you. On Linux the desktop-entry hint is the appId registerApplication() established, which is what makes the banner show up under your app in the shell's own list of them; appId overrides it. On macOS the bundle is the attribution, and there is nothing to pass.

What each rung cannot do

rung	update in place	close	actions	what the user did
cocoa	yes	yes	yes	action, dismissal
dbus	yes (replaces_id)	yes	if the daemon can	action, close reason
osascript	posts anew	nothing	no	nothing
notify-send	with an id, yes	nothing	no	nothing

The shell-out rungs are the crude floor: they show a banner and know nothing afterwards. handle.backend says which rung answered, so an app that cares can say "open the app to see it" there rather than promise a button it cannot deliver.

Actions on the bus are gated by the daemon. Whether it can show buttons is one of its GetCapabilities, and an app that sends actions to a daemon without actions has silently produced a notification nobody can act on — so they are dropped with a development warning rather than sent blind. Capabilities are read once per bus session.

A refusal is not fallen through. On the cocoa backend the first post asks the system's authorization prompt, once per bundle. A user who declined the app's notifications has said something, and notify() rejects with the platform's error (NotificationsDeniedError) rather than posting through osascript by a side door.

A refusal is an answer; a prompt nobody saw is not. The two look identical from the call — the authorization request comes back refused either way — so the rung reads the status again afterwards, because only a prompt moves it off notDetermined. A bundle macOS has not registered (an ad-hoc signature in a temp directory is the usual way to meet this) never gets the prompt, and reporting that as a denial would tell the user they declined something they were never shown. Nobody has turned anything off, so the ladder carries on to osascript; only backend: 'cocoa' turns it into an error, and that one says notDetermined rather than blaming the user.

Which is why an app that counts progress should ask before it decides. examples/notify.jsx (npm run examples:notify) is a download manager built around exactly that: it posts one banner and updates it in place where the rung can, and on osascript or notify-send stays quiet and posts once at the end, because posting per step there would be four banners for one download.

The bundle, on macOS

UNUserNotificationCenter attributes every banner to an app bundle — a CFBundleIdentifier Launch Services can see — and raises in a process that is none, which is what a bare node is. The bridge probes it once and reports available: false; notify() then takes the osascript rung, whose banners are Script Editor's. To get the real thing, run as a bundle: the executable in Name.app/Contents/MacOS/, an Info.plist that names it and carries CFBundleIdentifier, and a signature (codesign --force --deep --sign - Name.app is enough locally). See packaging.md.

useNotifier()

const { notify, available, backend } = useNotifier(defaults?);

notify is the bare function bound to the tree's connection; available settles once the ladder has been probed (one bus round trip, or one read of the centre) and is false where notify() would reject; backend is the rung. Options given to the hook are defaults for every call; options given to a call win.

Running it where there is no daemon

where	rung
macOS, the cocoa backend, as an app bundle	the centre
macOS, the cocoa backend, bare node	osascript
macOS + XQuartz	osascript (a D-Bus daemon on a Mac is rare)
GNOME / KDE / any desktop with a daemon	org.freedesktop.Notifications
Node 20 on Linux, where npm skips dbus-native	notify-send
ssh, startx, a container, CI	NoNotificationServiceError — draw your own toast

The bus rung holds one session-bus reference while any banner is live and releases it when the last one closes, so an app that closed its banners on the way out is free to exit.