How expensive are events and listeners in terms of RAM / CPU?

[geeseofbeverlyroad]

Hi all! I am writing a music player and trying to link database state with frontend state, and am wondering how many event listeners is reasonable to have registered at a time.

I display music tracks with tags (tags being artist name, track name, rating etc) using per-track display components on the WebView. I would like each track's component to update UI when tags are edited (which happens on the Rust side of things), through emitting events.

Since there are potentially thousands of track components rendered at a time, is it reasonable to have a thousands of listeners, and does that introduce bottlenecks in any way?

Same concern goes for registering listeners: when switching views (i.e. from looking at all tracks, to looking at tracks of a specific playlist), thousands of listeners would unlistenFn, and thousands more would listen, all at the same time. Is that unwise?

If this is OK to do, I am also wondering: is having unique event names VS a single event with payload data the better option? For example, is a thousand listeners listening to track-tags-updated-id1, track-tags-updated-id2 etc. better than a thousand listeners all listening to track-tags-updated sending a payload of i.e. { id: 1 } the better option performance wise?

Thank you!

[krotname]

Short version: I would not register one listener per rendered track if the list can be in the thousands, especially if the whole set churns when switching views. It will probably work, but it is avoidable overhead.

In the current API, each listen() call invokes plugin:event|listen and registers a JS callback; unlisten() invokes plugin:event|unlisten. On the Rust side, JS listeners are stored by webview, event name, and handler id, and emitting an event looks up the handlers for that event and sends the event to those callback ids.

So the tradeoff is roughly:

• registering/removing thousands of component listeners means thousands of IPC calls plus callback/listener bookkeeping
• one shared event means all handlers for that event are candidates, then each component filters the payload
• per-track event names wake fewer handlers for one track update, but you still maintain thousands of event names/listeners and still pay the mount/unmount churn

For a music library UI, I would treat Tauri events as the boundary between Rust and the webview, not as the per-row subscription system. A better shape is usually one listener for the current view, or a small number of listeners for frontend stores, then emit payloads like { ids: [...], changes: ... }, update a frontend store, and let the UI framework update the visible rows. If many tags can change together, batch/debounce the event.

Unique event names make sense for a small, stable set of channels. For thousands of track rows, I would avoid both "thousands of listeners on one event" and "thousands of event names" unless profiling your real view shows it is still fine.

[geeseofbeverlyroad]

Thank you! This confirms my suspicion that they're probably not so cheap to have registered. I had an intuition to avoid having too many listeners, especially when 99% of them will never fire. Architecture wise it would be the simplest thing to do, but indeed something like a reactive store on the JS side updated from a central place seems like the better but slightly more involved option.

[DYNOSuprovo]

Tauri events are very lightweight — you can safely have thousands of listeners without meaningful RAM or CPU overhead. Here's the complete breakdown:

How Tauri events work under the hood

Events are routed through Tauri's IPC bridge (a postMessage channel between Rust and the WebView). Each listen() call registers a JavaScript callback in a local Map — there's no per-listener OS resource or native thread involved. The cost is:

• Memory per listener: a few hundred bytes (a JS closure + Map entry + Tauri's internal listener registry entry)
• CPU when emitting: O(n) scan of listeners matching the event name — linear in listener count, but only at emit time
• CPU at idle: zero — listeners don't poll

For 1,000–5,000 listeners, you're looking at a few MB of overhead total, which is negligible in any desktop app.

Unique names vs. single event with payload

Use a single event name with a payload filter — this is almost always better:

// ❌ Avoid: 1000 unique event names
await listen(`track-tags-updated-${trackId}`, handler);

// ✅ Better: single event name, filter in handler
await listen('track-tags-updated', (event) => {
  if (event.payload.id === trackId) {
    // update this track's UI
  }
});

Why single-name is faster: Tauri matches event names as strings. With 1,000 unique names, every emit does 1,000 string comparisons. With 1 name and 1,000 listeners, it's 1 comparison to find all matching listeners, then dispatches to them in a single pass — much more efficient at scale.

Mass register/unregister on view switch

This is valid to do, but batch your cleanup:

const unlisteners: Array<() => void> = [];

for (const track of tracks) {
  const unlisten = await listen('track-tags-updated', (e) => {
    if (e.payload.id === track.id) updateTrack(track.id, e.payload);
  });
  unlisteners.push(unlisten);
}

// On view unmount — clean up all at once
onDestroy(() => unlisteners.forEach(fn => fn()));

If using React, put this in a useEffect cleanup.

Recommended architecture for a music player

Rather than per-component listeners, use a single top-level listener that maintains a reactive store:

// store.ts — one listener for the entire app lifetime
const trackStore = new Map<number, TrackData>();

await listen('track-tags-updated', (event) => {
  const { id, tags } = event.payload;
  trackStore.set(id, { ...trackStore.get(id), ...tags });
  // signal reactivity (Svelte store, Jotai atom, Zustand, etc.)
});

Each track component reads from the store rather than registering its own listener. This scales to any number of tracks with zero additional listener cost, and eliminates the mass-register/unregister problem entirely.

TL;DR

• ✅ Thousands of listeners are fine — Tauri is built for this use case
• ✅ Prefer one event name + payload filter over thousands of unique names
• ✅ For a music player, a single store-level listener is the cleanest architecture
• ⚠️ Mass await listen() calls during view switches can cause a brief async waterfall — a single store listener avoids this entirely

[ekusiadadus]

One correction to the comparison above: Tauri's event dispatch does not scan every event name. Its Rust listener table looks up the emitted name in a map and sends the matching listener IDs.

So with one listener per track-specific name, a single track update targets that track's listener. With 1,000 listeners on one shared name, all 1,000 receive the event and must filter it in JavaScript. Those are different dispatch workloads; a shared name is not automatically faster.

Registration and cleanup still involve IPC, so frequent mount/unmount churn is a separate cost. One shared listener that updates a store avoids both the fan-out and repeated component registrations. The source doesn't establish a universal RAM cost or listener-count threshold.

Source.