Tauri v2: Events and Channels (Rust → JS push)

Tauri v2 has three mechanisms for moving data from Rust to JS. Pick the right one — they have different cost and semantics.

Mechanism	Use for	Cost
#[tauri::command] return value	Request/response, JS asks, Rust answers once	Lowest for one shot
Events (emit / listen)	Unsolicited, low-frequency push (status changes, lifecycle, cross-window signals)	Per-event serialize + global dispatch
Channel<T>	Streaming, high-frequency, or large payloads tied to one command	One open pipe, minimal overhead

Quick rule: commands ask, events broadcast, channels stream. If a feature is one Rust task that produces many JS-side updates (download progress, tail logs, parsing stream), reach for a channel.

Global events

Rust side uses the Emitter trait on AppHandle, Window, Webview, or WebviewWindow. Payload must implement Serialize + Clone.

use serde::Serialize;
use tauri::{AppHandle, Emitter};

#[derive(Clone, Serialize)]
#[serde(rename_all = "camelCase")]
struct DownloadStarted<'a> {
  url: &'a str,
  download_id: usize,
}

#[tauri::command]
fn start_download(app: AppHandle, url: String) {
  app.emit("download-started", DownloadStarted { url: &url, download_id: 1 }).unwrap();
  app.emit("download-progress", 50).unwrap();
  app.emit("download-finished", 1).unwrap();
}

JS side uses @tauri-apps/api/event:

import { listen, type UnlistenFn } from '@tauri-apps/api/event';

const unlisten: UnlistenFn = await listen<number>('download-progress', (e) => {
  console.log('progress', e.payload, 'from window', e.windowLabel);
});

The callback receives an Event<T> with payload, event, id, and (when applicable) windowLabel.

Targeted emit

emit goes to every listener in every webview. To narrow it:

use tauri::{AppHandle, Emitter, EventTarget};

// Single labeled window:
app.emit_to("login", "login-result", "loggedIn").unwrap();

// Arbitrary target type:
app.emit_to(EventTarget::webview_window("settings"), "refresh", ()).unwrap();

// Multi-target filter:
app.emit_filter("open-file", path, |target| match target {
  EventTarget::WebviewWindow { label } => label == "main" || label == "file-viewer",
  _ => false,
}).unwrap();

Use targeted emit when you have multiple windows and an event is only meaningful to one — it avoids waking up unrelated listeners and prevents accidental cross-window coupling.

Listening in Rust

Rust can also subscribe — useful for cross-process signals or test harnesses:

use tauri::Listener;

let id = app.listen("frontend-ready", |event| {
  println!("got payload: {}", event.payload());
});

// One-shot:
app.once("first-paint", |_event| { /* ... */ });

// Drop the subscription when you're done:
app.unlisten(id);

event.payload() returns the raw JSON string — deserialize with serde_json::from_str if you need typed access.

Channels — streaming Channel<T>

When the frontend invokes a command that needs to stream back (download progress, log tail, SSE re-broadcast), pass a Channel as a command argument. The event system is explicitly not designed for low latency or high throughput — channels are.

use serde::Serialize;
use tauri::ipc::Channel;

#[derive(Clone, Serialize)]
#[serde(rename_all = "camelCase", tag = "event", content = "data")]
enum DownloadEvent<'a> {
  Started { url: &'a str, content_length: usize },
  Progress { chunk_length: usize },
  Finished,
}

#[tauri::command]
async fn download(url: String, on_event: Channel<DownloadEvent<'_>>) -> Result<(), String> {
  on_event.send(DownloadEvent::Started { url: &url, content_length: 1024 }).map_err(|e| e.to_string())?;
  for _ in 0..10 {
    on_event.send(DownloadEvent::Progress { chunk_length: 102 }).map_err(|e| e.to_string())?;
  }
  on_event.send(DownloadEvent::Finished).map_err(|e| e.to_string())?;
  Ok(())
}

import { invoke, Channel } from '@tauri-apps/api/core';

type DownloadEvent =
  | { event: 'started'; data: { url: string; contentLength: number } }
  | { event: 'progress'; data: { chunkLength: number } }
  | { event: 'finished' };

const onEvent = new Channel<DownloadEvent>();
onEvent.onmessage = (msg) => {
  if (msg.event === 'progress') updateBar(msg.data.chunkLength);
};
await invoke('download', { url: 'https://example.com/big.bin', onEvent });

Channels are unidirectional (Rust → JS), tied to one command invocation, and have no cross-window broadcast. Multiple concurrent downloads = multiple channels.

Events vs commands vs channels — decision guide

• Need a response to a user action right now? Command return value. Don’t emit an event the caller has to correlate.
• Multiple windows care about the same state change? Global event.
• One specific window cares? emit_to with that window label.
• Same logical task produces many updates over time? Channel. Don’t loop emit inside a command — you pay full dispatch cost per tick.
• Backpressure matters? Channel — send returns Result and respects the IPC transport. Events fire-and-forget.

Lifecycle and leaks

JS: every listen() returns an UnlistenFn. Call it on component unmount or you leak a subscription per mount. In React:

useEffect(() => {
  let unlisten: UnlistenFn | undefined;
  listen('download-progress', handler).then((fn) => { unlisten = fn; });
  return () => { unlisten?.(); };
}, []);

In vanilla TS, wire the unlisten into your teardown path (window close, view switch, etc.).

Rust: app.listen returns an EventId — call app.unlisten(id) when done, or use app.once for self-cleaning one-shots. Channels close automatically when the JS-side Channel is GC’d or the command future ends.

Common pitfalls

• Forgetting Clone on payload structs — emit requires it; the borrow checker will tell you with a confusing message about Emitter.
• Listening before the window exists — use app.once("tauri://webview-created", ...) or set up listeners inside setup once you have the handle.
• Emitting from a hot loop without batching — switch to a channel, or coalesce updates (throttle to ~60Hz / 16ms).
• Cross-window leakage — emit is global. If a payload contains window-specific state, use emit_to or filter on the JS side via e.windowLabel.
• Assuming order across mechanisms — events and channel messages are not ordered with respect to command return values.

Templates

See templates/events.rs for a complete Rust handler showing global emit, targeted emit, and a Channel<ProgressEvent> streaming command. See templates/events.ts for the matching JS — global listen with cleanup, and a streaming Channel consumer.