Stream

Lune includes a WebSocket-backed IPC stream for ordered, low-latency data delivery between Crystal and JavaScript. Use it when you need high-frequency or continuous streams — price ticks, log lines, LLM token output, sensor data — where firing a new evaluateJavaScript call per message would saturate the event loop.

For the full API reference see Stream plugin.

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Crystal → JavaScript

Call app.stream.send from any fiber:

app.stream.send("tick", { "price" => 45123.50 })
app.stream.send("log-line", "build finished in 4.2s")

Listen in JavaScript with lune.Stream.on:

import { lune } from "../lunejs/runtime/runtime.js";

lune.Stream.on("tick", ({ price }) => renderTicker(price));
lune.Stream.once("ready", () => showReadyState());

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JavaScript → Crystal

Fire-and-forget with lune.Stream.send — no await needed:

lune.Stream.send("stream-start");
lune.Stream.send("order", { symbol: "BTC", qty: 1 });

Listen in Crystal with app.stream.on:

app.stream.on("order") do |data|
  place_order(data["symbol"].as_s, data["qty"].as_i)
end

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Event vs Stream

Use Event for discrete, low-frequency signals; use Stream for sustained data flows.

	Event	Stream
Transport	evaluateJavaScript per call	WebSocket frames
JS → Crystal	await Event.emit(...)	lune.Stream.send(...) (no await)
Throughput	Low–medium	High (batched WS frames)
Ordering	Best-effort	Guaranteed per-connection
Best for	UI signals, one-off notifications	Tickers, log tails, token streams

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See Stream plugin for the full API reference including common patterns and listener management.