MotionStream

Streamed text revealed at a steady pace that catches up with bursts.

WebReact NativeNumbers and text

Try it

Preparing the example…

JavaScript is off. The code below is the complete example.

App.jsx

import { useEffect, useRef, useState } from 'react';
import { MotionStream } from '@kkwaenggwari/animated-chorus';
import './styles.css';

const answer =
  'Motion helps people follow change. When a total updates, a rolling number shows which way it moved. ' +
  'When a panel opens, the height that follows its content keeps the reader in place. ' +
  'When an answer arrives in bursts, a steady reveal turns the bursts into reading: nothing waits for the ' +
  'next packet, and nothing is dumped at once. The pace speeds up only when a backlog builds, so the text ' +
  'never falls far behind what the server has already sent.';

// Network-like bursts: long pauses, then a lot of text at once.
const pauses = [300, 1100, 250, 1500, 400, 900];

export default function App({ speed = 90, catchUp = 900, chunk = 160, effect = 'fade' }) {
  const [received, setReceived] = useState(0);
  const [chunks, setChunks] = useState(0);
  const renders = useRef(0);
  renders.current += 1;

  useEffect(() => {
    if (received >= answer.length) return;
    const timer = setTimeout(
      () => {
        setReceived((current) => Math.min(answer.length, current + chunk));
        setChunks((count) => count + 1);
      },
      pauses[chunks % pauses.length],
    );
    return () => clearTimeout(timer);
  }, [received, chunks, chunk]);

  const askAgain = () => {
    setReceived(0);
    setChunks(0);
  };

  return (
    <div className="chat">
      <p className="question">Why does motion help an interface?</p>
      <p className="answer">
        <MotionStream
          text={answer.slice(0, received)}
          speed={speed}
          catchUp={catchUp}
          effect={effect}
        />
      </p>
      <footer>
        <output>
          {chunks} chunks · App rendered {renders.current} times
        </output>
        <button onClick={askAgain}>Ask again</button>
      </footer>
    </div>
  );
}

styles.css

.chat {
  display: grid;
  gap: 16px;
  max-width: 620px;
  margin: 0 auto;
  padding: 24px 20px;
}

.question {
  justify-self: end;
  max-width: 80%;
  margin: 0;
  padding: 10px 16px;
  border-radius: 18px 18px 4px 18px;
  background: #176b52;
  color: #fff;
}

.answer {
  min-height: 180px;
  margin: 0;
  padding: 16px 18px;
  border: 1px solid #cfe8dc;
  border-radius: 18px 18px 18px 4px;
  background: #fff;
  color: #16181d;
  line-height: 1.65;
}

footer {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
}

output {
  font:
    600 13px ui-monospace,
    Consolas,
    monospace;
  color: #176b52;
}

footer button {
  min-height: 40px;
  padding: 0 16px;
  border: 1px solid #cfe8dc;
  border-radius: 999px;
  background: #fff;
  font:
    600 14px system-ui,
    sans-serif;
  cursor: pointer;
}

Shortest usage

Answer.jsx

<MotionStream text={answerSoFar} initial />

<MotionStream text={answerSoFar}>
  {visible => <Markdown>{visible}</Markdown>}
</MotionStream>

What it handles for you

Bursts become reading: text is revealed at speed characters per second instead of in chunks.
A backlog speeds the reveal up so it is shown within about catchUp milliseconds.
Emoji, flags and combining marks are never split, and a half-received emoji waits for the rest.
A rewritten answer continues from the shared start instead of starting over.
Revealed characters go straight to the page; React renders only a few times per second.

Use it for

An assistant's answer in a chat.
A build or deploy log.
Live captions.

Props

Prop	Type	Default	Description
`text`	`string`	`—`required	The text so far. Growing text is revealed at a steady pace; a rewrite continues from the shared start.
`speed`	`number`	`90`	Base reveal rate in characters per second.
`catchUp`	`number`	`900`	A backlog is revealed within about this many milliseconds by speeding up.
`initial`	`boolean`	`false`	Also reveal the text present on mount (otherwise it shows at once).
`effect`	`'fade' \| 'none'`	`'fade'`	`fade` fades new characters in; `none` shows them directly.
`children`	`(visible: string) => ReactNode`		A function that receives the paced text, for example to render Markdown.
`as`	`element`	`'span'`	The element to render.
`onMotionComplete`	`(text) => void`		Runs whenever the reveal catches up with `text`.

Other props go to the rendered element, so className, style and event handlers work as usual.

Web and React Native

On the web new characters fade in (effect="fade") and are folded into plain, selectable text. A function child receives the paced string for Markdown, at most every other frame. In a hidden tab everything shows at once.

On React Native the text is a real Text and appears without a fade.

Used in these examples

Chat answerSuggestions that fold away, an answer that streams at a steady pace and a status line that swaps.MotionStreamMotionListMotionSwapMotionHeight