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import { useCallback, useEffect, useMemo, useRef, useState } from "react";
import { useControllableState } from "@usefy/use-controllable-state";
import { useEventCallback } from "@usefy/use-event-callback";
import { useIsomorphicLayoutEffect } from "@usefy/use-isomorphic-layout-effect";
import { usePageVisibility } from "@usefy/use-page-visibility";
import { useUserMedia, type UserMediaStatus } from "@usefy/use-user-media";
import { QRCameraError, QRDecodeError } from "./errors";
import { decode, decodeFile, type DecodeOptions } from "./engine";
import { toImageData } from "./image/source";
import type { ImageSource, QRScanOptions, QRScanResult } from "./types";
import type { WorkerScanOptions } from "./worker-protocol";
/**
* The scanning loop: a camera, a frame pump, and a decoder.
*
* Everything here is about *pacing*. Decoding is not free, a camera delivers
* frames faster than any decoder needs them, and the one thing a user notices
* immediately is a preview that stutters. So the loop skips frames while a
* decode is in flight, throttles to a rate rather than chasing the camera, and
* stops entirely when the tab is hidden.
*/
/** How the scanner is currently behaving — what a UI needs to say out loud. */
export type QRScannerState =
/** No camera requested yet. */
| "idle"
/** The browser is asking the user for permission. */
| "prompting"
/** Running: frames are being decoded. */
| "scanning"
/** Started, but paused (by `paused`, or by a hidden tab). */
| "paused"
/** Permission was refused. */
| "denied"
/** No camera, or no way to use one here. */
| "unsupported"
/** Something failed; see `error`. */
| "error";
/**
* Something that can decode an image — a worker, or anything else.
*
* The options are the *transferable* subset: a custom decoder may well live
* behind `postMessage`, where a function `binarizer` cannot follow. Typing the
* boundary narrowly is what makes that a compile error rather than a silently
* ignored option.
*/
export interface Decoder {
decode(
source: ImageSource,
options?: WorkerScanOptions & { timestamp?: number },
): Promise<QRScanResult[]>;
}
export interface UseQRScannerOptions extends QRScanOptions {
/** Called once per distinct value, subject to `dedupeMs`. */
onScan?: (result: QRScanResult, all: readonly QRScanResult[]) => void;
/** Called for **every** decode, including repeats — for throughput readouts. */
onFrame?: (results: readonly QRScanResult[]) => void;
onError?: (error: Error) => void;
/** Open the camera as soon as the hook mounts. @default true */
autoStart?: boolean;
/**
* Hold the loop without releasing the camera.
*
* Controllable: pass `paused` to drive it yourself, `defaultPaused` to let
* the component own it while still hearing about changes through
* `onPausedChange`, or neither and use `pause()` / `resume()`.
*/
paused?: boolean;
/** Initial paused state when `paused` is not supplied. @default false */
defaultPaused?: boolean;
/** Fired whenever the paused state changes, from either side. */
onPausedChange?: (paused: boolean) => void;
facingMode?: "user" | "environment";
deviceId?: string;
constraints?: MediaTrackConstraints;
/**
* Maximum decodes per second.
* @default 12
*/
scanRate?: number;
/**
* How long the same value is suppressed after a hit, in milliseconds. `0`
* reports every frame.
* @default 1500
*/
dedupeMs?: number;
/** Release the camera while the tab is hidden, not just pause. @default false */
stopOnHide?: boolean;
/** Decode off the main thread (see {@link createWorkerDecoder}). */
decoder?: Decoder;
}
export interface UseQRScannerReturn {
/** Attach to a `<video>`; the hook wires up `srcObject` and playback. */
videoRef: React.RefObject<HTMLVideoElement | null>;
state: QRScannerState;
/** The most recent decode. */
result: QRScanResult | null;
/** Every symbol found in the most recent frame. */
results: readonly QRScanResult[];
error: Error | null;
start: () => Promise<void>;
stop: () => void;
/** Hold the loop; the camera stays open so resuming is instant. */
pause: () => void;
resume: () => void;
devices: MediaDeviceInfo[];
activeDeviceId: string | null;
selectDevice: (deviceId: string) => Promise<void>;
switchCamera: () => Promise<void>;
isTorchSupported: boolean;
torch: boolean;
setTorch: (on: boolean) => Promise<boolean>;
/** Decode a file the user picked, dropped or pasted. */
scanFile: (file: Blob) => Promise<QRScanResult[]>;
/** The current frame's pixels — for a "scan the still image" fallback. */
capture: () => Promise<ImageData | null>;
}
/** How many camera frames pass between attempts at an inverted (dark-mode) read. */
const INVERT_EVERY = 4;
/**
* Consecutive empty frames after which the session stops asking the platform
* detector and uses the internal engine for good.
*
* At the default twelve frames a second this is a second of pointing at
* nothing — long enough that a user simply looking for a code never triggers
* it, short enough that a detector which advertises QR and then never answers
* costs at most one second before the working engine takes over.
*/
const NATIVE_MISS_STREAK = 12;
/**
* Declared locally rather than pulled in from `@types/node` (qr-code's
* `render/warnings.ts` precedent): this package has no Node dependency, and the
* literal `process.env.NODE_ENV` text is what bundlers look for when stripping
* development-only code.
*/
declare const process: { env?: { NODE_ENV?: string } } | undefined;
let warnedAboutBinarizer = false;
function warnBinarizerNotTransferable(): void {
const isProduction = typeof process !== "undefined" && process.env?.NODE_ENV === "production";
Iif (warnedAboutBinarizer || isProduction) return;
warnedAboutBinarizer = true;
console.warn(
"[@usefy/qr-scanner] A function `binarizer` cannot be sent to a worker, so " +
"frames are being decoded on the main thread instead. Use `binarizer: " +
'"hybrid" | "otsu"` to keep the worker in play.',
);
}
/** Whether an error came from the camera layer (a `UserMediaError`). */
function isCameraFailure(error: unknown): boolean {
return (
typeof error === "object" &&
error !== null &&
"reason" in error &&
(error as { name?: unknown }).name === "UserMediaError"
);
}
function toScannerState(
media: UserMediaStatus,
paused: boolean,
hasStream: boolean,
): QRScannerState {
switch (media) {
case "idle":
return "idle";
case "prompting":
return "prompting";
case "denied":
return "denied";
case "unsupported":
return "unsupported";
case "error":
return "error";
case "granted":
return paused || !hasStream ? "paused" : "scanning";
}
}
/**
* Drive a camera and decode QR codes from its frames.
*
* @example
* ```tsx
* const scanner = useQRScanner({ onScan: (result) => setValue(result.text) });
* return <video ref={scanner.videoRef} playsInline muted />;
* ```
*/
export function useQRScanner(options: UseQRScannerOptions = {}): UseQRScannerReturn {
const {
onScan,
onFrame,
onError,
autoStart = true,
paused,
defaultPaused = false,
onPausedChange,
facingMode = "environment",
deviceId,
constraints,
scanRate = 12,
dedupeMs = 1500,
stopOnHide = false,
decoder,
...scanOptions
} = options;
const videoRef = useRef<HTMLVideoElement | null>(null);
const [results, setResults] = useState<readonly QRScanResult[]>([]);
const [result, setResult] = useState<QRScanResult | null>(null);
const [error, setError] = useState<Error | null>(null);
// One paused state, whoever owns it: a `paused` prop, a `defaultPaused` plus
// `onPausedChange`, or `pause()`/`resume()`. Two separate flags — a prop and
// an internal one — would let them disagree, and "I called resume() but the
// prop still says paused" is a bug report nobody enjoys.
const [effectivelyPaused, setPaused] = useControllableState<boolean>({
value: paused,
defaultValue: defaultPaused,
onChange: onPausedChange,
});
const isVisible = usePageVisibility();
const handleError = useEventCallback((caught: unknown) => {
// Camera failures arrive as the hook's `UserMediaError`; they are re-typed
// as this package's `QRCameraError` so a consumer narrows on one error type
// from one package rather than importing a sibling's to write a `catch`.
const normalized =
isCameraFailure(caught) || caught instanceof QRCameraError
? QRCameraError.from(caught)
: caught instanceof Error
? caught
: new Error(String(caught));
setError(normalized);
onError?.(normalized);
});
const mediaConstraints = useMemo<MediaStreamConstraints>(
() => ({ video: constraints ?? true, audio: false }),
[constraints],
);
const camera = useUserMedia({
constraints: mediaConstraints,
autoStart,
facingMode,
deviceId,
onError: handleError,
});
/* ───────────────────────── the frame pump ───────────────────────── */
// Written in a layout effect rather than during render: a render React throws
// away must not mutate a ref, and the house `useEventCallback` uses the same
// pattern for exactly this reason.
const decodeOptionsRef = useRef(scanOptions);
useIsomorphicLayoutEffect(() => {
decodeOptionsRef.current = scanOptions;
});
const emitScan = useEventCallback((frame: readonly QRScanResult[]) => {
onFrame?.(frame);
if (frame.length === 0) return;
setResults(frame);
const now = frame[0]!.timestamp;
for (const candidate of frame) {
const previous = seenRef.current.get(candidate.text);
if (previous !== undefined && dedupeMs > 0 && now - previous < dedupeMs) continue;
seenRef.current.set(candidate.text, now);
setResult(candidate);
onScan?.(candidate, frame);
}
// The dedupe map is per-session, and a long shift at a checkout desk would
// otherwise grow it without bound.
Iif (seenRef.current.size > 256) {
for (const [value, at] of seenRef.current) {
if (now - at > dedupeMs) seenRef.current.delete(value);
}
}
});
const seenRef = useRef(new Map<string, number>());
const runningRef = useRef(false);
const inFlightRef = useRef(false);
const frameCountRef = useRef(0);
const lastDecodeRef = useRef(0);
const mountedRef = useRef(true);
const bufferRef = useRef<Uint8ClampedArray | undefined>(undefined);
/** Consecutive frames the native detector found nothing in. */
const nativeMissesRef = useRef(0);
/** Once the native path has proven unhelpful, the session stays internal. */
const nativeGaveUpRef = useRef(false);
useEffect(() => {
mountedRef.current = true;
return () => {
mountedRef.current = false;
};
}, []);
const decodeFrame = useEventCallback(async (video: HTMLVideoElement) => {
Iif (inFlightRef.current) return;
inFlightRef.current = true;
try {
const frameIndex = frameCountRef.current++;
// The inverted (light-on-dark) pass doubles the cost of a frame that
// found nothing, so the live loop pays for it once every few frames
// rather than on every one. A still image gets it every time — that is
// `decodeFile`'s call, not this one's.
const invert = frameIndex % INVERT_EVERY === 0 ? "auto" : "never";
// The luminance plane is the biggest per-frame allocation, and at twelve
// frames a second a fresh one is megabytes of garbage a minute. One
// buffer sized for the full frame serves every working resolution,
// because `toGray` takes a subarray of whatever it is handed.
const pixels = video.videoWidth * video.videoHeight;
if (pixels > 0 && (!bufferRef.current || bufferRef.current.length < pixels)) {
bufferRef.current = new Uint8ClampedArray(pixels);
}
const request: DecodeOptions = {
...decodeOptionsRef.current,
invert: decodeOptionsRef.current.invert ?? invert,
buffer: bufferRef.current,
timestamp: performance.now(),
// On a live camera the native detector gets the frame to itself: running
// the internal engine too, on every frame that found nothing, doubles
// the cost of the most common frame there is. A still image is the
// opposite trade and keeps the fallback (`engine.ts`).
fallbackToInternal: false,
forceInternal: nativeGaveUpRef.current || undefined,
};
// A custom binarizer is a function, and functions do not survive
// `postMessage`. Rather than dropping it silently — which would leave a
// consumer debugging why their thresholding never runs — the frame goes
// to the main thread, honouring the explicit choice over the implicit
// performance preference, and dev mode says so once.
const workerCanRun = typeof request.binarizer !== "function";
if (decoder && !workerCanRun) warnBinarizerNotTransferable();
const frame =
decoder && workerCanRun
? await decoder.decode(video, request as WorkerScanOptions & { timestamp?: number })
: await decode(video, request);
// Decoding is async; the component can be gone by the time it answers,
// and delivering a scan to a component that no longer exists is a bug in
// the consumer's app that they did not write.
Iif (!mountedRef.current) return;
// ── The native miss-streak switch ──
// `BarcodeDetector` is faster when it works, but some builds advertise QR
// and then quietly find nothing (a symbology they only partly implement,
// a frame format they dislike). Rather than scanning forever with a
// detector that never answers, the session gives up on it after a run of
// empty frames and stays internal.
if (!nativeGaveUpRef.current && !decoder) {
if (frame.length > 0) {
nativeMissesRef.current = 0;
E} else if (++nativeMissesRef.current >= NATIVE_MISS_STREAK) {
nativeGaveUpRef.current = true;
}
}
emitScan(frame);
} catch (caught) {
// A frame that cannot be decoded is the normal case, not an error — only
// a camera or environment failure is worth surfacing.
if (mountedRef.current && !(caught instanceof QRDecodeError)) handleError(caught);
} finally {
inFlightRef.current = false;
}
});
useEffect(() => {
const video = videoRef.current;
const stream = camera.stream;
if (!video || !stream) return;
video.srcObject = stream;
// Two guards in one line. `play()` rejects if the element is removed
// mid-call, which is routine during navigation and not worth reporting —
// and it returns `undefined` rather than a promise in older engines, where
// an unguarded `.catch` is a TypeError that takes the scanner down.
void video.play()?.catch(() => {});
return () => {
video.srcObject = null;
};
}, [camera.stream]);
// A hidden tab stops the loop whether or not the camera is released: decoding
// frames nobody can see is pure battery.
const active = !effectivelyPaused && isVisible && camera.status === "granted";
useEffect(() => {
if (!active) return;
const video = videoRef.current;
Iif (!video) return;
runningRef.current = true;
const interval = 1000 / Math.max(1, scanRate);
let handle = 0;
let frameHandle = 0;
const tick = (): void => {
if (!runningRef.current) return;
const now = performance.now();
Eif (now - lastDecodeRef.current >= interval && video.readyState >= 2) {
lastDecodeRef.current = now;
void decodeFrame(video);
}
schedule();
};
const schedule = (): void => {
Iif (!runningRef.current) return;
// `requestVideoFrameCallback` fires once per *presented* frame, so the
// loop never decodes the same picture twice and never spins between
// frames. rAF is the fallback where it does not exist (Firefox).
if (typeof video.requestVideoFrameCallback === "function") {
frameHandle = video.requestVideoFrameCallback(() => tick());
} else E{
handle = requestAnimationFrame(() => tick());
}
};
schedule();
return () => {
runningRef.current = false;
Iif (handle) cancelAnimationFrame(handle);
Eif (frameHandle && typeof video.cancelVideoFrameCallback === "function") {
video.cancelVideoFrameCallback(frameHandle);
}
};
}, [active, scanRate, decodeFrame]);
// Releasing the camera while hidden is opt-in: it frees the LED and the
// sensor, but re-acquiring costs a permission-free round trip and a visible
// black frame, which is the wrong trade for a scanner the user is switching
// back to in a second.
const hiddenReleaseRef = useRef(false);
useEffect(() => {
Eif (!stopOnHide) return;
if (!isVisible) {
hiddenReleaseRef.current = true;
camera.stop();
return;
}
// Only re-acquire what *this* effect released. On mount the tab is visible
// and `useUserMedia` has already acquired (or deliberately not, if
// `autoStart` is false); starting again here would mean two `getUserMedia`
// calls, a superseded stream, and a visible blink of the camera LED.
if (hiddenReleaseRef.current) {
hiddenReleaseRef.current = false;
if (autoStart) void camera.start();
}
}, [isVisible, stopOnHide, autoStart, camera]);
/* ───────────────────────── imperative API ───────────────────────── */
// Each of these depends on one *stable* function from the camera hook rather
// than on the whole object. Depending on `camera` would make every action's
// identity change on every render — and a consumer's `useEffect([scanner])`
// would then re-run on every decoded frame.
const cameraStart = camera.start;
const cameraStop = camera.stop;
const cameraSelect = camera.selectDevice;
const cameraSwitch = camera.switchDevice;
const start = useCallback(async () => {
setError(null);
await cameraStart();
}, [cameraStart]);
const stop = useCallback(() => {
runningRef.current = false;
cameraStop();
}, [cameraStop]);
const pause = useCallback(() => setPaused(true), [setPaused]);
const resume = useCallback(() => setPaused(false), [setPaused]);
const selectDevice = useCallback(
async (id: string) => {
await cameraSelect(id);
},
[cameraSelect],
);
const switchCamera = useCallback(async () => {
await cameraSwitch();
}, [cameraSwitch]);
const scanFile = useCallback(
async (file: Blob) => {
setError(null);
try {
const found = await decodeFile(file, decodeOptionsRef.current);
emitScan(found);
return found;
} catch (caught) {
handleError(caught);
return [];
}
},
[emitScan, handleError],
);
const capture = useCallback(async () => {
const video = videoRef.current;
if (!video || video.readyState < 2) return null;
try {
return await toImageData(video);
} catch {
return null;
}
}, []);
const state = toScannerState(camera.status, effectivelyPaused || !isVisible, camera.stream !== null);
// Memoized for the same reason the camera hook's return is: a scanner
// re-renders on every decoded frame, and a fresh object each time would churn
// every consumer's dependency array twelve times a second.
return useMemo(
() => ({
videoRef,
state,
result,
results,
// `camera.error` is only reached if the hook set one without calling
// `onError` — it is re-typed here for the same reason as above, so `error`
// is always this package's own type whatever route it took.
error: error ?? (camera.error ? QRCameraError.from(camera.error) : null),
start,
stop,
pause,
resume,
devices: camera.devices,
activeDeviceId: camera.activeDeviceId,
selectDevice,
switchCamera,
isTorchSupported: camera.isTorchSupported,
torch: camera.torch,
setTorch: camera.setTorch,
scanFile,
capture,
}),
[
state,
result,
results,
error,
camera.error,
camera.devices,
camera.activeDeviceId,
camera.isTorchSupported,
camera.torch,
camera.setTorch,
start,
stop,
pause,
resume,
selectDevice,
switchCamera,
scanFile,
capture,
],
);
}
/** Re-exported so a consumer can narrow on the camera's own failure type. */
export { QRCameraError };
|