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import type {
OnMutationCallback,
UseMutationObserverOptions,
UseMutationObserverReturn,
} from "./types";
import {
EMPTY_RECORDS,
createNoopRef,
getMutationConfigKey,
isMutationObserverSupported,
resolveMutationConfig,
} from "./utils";
/**
* React hook that wraps the DOM `MutationObserver` API to watch an element for
* DOM mutations — child additions/removals, attribute changes, and character
* data changes.
*
* `useMutationObserver` is the low-level observer primitive, a sibling to
* {@link useResizeObserver} and `useIntersectionObserver`. Attach the returned
* `ref` to the element you want to watch; the hook (dis)connects a
* `MutationObserver` for you. You can react to mutations either via the
* `onMutation` callback, or via the reactive `records` state (the latest batch
* of `MutationRecord`s), or both.
*
* Behaviour notes:
* - **SSR-safe**: no `MutationObserver`/DOM access on the server. `isSupported`
* is `false` and the returned `ref`/methods are inert no-ops.
* - **StrictMode / concurrent-safe**: mount/unmount cleanly connect/disconnect
* with no leaked observers. The user callback is never fired from inside a
* `setState` updater.
* - **Stable callback**: `onMutation` is stored in a ref, so changing its
* identity every render does **not** tear down and re-create the observer.
* The observer is only re-registered when the observation config (`childList`,
* `attributes`, `subtree`, …) or `enabled` actually changes.
* - At least one of `childList`/`attributes`/`characterData` must be watched.
* When you specify none, the hook defaults to `childList: true` so `observe()`
* never throws.
*
* @typeParam T - The element type being observed (defaults to `Element`).
* @param options - Configuration: the standard `MutationObserverInit` knobs plus
* `onMutation`, `enabled`, and `updateState`.
* @returns An object with the callback `ref`, the latest `records`, support and
* observing flags, and manual `observe`/`disconnect`/`takeRecords` controls.
*
* @example
* ```tsx
* // Watch child list + attributes and log every mutation.
* import { useMutationObserver } from "@usefy/use-mutation-observer";
*
* function Watched() {
* const { ref, records } = useMutationObserver<HTMLDivElement>({
* childList: true,
* attributes: true,
* subtree: true,
* onMutation: (mutations) => {
* for (const m of mutations) console.log(m.type, m.target);
* },
* });
*
* return <div ref={ref}>{records.length} recent mutations</div>;
* }
* ```
*
* @example
* ```tsx
* // Callback-only mode (zero re-renders) — watch a single attribute.
* function ClassWatcher() {
* const { ref } = useMutationObserver({
* attributeFilter: ["class"],
* updateState: false,
* onMutation: (mutations) => {
* const el = mutations[0]?.target as HTMLElement;
* el?.dataset && console.log("class is now", el.className);
* },
* });
* return <div ref={ref} className="box" />;
* }
* ```
*/
export function useMutationObserver<T extends Element = Element>(
options: UseMutationObserverOptions<T> = {}
): UseMutationObserverReturn<T> {
const { onMutation, enabled = true, updateState = true } = options;
// ============ SSR Check ============
const isSupported = isMutationObserverSupported();
// ============ Resolved config ============
// Recomputed each render, but the effect only re-registers when `configKey`
// (a stable string of the config values) actually changes.
const resolvedConfig = resolveMutationConfig(options);
const configKey = getMutationConfigKey(resolvedConfig);
// ============ Refs ============
const observerRef = useRef<MutationObserver | null>(null);
const targetRef = useRef<T | null>(null);
const onMutationRef = useRef<OnMutationCallback | undefined>(onMutation);
const configRef = useRef<MutationObserverInit>(resolvedConfig);
const enabledRef = useRef<boolean>(enabled);
const updateStateRef = useRef<boolean>(updateState);
const isObservingRef = useRef<boolean>(false);
// Keep refs current so stable callbacks always read the latest values.
onMutationRef.current = onMutation;
configRef.current = resolvedConfig;
enabledRef.current = enabled;
updateStateRef.current = updateState;
// ============ State ============
const [records, setRecords] = useState<readonly MutationRecord[]>(
() => EMPTY_RECORDS
);
const [isObserving, setIsObserving] = useState<boolean>(false);
// ============ Observer Callback (stable via ref indirection) ============
const observerCallbackRef = useRef<MutationCallback>(() => {});
observerCallbackRef.current = (mutations, observer) => {
// Fire the user callback from the event handler, never inside a setState
// updater (StrictMode / concurrent safety).
if (onMutationRef.current) {
onMutationRef.current(mutations, observer);
}
if (updateStateRef.current && mutations.length > 0) {
setRecords(mutations);
}
};
// ============ Ref Callback ============
const setRef = useCallback((node: T | null) => {
const prevTarget = targetRef.current;
// Skip redundant work when the same node is attached again.
if (node === prevTarget) return;
// MutationObserver has no per-target unobserve — disconnect drops all
// observation, then we re-observe the new node below.
if (observerRef.current) {
observerRef.current.disconnect();
isObservingRef.current = false;
}
targetRef.current = node;
if (node && observerRef.current && enabledRef.current) {
observerRef.current.observe(node, configRef.current);
isObservingRef.current = true;
setIsObserving(true);
} else {
setIsObserving(false);
}
}, []);
// ============ Manual Control Methods ============
const observe = useCallback(
(element: T) => {
Iif (!isSupported) return;
if (!observerRef.current) {
observerRef.current = new MutationObserver((mutations, observer) => {
observerCallbackRef.current(mutations, observer);
});
}
// Mirror the ref path: drop any prior observation first (MutationObserver
// has no per-target unobserve, so a single observer would otherwise watch
// both the old and new element), record the target so the reconcile effect
// manages it, and honor `enabled` — record but don't observe while disabled.
observerRef.current.disconnect();
isObservingRef.current = false;
targetRef.current = element;
if (enabledRef.current) {
observerRef.current.observe(element, configRef.current);
isObservingRef.current = true;
setIsObserving(true);
} else {
setIsObserving(false);
}
},
[isSupported]
);
const disconnect = useCallback(() => {
if (!observerRef.current) return;
observerRef.current.disconnect();
observerRef.current = null;
isObservingRef.current = false;
setIsObserving(false);
}, []);
const takeRecords = useCallback((): MutationRecord[] => {
return observerRef.current?.takeRecords() ?? [];
}, []);
// ============ Effect: Create Observer on Mount ============
useEffect(() => {
if (!isSupported) return;
const observer = new MutationObserver((mutations, obs) => {
observerCallbackRef.current(mutations, obs);
});
observerRef.current = observer;
// Observe the already-attached target (ref callbacks run before effects).
if (targetRef.current && enabledRef.current) {
observer.observe(targetRef.current, configRef.current);
isObservingRef.current = true;
setIsObserving(true);
}
return () => {
// Disconnect the *live* observer (the ref may have been swapped by a
// manual disconnect()+observe() cycle), not just the one created here.
observerRef.current?.disconnect();
observer.disconnect();
observerRef.current = null;
isObservingRef.current = false;
};
// Only depend on isSupported so the observer is created once. `enabled` and
// config changes are reconciled in the separate effect below.
// eslint-disable-next-line react-hooks/exhaustive-deps
}, [isSupported]);
// ============ Effect: Reconcile enabled / config changes ============
useEffect(() => {
const observer = observerRef.current;
if (!isSupported || !observer || !targetRef.current) return;
if (enabled) {
// Re-observe with the latest config (disconnect first so a changed config
// fully replaces the old one).
observer.disconnect();
observer.observe(targetRef.current, resolvedConfig);
isObservingRef.current = true;
setIsObserving(true);
E} else if (isObservingRef.current) {
observer.disconnect();
isObservingRef.current = false;
setIsObserving(false);
}
// resolvedConfig is captured via configKey (its stable string identity).
// eslint-disable-next-line react-hooks/exhaustive-deps
}, [enabled, isSupported, configKey]);
// ============ SSR / Unsupported Return ============
const inertReturn = useMemo<UseMutationObserverReturn<T>>(
() => ({
ref: createNoopRef<T>(),
records: EMPTY_RECORDS,
isSupported: false,
isObserving: false,
observe: () => {},
disconnect: () => {},
takeRecords: () => [],
}),
[]
);
if (!isSupported) {
return inertReturn;
}
// ============ Client Return ============
return {
ref: setRef,
records,
isSupported,
isObserving,
observe,
disconnect,
takeRecords,
};
}
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