next/dist/esm/client/use-merged-ref.js

The React Framework

import { useCallback, useRef } from 'react';
// This is a compatibility hook to support React 18 and 19 refs.
// In 19, a cleanup function from refs may be returned.
// In 18, returning a cleanup function creates a warning.
// Since we take userspace refs, we don't know ahead of time if a cleanup function will be returned.
// This implements cleanup functions with the old behavior in 18.
// We know refs are always called alternating with `null` and then `T`.
// So a call with `null` means we need to call the previous cleanup functions.
export function useMergedRef(refA, refB) {
    const cleanupA = useRef(null);
    const cleanupB = useRef(null);
    // NOTE: In theory, we could skip the wrapping if only one of the refs is non-null.
    // (this happens often if the user doesn't pass a ref to Link/Form/Image)
    // But this can cause us to leak a cleanup-ref into user code (e.g. via `<Link legacyBehavior>`),
    // and the user might pass that ref into ref-merging library that doesn't support cleanup refs
    // (because it hasn't been updated for React 19)
    // which can then cause things to blow up, because a cleanup-returning ref gets called with `null`.
    // So in practice, it's safer to be defensive and always wrap the ref, even on React 19.
    return useCallback((current)=>{
        if (current === null) {
            const cleanupFnA = cleanupA.current;
            if (cleanupFnA) {
                cleanupA.current = null;
                cleanupFnA();
            }
            const cleanupFnB = cleanupB.current;
            if (cleanupFnB) {
                cleanupB.current = null;
                cleanupFnB();
            }
        } else {
            if (refA) {
                cleanupA.current = applyRef(refA, current);
            }
            if (refB) {
                cleanupB.current = applyRef(refB, current);
            }
        }
    }, [
        refA,
        refB
    ]);
}
function applyRef(refA, current) {
    if (typeof refA === 'function') {
        const cleanup = refA(current);
        if (typeof cleanup === 'function') {
            return cleanup;
        } else {
            return ()=>refA(null);
        }
    } else {
        refA.current = current;
        return ()=>{
            refA.current = null;
        };
    }
}

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