/// <reference types="react" />
/**
 * Measures the line height of the element referenced by `ref` in pixels.
 * Re-measures on resize and after fonts load. Returns `null` until measured.
 *
 * @param ref - Ref to the element whose line height should be measured.
 * @returns The line height in pixels, or `null` if not yet measured.
 * @internal
 */
export declare function useLineHeight<T extends HTMLElement>(ref: React.RefObject<T | null>): number | null;