Overleaf/services/web/frontend/js/shared/hooks/use-pdf-scroll-position.ts

import { Dispatch, SetStateAction, useEffect, useRef, useState } from 'react'
import customLocalStorage from '@/infrastructure/local-storage'
import { debugConsole } from '@/utils/debugging'

export type PdfScrollPosition = Record<string, any> | undefined

export const usePdfScrollPosition = (
  lastCompileRootDocId: string | null | undefined
): [PdfScrollPosition, Dispatch<SetStateAction<PdfScrollPosition>>] => {
  // scroll position of the PDF
  const [position, setPosition] = useState<PdfScrollPosition>()

  const lastCompileRootDocIdRef = useRef<string | null | undefined>(
    lastCompileRootDocId
  )
  useEffect(() => {
    lastCompileRootDocIdRef.current = lastCompileRootDocId
  }, [lastCompileRootDocId])

  const initialScrollPositionRef = useRef<PdfScrollPosition | null>(null)

  // load the stored PDF scroll position when the compiled root doc changes
  useEffect(() => {
    if (lastCompileRootDocId) {
      const position = customLocalStorage.getItem(
        `pdf.position.${lastCompileRootDocId}`
      )
      if (position) {
        debugConsole.log('loaded position for', lastCompileRootDocId, position)
        initialScrollPositionRef.current = position
        setPosition(position)
      }
    }
  }, [lastCompileRootDocId])

  // store the current root doc's PDF scroll position when it changes
  useEffect(() => {
    if (
      lastCompileRootDocIdRef.current &&
      position &&
      position !== initialScrollPositionRef.current
    ) {
      debugConsole.log(
        'storing position for',
        lastCompileRootDocIdRef.current,
        position
      )
      customLocalStorage.setItem(
        `pdf.position.${lastCompileRootDocIdRef.current}`,
        position
      )
    }
  }, [position])

  return [position, setPosition]
}