第九章状态话中通过自定义状态制作页眉能否更详细一点？ #42

#let first-heading = state("first-heading", ())
#let last-heading = state("last-heading", ())
#show heading.where(level: 2): curr-heading => {
  curr-heading //返回值不变
  context first-heading.update(headings => {
    //字典没有这个键才插入一次，有的话就不插入了，所以得到第一个
    let page = str(here().page())
    if page not in headings {
      headings.insert(page, curr-heading.body)
    }
    headings //返回更新值
  })
  context last-heading.update(headings => {
    //字典特性，比如在第三页持续覆盖旧值
    headings.insert(str(here().page()), curr-heading.body)
    headings
  })
}
#let get-heading-at-page() = context {
  let find-headings(headings, page) = if page > 0 {
    headings.at(str(page), default: find-headings(headings, page - 1))
  }
  let first-headings = first-heading.final(here())
  let last-headings = last-heading.get()
  let page = here().page()
  first-headings.at(str(page), default: find-headings(last-headings, page))
}
#set page(header: emph(get-heading-at-page()))