import * as Solid from 'solid-js'

/**
 * Observe a Link's element while it preloads on `'viewport'`: `callback`
 * receives each intersection entry. Whenever the element or preload mode
 * changes, and on cleanup, a preloading Link gets `callback()` so it can
 * cancel a pending preload.
 */
export function useIntersectionObserver<T extends Element>(
  ref: Solid.Accessor<T | null>,
  callback: (entry?: IntersectionObserverEntry) => void,
  preload: Solid.Accessor<unknown>,
): void {
  Solid.createEffect(() => {
    const r = ref()
    const mode = preload()
    if (
      mode === 'viewport' &&
      r &&
      typeof IntersectionObserver === 'function'
    ) {
      let active = true
      const observer = new IntersectionObserver(
        (entries) => {
          // Queued notifications can arrive after this effect has cleaned up.
          if (active) {
            callback(entries.pop())
          }
        },
        { rootMargin: '100px' },
      )
      observer.observe(r)
      Solid.onCleanup(() => {
        active = false
        observer.disconnect()
        callback()
      })
    } else if (mode) {
      // Intent preloading still needs timer cleanup without an observer.
      Solid.onCleanup(() => callback())
    }
  })
}