lazyMap.ts

import { addItemMutable } from "./collectionMethods"
import { AnyFn } from "./typings"

const invokedRecord = new Map<string, (LazyMapSettings<any, any> & { idleId: number })[]>()

type LazyMapSettings<T, U> = {
  source: T[]
  sourceKey: string
  loopFn: (item: T, index: number, source: T[]) => U
  onListChange: (list: U[]) => void
}

/**
 * like Array's map(), but each loop will check if new task is pushed in todo queue
 * inspired by `window.requestIdleCallback()`
 * @param settings.source arr
 * @param settings.sourceKey flag for todo queue
 * @param settings.loopFn like js: array::map
 */
export function lazyMap<T, U>(setting: LazyMapSettings<T, U>) {
  const idleId = requestIdleCallback(() => {
    // console.time(`lazy map (${setting.sourceKey})`)
    const result = setting.source.map(setting.loopFn)
    setting.onListChange(result)
    // console.timeEnd(`lazy map (${setting.sourceKey})`)
  })

  // cancel the last idle callback, and record new setting
  const currentKeySettings = invokedRecord.get(setting.sourceKey) ?? []
  const lastIdleId = currentKeySettings[currentKeySettings.length - 1]?.idleId
  if (lastIdleId) cancelIdleCallback(lastIdleId)
  invokedRecord.set(setting.sourceKey, addItemMutable(invokedRecord.get(setting.sourceKey) ?? [], { ...setting, idleId }))
}

function requestIdleCallback(fn: AnyFn): number {
  return globalThis.requestIdleCallback?.(fn) ?? globalThis.setTimeout?.(fn) // Safari no't support `window.requestIdleCallback()`, so have to check first
}

function cancelIdleCallback(handleId: number): void {
  // @ts-ignore
  globalThis.cancelIdleCallback?.(handleId) ?? globalThis.clearTimeout?.(handleId)
}