사용했던 오픈소스 읽고 이해하기 - SWR

if (constructor == Array) {
      // Array.
      result = '@'
      for (index = 0; index < arg.length; index++) {
        result += stableHash(arg[index]) + ','
      }
      table.set(arg, result)
    }
    if (constructor == OBJECT) {
      // Object, sort keys.
      result = '#'
      const keys = OBJECT.keys(arg).sort()
      while (!isUndefined((index = keys.pop() as string))) {
        if (!isUndefined(arg[index])) {
          result += index + ':' + stableHash(arg[index]) + ','
        }
      }
      table.set(arg, result)
    }

const keyRef = useRef(key)
const fetcherRef = useRef(fetcher)
const configRef = useRef(config)

const onErrorRetry = (
  _: unknown,
  __: string,
  config: Readonly<PublicConfiguration>,
  revalidate: Revalidator,
  opts: Required<RevalidatorOptions>
): void => {
  const maxRetryCount = config.errorRetryCount
  const currentRetryCount = opts.retryCount

  // Exponential backoff
  const timeout =
    ~~( //소숫점 이하가 버려짐
      (Math.random() + 0.5) * //난수 생성
      (1 << (currentRetryCount < 8 ? currentRetryCount : 8))
			// exponential backoff, 최대 8회까지만
    ) * config.errorRetryInterval //사용자가 설정한 interval

  if (!isUndefined(maxRetryCount) && currentRetryCount > maxRetryCount) {
    return
  }

  setTimeout(revalidate, timeout, opts)
}

function Profile() {
  const { trigger, data, error } = useSWRMutation('/api/user', fetcher, options)
 
	//trigger 안의 인수는 options 안의 객체로 전달됨
	//렌더링 결과로 mutation의 결과를 사용할 수도, 사용하지 않을 수도 있음
  return <button onClick={() => {
    trigger('data')
  }}>Update User</button>
}

//   req1------------------>res1        
//        req2---------------->res2

//   req1-------------------------->res1    (useSWR: get)
//        req2---------------->res2         (mutation: post, ...etc)

//use-swr.ts

// If there're other ongoing request(s), started after the current one,
// we need to ignore the current one to avoid possible race conditions:
//   req1------------------>res1        (current one)
//        req2---------------->res2
// the request that fired later will always be kept.
// The timestamp maybe be undefined or a number

if (!FETCH[key] || FETCH[key][1] !== startAt) {
		if (shouldStartNewRequest) {
			if (callbackSafeguard()) { //key가 변경되었는지와, 최초 렌더링인지의 여부
				getConfig().onDiscarded(key)
			}
		}
	return false
}