use-swr.ts

import { useCallback, useRef, useDebugValue } from 'react'
import { defaultConfig } from './utils/config'
import { SWRGlobalState, GlobalState } from './utils/global-state'
import {
  IS_REACT_LEGACY,
  IS_SERVER,
  rAF,
  useIsomorphicLayoutEffect
} from './utils/env'
import { serialize } from './utils/serialize'
import {
  isUndefined,
  UNDEFINED,
  OBJECT,
  mergeObjects,
  isFunction
} from './utils/helper'
import ConfigProvider from './utils/config-context'
import { useStateWithDeps } from './utils/state'
import { withArgs } from './utils/resolve-args'
import { subscribeCallback } from './utils/subscribe-key'
import { broadcastState } from './utils/broadcast-state'
import { getTimestamp } from './utils/timestamp'
import { internalMutate } from './utils/mutate'
import * as revalidateEvents from './constants'
import { createCacheHelper } from './utils/cache'

import {
  State,
  Fetcher,
  Key,
  SWRResponse,
  RevalidatorOptions,
  FullConfiguration,
  SWRConfiguration,
  SWRHook,
  StateUpdateCallback,
  RevalidateEvent
} from './types'

const WITH_DEDUPE = { dedupe: true }

type DefinitelyTruthy<T> = false extends T
  ? never
  : 0 extends T
  ? never
  : '' extends T
  ? never
  : null extends T
  ? never
  : undefined extends T
  ? never
  : T

export const useSWRHandler = <Data = any, Error = any>(
  _key: Key,
  fetcher: Fetcher<Data> | null,
  config: typeof defaultConfig & SWRConfiguration<Data, Error>
) => {
  const {
    cache,
    compare,
    suspense,
    fallbackData,
    revalidateOnMount,
    refreshInterval,
    refreshWhenHidden,
    refreshWhenOffline,
    keepPreviousData
  } = config

  const [EVENT_REVALIDATORS, STATE_UPDATERS, MUTATION, FETCH] =
    SWRGlobalState.get(cache) as GlobalState

  // `key` is the identifier of the SWR `data` state, `keyInfo` holds extra
  // states such as `error` and `isValidating` inside,
  // all of them are derived from `_key`.
  // `fnArg` is the argument/arguments parsed from the key, which will be passed
  // to the fetcher.
  const [key, fnArg] = serialize(_key)

  // If it's the initial render of this hook.
  const initialMountedRef = useRef(false)

  // If the hook is unmounted already. This will be used to prevent some effects
  // to be called after unmounting.
  const unmountedRef = useRef(false)

  // Refs to keep the key and config.
  const keyRef = useRef(key)
  const fetcherRef = useRef(fetcher)
  const configRef = useRef(config)
  const getConfig = () => configRef.current
  const isActive = () => getConfig().isVisible() && getConfig().isOnline()

  const [getCache, setCache] = createCacheHelper<Data>(cache, key)

  // Get the current state that SWR should return.
  const cached = getCache()
  const cachedData = cached.data
  const fallback = isUndefined(fallbackData)
    ? config.fallback[key]
    : fallbackData
  const data = isUndefined(cachedData) ? fallback : cachedData
  const error = cached.error

  // Use a ref to store previous returned data. Use the inital data as its inital value.
  const laggyDataRef = useRef(data)

  const isInitialMount = !initialMountedRef.current
  const returnedData = keepPreviousData
    ? isUndefined(cachedData)
      ? laggyDataRef.current
      : cachedData
    : data

  // - Suspense mode and there's stale data for the initial render.
  // - Not suspense mode and there is no fallback data and `revalidateIfStale` is enabled.
  // - `revalidateIfStale` is enabled but `data` is not defined.
  const shouldDoInitialRevalidation = (() => {
    // If `revalidateOnMount` is set, we take the value directly.
    if (isInitialMount && !isUndefined(revalidateOnMount))
      return revalidateOnMount

    // If it's paused, we skip revalidation.
    if (getConfig().isPaused()) return false

    // Under suspense mode, it will always fetch on render if there is no
    // stale data so no need to revalidate immediately on mount again.
    // If data exists, only revalidate if `revalidateIfStale` is true.
    if (suspense) return isUndefined(data) ? false : config.revalidateIfStale

    // If there is no stale data, we need to revalidate on mount;
    // If `revalidateIfStale` is set to true, we will always revalidate.
    return isUndefined(data) || config.revalidateIfStale
  })()

  // Resolve the default validating state:
  // If it's able to validate, and it should revalidate on mount, this will be true.
  const defaultValidatingState = !!(
    key &&
    fetcher &&
    isInitialMount &&
    shouldDoInitialRevalidation
  )
  const isValidating = cached.isValidating || defaultValidatingState
  const isLoading = cached.isLoading || defaultValidatingState

  const currentState = {
    data,
    error,
    isValidating,
    isLoading
  }
  const [stateRef, stateDependencies, setState] = useStateWithDeps(currentState)

  // The revalidation function is a carefully crafted wrapper of the original
  // `fetcher`, to correctly handle the many edge cases.
  const revalidate = useCallback(
    async (revalidateOpts?: RevalidatorOptions): Promise<boolean> => {
      const currentFetcher = fetcherRef.current

      if (
        !key ||
        !currentFetcher ||
        unmountedRef.current ||
        getConfig().isPaused()
      ) {
        return false
      }

      let newData: Data
      let startAt: number
      let loading = true
      const opts = revalidateOpts || {}

      // If there is no ongoing concurrent request, or `dedupe` is not set, a
      // new request should be initiated.
      const shouldStartNewRequest = !FETCH[key] || !opts.dedupe

      /* 
         For React 17
         Do unmount check for calls:
         If key has changed during the revalidation, or the component has been
         unmounted, old dispatch and old event callbacks should not take any
         effect

        For React 18
        only check if key has changed
        https://github.com/reactwg/react-18/discussions/82
      */
      const callbackSafeguard = () => {
        if (IS_REACT_LEGACY) {
          return (
            !unmountedRef.current &&
            key === keyRef.current &&
            initialMountedRef.current
          )
        }
        return key === keyRef.current
      }

      // The final state object when request finishes.
      const finalState: State<Data, Error> = {
        isValidating: false,
        isLoading: false
      }
      const finishRequestAndUpdateState = () => {
        // Set the global cache.
        setCache(finalState)

        // We can only set the local state if it's safe (still mounted with the same key).
        if (callbackSafeguard()) {
          setState(finalState)
        }
      }
      const cleanupState = () => {
        // Check if it's still the same request before deleting.
        const requestInfo = FETCH[key]
        if (requestInfo && requestInfo[1] === startAt) {
          delete FETCH[key]
        }
      }

      // Start fetching. Change the `isValidating` state, update the cache.
      const initialState: State<Data, Error> = { isValidating: true }
      // It is in the `isLoading` state, if and only if there is no cached data.
      // This bypasses fallback data and laggy data.
      if (isUndefined(getCache().data)) {
        initialState.isLoading = true
      }
      setCache(initialState)
      setState(initialState)

      try {
        if (shouldStartNewRequest) {
          // Tell all other hooks to change the `isValidating` state.
          broadcastState(cache, key, {
            ...stateRef.current,
            isValidating: true
          })

          // If no cache being rendered currently (it shows a blank page),
          // we trigger the loading slow event.
          if (config.loadingTimeout && isUndefined(getCache().data)) {
            setTimeout(() => {
              if (loading && callbackSafeguard()) {
                getConfig().onLoadingSlow(key, config)
              }
            }, config.loadingTimeout)
          }

          // Start the request and save the timestamp.
          // Key must be truthy if entering here.
          FETCH[key] = [
            currentFetcher(fnArg as DefinitelyTruthy<Key>),
            getTimestamp()
          ]
        }

        // Wait until the ongoing request is done. Deduplication is also
        // considered here.
        ;[newData, startAt] = FETCH[key]
        newData = await newData

        if (shouldStartNewRequest) {
          // If the request isn't interrupted, clean it up after the
          // deduplication interval.
          setTimeout(cleanupState, config.dedupingInterval)
        }

        // 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()) {
              getConfig().onDiscarded(key)
            }
          }
          return false
        }

        // Clear error.
        finalState.error = UNDEFINED

        // If there're other mutations(s), overlapped with the current revalidation:
        // case 1:
        //   req------------------>res
        //       mutate------>end
        // case 2:
        //         req------------>res
        //   mutate------>end
        // case 3:
        //   req------------------>res
        //       mutate-------...---------->
        // we have to ignore the revalidation result (res) because it's no longer fresh.
        // meanwhile, a new revalidation should be triggered when the mutation ends.
        const mutationInfo = MUTATION[key]
        if (
          !isUndefined(mutationInfo) &&
          // case 1
          (startAt <= mutationInfo[0] ||
            // case 2
            startAt <= mutationInfo[1] ||
            // case 3
            mutationInfo[1] === 0)
        ) {
          finishRequestAndUpdateState()
          if (shouldStartNewRequest) {
            if (callbackSafeguard()) {
              getConfig().onDiscarded(key)
            }
          }
          return false
        }

        // Deep compare with latest state to avoid extra re-renders.
        // For local state, compare and assign.
        if (!compare(stateRef.current.data, newData)) {
          finalState.data = newData
        } else {
          // `data` and `newData` are deeply equal (serialized value).
          // So it should be safe to broadcast the stale data to keep referential equality (===).
          finalState.data = stateRef.current.data
          // At the end of this function, `broadcastState` invokes the `onStateUpdate` function,
          // which takes care of avoiding the re-render.
        }

        // Trigger the successful callback if it's the original request.
        if (shouldStartNewRequest) {
          if (callbackSafeguard()) {
            getConfig().onSuccess(newData, key, config)
          }
        }
      } catch (err) {
        cleanupState()

        const currentConfig = getConfig()
        const { shouldRetryOnError } = currentConfig

        // Not paused, we continue handling the error. Otherwise discard it.
        if (!currentConfig.isPaused()) {
          // Get a new error, don't use deep comparison for errors.
          finalState.error = err as Error

          // Error event and retry logic. Only for the actual request, not
          // deduped ones.
          if (shouldStartNewRequest && callbackSafeguard()) {
            currentConfig.onError(err, key, currentConfig)
            if (
              shouldRetryOnError === true ||
              (isFunction(shouldRetryOnError) &&
                shouldRetryOnError(err as Error))
            ) {
              if (isActive()) {
                // If it's inactive, stop. It will auto revalidate when
                // refocusing or reconnecting.
                // When retrying, deduplication is always enabled.
                currentConfig.onErrorRetry(
                  err,
                  key,
                  currentConfig,
                  revalidate,
                  {
                    retryCount: (opts.retryCount || 0) + 1,
                    dedupe: true
                  }
                )
              }
            }
          }
        }
      }

      // Mark loading as stopped.
      loading = false

      // Update the current hook's state.
      finishRequestAndUpdateState()

      // Here is the source of the request, need to tell all other hooks to
      // update their states.
      if (callbackSafeguard() && shouldStartNewRequest) {
        broadcastState(cache, key, finalState)
      }

      return true
    },
    // `setState` is immutable, and `eventsCallback`, `fnArg`, and
    // `keyValidating` are depending on `key`, so we can exclude them from
    // the deps array.
    //
    // FIXME:
    // `fn` and `config` might be changed during the lifecycle,
    // but they might be changed every render like this.
    // `useSWR('key', () => fetch('/api/'), { suspense: true })`
    // So we omit the values from the deps array
    // even though it might cause unexpected behaviors.
    // eslint-disable-next-line react-hooks/exhaustive-deps
    [key]
  )

  // Similar to the global mutate, but bound to the current cache and key.
  // `cache` isn't allowed to change during the lifecycle.
  // eslint-disable-next-line react-hooks/exhaustive-deps
  const boundMutate: SWRResponse<Data, Error>['mutate'] = useCallback(
    // By using `bind` we don't need to modify the size of the rest arguments.
    // Due to https://github.com/microsoft/TypeScript/issues/37181, we have to
    // cast it to any for now.
    internalMutate.bind(UNDEFINED, cache, () => keyRef.current) as any,
    // eslint-disable-next-line react-hooks/exhaustive-deps
    []
  )

  // Logic for updating refs.
  useIsomorphicLayoutEffect(() => {
    fetcherRef.current = fetcher
    configRef.current = config
    stateRef.current = currentState

    // Handle laggy data updates. If there's cached data of the current key,
    // it'll be the correct reference.
    if (!isUndefined(cachedData)) {
      laggyDataRef.current = cachedData
    }
  })

  // After mounted or key changed.
  useIsomorphicLayoutEffect(() => {
    if (!key) return

    const softRevalidate = revalidate.bind(UNDEFINED, WITH_DEDUPE)

    // Expose state updater to global event listeners. So we can update hook's
    // internal state from the outside.
    const onStateUpdate: StateUpdateCallback<Data, Error> = (state = {}) => {
      setState(
        mergeObjects(
          {
            error: state.error,
            isValidating: state.isValidating
          },
          // Since `setState` only shallowly compares states, we do a deep
          // comparison here.
          compare(stateRef.current.data, state.data)
            ? UNDEFINED
            : {
                data: state.data
              }
        )
      )
    }

    // Expose revalidators to global event listeners. So we can trigger
    // revalidation from the outside.
    let nextFocusRevalidatedAt = 0
    const onRevalidate = (type: RevalidateEvent) => {
      if (type == revalidateEvents.FOCUS_EVENT) {
        const now = Date.now()
        if (
          getConfig().revalidateOnFocus &&
          now > nextFocusRevalidatedAt &&
          isActive()
        ) {
          nextFocusRevalidatedAt = now + getConfig().focusThrottleInterval
          softRevalidate()
        }
      } else if (type == revalidateEvents.RECONNECT_EVENT) {
        if (getConfig().revalidateOnReconnect && isActive()) {
          softRevalidate()
        }
      } else if (type == revalidateEvents.MUTATE_EVENT) {
        return revalidate()
      }
      return
    }

    const unsubUpdate = subscribeCallback(key, STATE_UPDATERS, onStateUpdate)
    const unsubEvents = subscribeCallback(key, EVENT_REVALIDATORS, onRevalidate)

    // Mark the component as mounted and update corresponding refs.
    unmountedRef.current = false
    keyRef.current = key
    initialMountedRef.current = true

    // Trigger a revalidation.
    if (shouldDoInitialRevalidation) {
      if (isUndefined(data) || IS_SERVER) {
        // Revalidate immediately.
        softRevalidate()
      } else {
        // Delay the revalidate if we have data to return so we won't block
        // rendering.
        rAF(softRevalidate)
      }
    }

    return () => {
      // Mark it as unmounted.
      unmountedRef.current = true

      unsubUpdate()
      unsubEvents()
    }
  }, [key])

  // Polling
  useIsomorphicLayoutEffect(() => {
    let timer: any

    function next() {
      // Use the passed interval
      // ...or invoke the function with the updated data to get the interval
      const interval = isFunction(refreshInterval)
        ? refreshInterval(data)
        : refreshInterval

      // We only start next interval if `refreshInterval` is not 0, and:
      // - `force` is true, which is the start of polling
      // - or `timer` is not 0, which means the effect wasn't canceled
      if (interval && timer !== -1) {
        timer = setTimeout(execute, interval)
      }
    }

    function execute() {
      // Check if it's OK to execute:
      // Only revalidate when the page is visible, online and not errored.
      if (
        !stateRef.current.error &&
        (refreshWhenHidden || getConfig().isVisible()) &&
        (refreshWhenOffline || getConfig().isOnline())
      ) {
        revalidate(WITH_DEDUPE).then(next)
      } else {
        // Schedule next interval to check again.
        next()
      }
    }

    next()

    return () => {
      if (timer) {
        clearTimeout(timer)
        timer = -1
      }
    }
  }, [refreshInterval, refreshWhenHidden, refreshWhenOffline, key])

  // Display debug info in React DevTools.
  useDebugValue(returnedData)

  // In Suspense mode, we can't return the empty `data` state.
  // If there is `error`, the `error` needs to be thrown to the error boundary.
  // If there is no `error`, the `revalidation` promise needs to be thrown to
  // the suspense boundary.
  if (suspense && isUndefined(data) && key) {
    // SWR should throw when trying to use Suspense on the server with React 18,
    // without providing any initial data. See:
    // https://github.com/vercel/swr/issues/1832
    if (!IS_REACT_LEGACY && IS_SERVER) {
      throw new Error('Fallback data is required when using suspense in SSR.')
    }

    // Always update fetcher and config refs even with the Suspense mode.
    fetcherRef.current = fetcher
    configRef.current = config
    unmountedRef.current = false
    throw isUndefined(error) ? revalidate(WITH_DEDUPE) : error
  }

  return {
    mutate: boundMutate,
    get data() {
      stateDependencies.data = true
      return returnedData
    },
    get error() {
      stateDependencies.error = true
      return error
    },
    get isValidating() {
      stateDependencies.isValidating = true
      return isValidating
    },
    get isLoading() {
      stateDependencies.isLoading = true
      return isLoading
    }
  } as SWRResponse<Data, Error>
}

export const SWRConfig = OBJECT.defineProperty(ConfigProvider, 'default', {
  value: defaultConfig
}) as typeof ConfigProvider & {
  default: FullConfiguration
}

export const unstable_serialize = (key: Key) => serialize(key)[0]

export default withArgs<SWRHook>(useSWRHandler)