ReactFiberHooks.js

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

import type {ReactContext} from 'shared/ReactTypes';
import type {SideEffectTag} from 'shared/ReactSideEffectTags';
import type {Fiber} from './ReactFiber';
import type {ExpirationTime} from './ReactFiberExpirationTime';
import type {HookEffectTag} from './ReactHookEffectTags';

import ReactSharedInternals from 'shared/ReactSharedInternals';

import {NoWork} from './ReactFiberExpirationTime';
import {readContext} from './ReactFiberNewContext';
import {
  Update as UpdateEffect,
  Passive as PassiveEffect,
} from 'shared/ReactSideEffectTags';
import {
  NoEffect as NoHookEffect,
  UnmountMutation,
  MountLayout,
  UnmountPassive,
  MountPassive,
} from './ReactHookEffectTags';
import {
  scheduleWork,
  warnIfNotCurrentlyBatchingInDev,
  computeExpirationForFiber,
  flushPassiveEffects,
  requestCurrentTime,
} from './ReactFiberScheduler';

import invariant from 'shared/invariant';
import warning from 'shared/warning';
import getComponentName from 'shared/getComponentName';
import is from 'shared/objectIs';
import {markWorkInProgressReceivedUpdate} from './ReactFiberBeginWork';

const {ReactCurrentDispatcher} = ReactSharedInternals;

export type Dispatcher = {
  readContext<T>(
    context: ReactContext<T>,
    observedBits: void | number | boolean,
  ): T,
  useState<S>(initialState: (() => S) | S): [S, Dispatch<BasicStateAction<S>>],
  useReducer<S, I, A>(
    reducer: (S, A) => S,
    initialArg: I,
    init?: (I) => S,
  ): [S, Dispatch<A>],
  useContext<T>(
    context: ReactContext<T>,
    observedBits: void | number | boolean,
  ): T,
  useRef<T>(initialValue: T): {current: T},
  useEffect(
    create: () => (() => void) | void,
    deps: Array<mixed> | void | null,
  ): void,
  useLayoutEffect(
    create: () => (() => void) | void,
    deps: Array<mixed> | void | null,
  ): void,
  useCallback<T>(callback: T, deps: Array<mixed> | void | null): T,
  useMemo<T>(nextCreate: () => T, deps: Array<mixed> | void | null): T,
  useImperativeHandle<T>(
    ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
    create: () => T,
    deps: Array<mixed> | void | null,
  ): void,
  useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void,
};

type Update<S, A> = {
  expirationTime: ExpirationTime,
  action: A,
  eagerReducer: ((S, A) => S) | null,
  eagerState: S | null,
  next: Update<S, A> | null,
};

type UpdateQueue<S, A> = {
  last: Update<S, A> | null,
  dispatch: (A => mixed) | null,
  eagerReducer: ((S, A) => S) | null,
  eagerState: S | null,
};

type HookType =
  | 'useState'
  | 'useReducer'
  | 'useContext'
  | 'useRef'
  | 'useEffect'
  | 'useLayoutEffect'
  | 'useCallback'
  | 'useMemo'
  | 'useImperativeHandle'
  | 'useDebugValue';

let didWarnAboutMismatchedHooksForComponent;
if (__DEV__) {
  didWarnAboutMismatchedHooksForComponent = new Set();
}

export type Hook = {
  memoizedState: any,

  baseState: any,
  baseUpdate: Update<any, any> | null,
  queue: UpdateQueue<any, any> | null,

  next: Hook | null,
};

type HookDev = Hook & {
  _debugType: HookType,
};

type Effect = {
  tag: HookEffectTag,
  create: () => (() => void) | void,
  destroy: (() => void) | void,
  deps: Array<mixed> | null,
  next: Effect,
};

export type FunctionComponentUpdateQueue = {
  lastEffect: Effect | null,
};

type BasicStateAction<S> = (S => S) | S;

type Dispatch<A> = A => void;

// These are set right before calling the component.
let renderExpirationTime: ExpirationTime = NoWork;
// The work-in-progress fiber. I've named it differently to distinguish it from
// the work-in-progress hook.
let currentlyRenderingFiber: Fiber | null = null;

// Hooks are stored as a linked list on the fiber's memoizedState field. The
// current hook list is the list that belongs to the current fiber. The
// work-in-progress hook list is a new list that will be added to the
// work-in-progress fiber.
let firstCurrentHook: Hook | null = null;
let currentHook: Hook | null = null;
let nextCurrentHook: Hook | null = null;
let firstWorkInProgressHook: Hook | null = null;
let workInProgressHook: Hook | null = null;
let nextWorkInProgressHook: Hook | null = null;

let remainingExpirationTime: ExpirationTime = NoWork;
let componentUpdateQueue: FunctionComponentUpdateQueue | null = null;
let sideEffectTag: SideEffectTag = 0;

// Updates scheduled during render will trigger an immediate re-render at the
// end of the current pass. We can't store these updates on the normal queue,
// because if the work is aborted, they should be discarded. Because this is
// a relatively rare case, we also don't want to add an additional field to
// either the hook or queue object types. So we store them in a lazily create
// map of queue -> render-phase updates, which are discarded once the component
// completes without re-rendering.

// Whether an update was scheduled during the currently executing render pass.
let didScheduleRenderPhaseUpdate: boolean = false;
// Lazily created map of render-phase updates
let renderPhaseUpdates: Map<
  UpdateQueue<any, any>,
  Update<any, any>,
> | null = null;
// Counter to prevent infinite loops.
let numberOfReRenders: number = 0;
const RE_RENDER_LIMIT = 25;

// In DEV, this is the name of the currently executing primitive hook
let currentHookNameInDev: ?HookType = null;

function warnOnHookMismatchInDev() {
  if (__DEV__) {
    const componentName = getComponentName(
      ((currentlyRenderingFiber: any): Fiber).type,
    );
    if (!didWarnAboutMismatchedHooksForComponent.has(componentName)) {
      didWarnAboutMismatchedHooksForComponent.add(componentName);

      const secondColumnStart = 22;

      let table = '';
      let prevHook: HookDev | null = (firstCurrentHook: any);
      let nextHook: HookDev | null = (firstWorkInProgressHook: any);
      let n = 1;
      while (prevHook !== null && nextHook !== null) {
        const oldHookName = prevHook._debugType;
        const newHookName = nextHook._debugType;

        let row = `${n}. ${oldHookName}`;

        // Extra space so second column lines up
        // lol @ IE not supporting String#repeat
        while (row.length < secondColumnStart) {
          row += ' ';
        }

        row += newHookName + '\n';

        table += row;
        prevHook = (prevHook.next: any);
        nextHook = (nextHook.next: any);
        n++;
      }

      warning(
        false,
        'React has detected a change in the order of Hooks called by %s. ' +
          'This will lead to bugs and errors if not fixed. ' +
          'For more information, read the Rules of Hooks: https://fb.me/rules-of-hooks\n\n' +
          '   Previous render    Next render\n' +
          '   -------------------------------\n' +
          '%s' +
          '   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n',
        componentName,
        table,
      );
    }
  }
}

function throwInvalidHookError() {
  invariant(
    false,
    'Hooks can only be called inside the body of a function component. ' +
      '(https://fb.me/react-invalid-hook-call)',
  );
}

function areHookInputsEqual(
  nextDeps: Array<mixed>,
  prevDeps: Array<mixed> | null,
) {
  if (prevDeps === null) {
    if (__DEV__) {
      warning(
        false,
        '%s received a final argument during this render, but not during ' +
          'the previous render. Even though the final argument is optional, ' +
          'its type cannot change between renders.',
        currentHookNameInDev,
      );
    }
    return false;
  }

  if (__DEV__) {
    // Don't bother comparing lengths in prod because these arrays should be
    // passed inline.
    if (nextDeps.length !== prevDeps.length) {
      warning(
        false,
        'The final argument passed to %s changed size between renders. The ' +
          'order and size of this array must remain constant.\n\n' +
          'Previous: %s\n' +
          'Incoming: %s',
        currentHookNameInDev,
        `[${nextDeps.join(', ')}]`,
        `[${prevDeps.join(', ')}]`,
      );
    }
  }
  for (let i = 0; i < prevDeps.length && i < nextDeps.length; i++) {
    if (is(nextDeps[i], prevDeps[i])) {
      continue;
    }
    return false;
  }
  return true;
}

export function renderWithHooks(
  current: Fiber | null,
  workInProgress: Fiber,
  Component: any,
  props: any,
  refOrContext: any,
  nextRenderExpirationTime: ExpirationTime,
): any {
  renderExpirationTime = nextRenderExpirationTime;
  currentlyRenderingFiber = workInProgress;
  firstCurrentHook = nextCurrentHook =
    current !== null ? current.memoizedState : null;

  // The following should have already been reset
  // currentHook = null;
  // workInProgressHook = null;

  // remainingExpirationTime = NoWork;
  // componentUpdateQueue = null;

  // didScheduleRenderPhaseUpdate = false;
  // renderPhaseUpdates = null;
  // numberOfReRenders = 0;
  // sideEffectTag = 0;

  if (__DEV__) {
    ReactCurrentDispatcher.current =
      nextCurrentHook === null
        ? HooksDispatcherOnMountInDEV
        : HooksDispatcherOnUpdateInDEV;
  } else {
    ReactCurrentDispatcher.current =
      nextCurrentHook === null
        ? HooksDispatcherOnMount
        : HooksDispatcherOnUpdate;
  }

  let children = Component(props, refOrContext);

  if (didScheduleRenderPhaseUpdate) {
    do {
      didScheduleRenderPhaseUpdate = false;
      numberOfReRenders += 1;

      // Start over from the beginning of the list
      firstCurrentHook = nextCurrentHook =
        current !== null ? current.memoizedState : null;
      nextWorkInProgressHook = firstWorkInProgressHook;

      currentHook = null;
      workInProgressHook = null;
      componentUpdateQueue = null;

      ReactCurrentDispatcher.current = __DEV__
        ? HooksDispatcherOnUpdateInDEV
        : HooksDispatcherOnUpdate;

      children = Component(props, refOrContext);
    } while (didScheduleRenderPhaseUpdate);

    renderPhaseUpdates = null;
    numberOfReRenders = 0;
  }

  if (__DEV__) {
    currentHookNameInDev = null;
  }

  // We can assume the previous dispatcher is always this one, since we set it
  // at the beginning of the render phase and there's no re-entrancy.
  ReactCurrentDispatcher.current = ContextOnlyDispatcher;

  const renderedWork: Fiber = (currentlyRenderingFiber: any);

  renderedWork.memoizedState = firstWorkInProgressHook;
  renderedWork.expirationTime = remainingExpirationTime;
  renderedWork.updateQueue = (componentUpdateQueue: any);
  renderedWork.effectTag |= sideEffectTag;

  const didRenderTooFewHooks =
    currentHook !== null && currentHook.next !== null;

  renderExpirationTime = NoWork;
  currentlyRenderingFiber = null;

  firstCurrentHook = null;
  currentHook = null;
  nextCurrentHook = null;
  firstWorkInProgressHook = null;
  workInProgressHook = null;
  nextWorkInProgressHook = null;

  remainingExpirationTime = NoWork;
  componentUpdateQueue = null;
  sideEffectTag = 0;

  // These were reset above
  // didScheduleRenderPhaseUpdate = false;
  // renderPhaseUpdates = null;
  // numberOfReRenders = 0;

  invariant(
    !didRenderTooFewHooks,
    'Rendered fewer hooks than expected. This may be caused by an accidental ' +
      'early return statement.',
  );

  return children;
}

export function bailoutHooks(
  current: Fiber,
  workInProgress: Fiber,
  expirationTime: ExpirationTime,
) {
  workInProgress.updateQueue = current.updateQueue;
  workInProgress.effectTag &= ~(PassiveEffect | UpdateEffect);
  if (current.expirationTime <= expirationTime) {
    current.expirationTime = NoWork;
  }
}

export function resetHooks(): void {
  // We can assume the previous dispatcher is always this one, since we set it
  // at the beginning of the render phase and there's no re-entrancy.
  ReactCurrentDispatcher.current = ContextOnlyDispatcher;

  // This is used to reset the state of this module when a component throws.
  // It's also called inside mountIndeterminateComponent if we determine the
  // component is a module-style component.
  renderExpirationTime = NoWork;
  currentlyRenderingFiber = null;

  firstCurrentHook = null;
  currentHook = null;
  nextCurrentHook = null;
  firstWorkInProgressHook = null;
  workInProgressHook = null;
  nextWorkInProgressHook = null;

  remainingExpirationTime = NoWork;
  componentUpdateQueue = null;
  sideEffectTag = 0;

  if (__DEV__) {
    currentHookNameInDev = null;
  }

  didScheduleRenderPhaseUpdate = false;
  renderPhaseUpdates = null;
  numberOfReRenders = 0;
}

function mountWorkInProgressHook(): Hook {
  const hook: Hook = {
    memoizedState: null,

    baseState: null,
    queue: null,
    baseUpdate: null,

    next: null,
  };

  if (__DEV__) {
    (hook: any)._debugType = (currentHookNameInDev: any);
  }
  if (workInProgressHook === null) {
    // This is the first hook in the list
    firstWorkInProgressHook = workInProgressHook = hook;
  } else {
    // Append to the end of the list
    workInProgressHook = workInProgressHook.next = hook;
  }
  return workInProgressHook;
}

function updateWorkInProgressHook(): Hook {
  // This function is used both for updates and for re-renders triggered by a
  // render phase update. It assumes there is either a current hook we can
  // clone, or a work-in-progress hook from a previous render pass that we can
  // use as a base. When we reach the end of the base list, we must switch to
  // the dispatcher used for mounts.
  if (nextWorkInProgressHook !== null) {
    // There's already a work-in-progress. Reuse it.
    workInProgressHook = nextWorkInProgressHook;
    nextWorkInProgressHook = workInProgressHook.next;

    currentHook = nextCurrentHook;
    nextCurrentHook = currentHook !== null ? currentHook.next : null;
  } else {
    // Clone from the current hook.
    invariant(
      nextCurrentHook !== null,
      'Rendered more hooks than during the previous render.',
    );
    currentHook = nextCurrentHook;

    const newHook: Hook = {
      memoizedState: currentHook.memoizedState,

      baseState: currentHook.baseState,
      queue: currentHook.queue,
      baseUpdate: currentHook.baseUpdate,

      next: null,
    };

    if (workInProgressHook === null) {
      // This is the first hook in the list.
      workInProgressHook = firstWorkInProgressHook = newHook;
    } else {
      // Append to the end of the list.
      workInProgressHook = workInProgressHook.next = newHook;
    }
    nextCurrentHook = currentHook.next;

    if (__DEV__) {
      (newHook: any)._debugType = (currentHookNameInDev: any);
      if (currentHookNameInDev !== ((currentHook: any): HookDev)._debugType) {
        warnOnHookMismatchInDev();
      }
    }
  }
  return workInProgressHook;
}

function createFunctionComponentUpdateQueue(): FunctionComponentUpdateQueue {
  return {
    lastEffect: null,
  };
}

function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  return typeof action === 'function' ? action(state) : action;
}

function mountContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
  mountWorkInProgressHook();
  return readContext(context, observedBits);
}

function updateContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
  updateWorkInProgressHook();
  return readContext(context, observedBits);
}

function mountReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = mountWorkInProgressHook();
  let initialState;
  if (init !== undefined) {
    initialState = init(initialArg);
  } else {
    initialState = ((initialArg: any): S);
  }
  hook.memoizedState = hook.baseState = initialState;
  const queue = (hook.queue = {
    last: null,
    dispatch: null,
    eagerReducer: reducer,
    eagerState: (initialState: any),
  });
  const dispatch: Dispatch<A> = (queue.dispatch = (dispatchAction.bind(
    null,
    // Flow doesn't know this is non-null, but we do.
    ((currentlyRenderingFiber: any): Fiber),
    queue,
  ): any));
  return [hook.memoizedState, dispatch];
}

function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  invariant(
    queue !== null,
    'Should have a queue. This is likely a bug in React. Please file an issue.',
  );

  if (numberOfReRenders > 0) {
    // This is a re-render. Apply the new render phase updates to the previous
    // work-in-progress hook.
    const dispatch: Dispatch<A> = (queue.dispatch: any);
    if (renderPhaseUpdates !== null) {
      // Render phase updates are stored in a map of queue -> linked list
      const firstRenderPhaseUpdate = renderPhaseUpdates.get(queue);
      if (firstRenderPhaseUpdate !== undefined) {
        renderPhaseUpdates.delete(queue);
        let newState = hook.memoizedState;
        let update = firstRenderPhaseUpdate;
        do {
          // Process this render phase update. We don't have to check the
          // priority because it will always be the same as the current
          // render's.
          const action = update.action;
          newState = reducer(newState, action);
          update = update.next;
        } while (update !== null);

        // Mark that the fiber performed work, but only if the new state is
        // different from the current state.
        if (!is(newState, hook.memoizedState)) {
          markWorkInProgressReceivedUpdate();
        }

        hook.memoizedState = newState;
        // Don't persist the state accumlated from the render phase updates to
        // the base state unless the queue is empty.
        // TODO: Not sure if this is the desired semantics, but it's what we
        // do for gDSFP. I can't remember why.
        if (hook.baseUpdate === queue.last) {
          hook.baseState = newState;
        }

        queue.eagerReducer = reducer;
        queue.eagerState = newState;

        return [newState, dispatch];
      }
    }
    return [hook.memoizedState, dispatch];
  }

  // The last update in the entire queue
  const last = queue.last;
  // The last update that is part of the base state.
  const baseUpdate = hook.baseUpdate;
  const baseState = hook.baseState;

  // Find the first unprocessed update.
  let first;
  if (baseUpdate !== null) {
    if (last !== null) {
      // For the first update, the queue is a circular linked list where
      // `queue.last.next = queue.first`. Once the first update commits, and
      // the `baseUpdate` is no longer empty, we can unravel the list.
      last.next = null;
    }
    first = baseUpdate.next;
  } else {
    first = last !== null ? last.next : null;
  }
  if (first !== null) {
    let newState = baseState;
    let newBaseState = null;
    let newBaseUpdate = null;
    let prevUpdate = baseUpdate;
    let update = first;
    let didSkip = false;
    do {
      const updateExpirationTime = update.expirationTime;
      if (updateExpirationTime < renderExpirationTime) {
        // Priority is insufficient. Skip this update. If this is the first
        // skipped update, the previous update/state is the new base
        // update/state.
        if (!didSkip) {
          didSkip = true;
          newBaseUpdate = prevUpdate;
          newBaseState = newState;
        }
        // Update the remaining priority in the queue.
        if (updateExpirationTime > remainingExpirationTime) {
          remainingExpirationTime = updateExpirationTime;
        }
      } else {
        // Process this update.
        if (update.eagerReducer === reducer) {
          // If this update was processed eagerly, and its reducer matches the
          // current reducer, we can use the eagerly computed state.
          newState = ((update.eagerState: any): S);
        } else {
          const action = update.action;
          newState = reducer(newState, action);
        }
      }
      prevUpdate = update;
      update = update.next;
    } while (update !== null && update !== first);

    if (!didSkip) {
      newBaseUpdate = prevUpdate;
      newBaseState = newState;
    }

    // Mark that the fiber performed work, but only if the new state is
    // different from the current state.
    if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }

    hook.memoizedState = newState;
    hook.baseUpdate = newBaseUpdate;
    hook.baseState = newBaseState;

    queue.eagerReducer = reducer;
    queue.eagerState = newState;
  }

  const dispatch: Dispatch<A> = (queue.dispatch: any);
  return [hook.memoizedState, dispatch];
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  const hook = mountWorkInProgressHook();
  if (typeof initialState === 'function') {
    initialState = initialState();
  }
  hook.memoizedState = hook.baseState = initialState;
  const queue = (hook.queue = {
    last: null,
    dispatch: null,
    eagerReducer: basicStateReducer,
    eagerState: (initialState: any),
  });
  const dispatch: Dispatch<
    BasicStateAction<S>,
  > = (queue.dispatch = (dispatchAction.bind(
    null,
    // Flow doesn't know this is non-null, but we do.
    ((currentlyRenderingFiber: any): Fiber),
    queue,
  ): any));
  return [hook.memoizedState, dispatch];
}

function updateState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return updateReducer(basicStateReducer, (initialState: any));
}

function pushEffect(tag, create, destroy, deps) {
  const effect: Effect = {
    tag,
    create,
    destroy,
    deps,
    // Circular
    next: (null: any),
  };
  if (componentUpdateQueue === null) {
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    componentUpdateQueue.lastEffect = effect.next = effect;
  } else {
    const lastEffect = componentUpdateQueue.lastEffect;
    if (lastEffect === null) {
      componentUpdateQueue.lastEffect = effect.next = effect;
    } else {
      const firstEffect = lastEffect.next;
      lastEffect.next = effect;
      effect.next = firstEffect;
      componentUpdateQueue.lastEffect = effect;
    }
  }
  return effect;
}

function mountRef<T>(initialValue: T): {current: T} {
  const hook = mountWorkInProgressHook();
  const ref = {current: initialValue};
  if (__DEV__) {
    Object.seal(ref);
  }
  hook.memoizedState = ref;
  return ref;
}

function updateRef<T>(initialValue: T): {current: T} {
  const hook = updateWorkInProgressHook();
  return hook.memoizedState;
}

function mountEffectImpl(fiberEffectTag, hookEffectTag, create, deps): void {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  sideEffectTag |= fiberEffectTag;
  hook.memoizedState = pushEffect(hookEffectTag, create, undefined, nextDeps);
}

function updateEffectImpl(fiberEffectTag, hookEffectTag, create, deps): void {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  let destroy = undefined;

  if (currentHook !== null) {
    const prevEffect = currentHook.memoizedState;
    destroy = prevEffect.destroy;
    if (nextDeps !== null) {
      const prevDeps = prevEffect.deps;
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        pushEffect(NoHookEffect, create, destroy, nextDeps);
        return;
      }
    }
  }

  sideEffectTag |= fiberEffectTag;
  hook.memoizedState = pushEffect(hookEffectTag, create, destroy, nextDeps);
}

function mountEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return mountEffectImpl(
    UpdateEffect | PassiveEffect,
    UnmountPassive | MountPassive,
    create,
    deps,
  );
}

function updateEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return updateEffectImpl(
    UpdateEffect | PassiveEffect,
    UnmountPassive | MountPassive,
    create,
    deps,
  );
}

function mountLayoutEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return mountEffectImpl(
    UpdateEffect,
    UnmountMutation | MountLayout,
    create,
    deps,
  );
}

function updateLayoutEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return updateEffectImpl(
    UpdateEffect,
    UnmountMutation | MountLayout,
    create,
    deps,
  );
}

function imperativeHandleEffect<T>(
  create: () => T,
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
) {
  if (typeof ref === 'function') {
    const refCallback = ref;
    const inst = create();
    refCallback(inst);
    return () => {
      refCallback(null);
    };
  } else if (ref !== null && ref !== undefined) {
    const refObject = ref;
    if (__DEV__) {
      warning(
        refObject.hasOwnProperty('current'),
        'Expected useImperativeHandle() first argument to either be a ' +
          'ref callback or React.createRef() object. Instead received: %s.',
        'an object with keys {' + Object.keys(refObject).join(', ') + '}',
      );
    }
    const inst = create();
    refObject.current = inst;
    return () => {
      refObject.current = null;
    };
  }
}

function mountImperativeHandle<T>(
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
  create: () => T,
  deps: Array<mixed> | void | null,
): void {
  if (__DEV__) {
    warning(
      typeof create === 'function',
      'Expected useImperativeHandle() second argument to be a function ' +
        'that creates a handle. Instead received: %s.',
      create !== null ? typeof create : 'null',
    );
  }

  // TODO: If deps are provided, should we skip comparing the ref itself?
  const effectDeps =
    deps !== null && deps !== undefined ? deps.concat([ref]) : null;

  return mountEffectImpl(
    UpdateEffect,
    UnmountMutation | MountLayout,
    imperativeHandleEffect.bind(null, create, ref),
    effectDeps,
  );
}

function updateImperativeHandle<T>(
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
  create: () => T,
  deps: Array<mixed> | void | null,
): void {
  if (__DEV__) {
    warning(
      typeof create === 'function',
      'Expected useImperativeHandle() second argument to be a function ' +
        'that creates a handle. Instead received: %s.',
      create !== null ? typeof create : 'null',
    );
  }

  // TODO: If deps are provided, should we skip comparing the ref itself?
  const effectDeps =
    deps !== null && deps !== undefined ? deps.concat([ref]) : null;

  return updateEffectImpl(
    UpdateEffect,
    UnmountMutation | MountLayout,
    imperativeHandleEffect.bind(null, create, ref),
    effectDeps,
  );
}

function mountDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void {
  // This hook is normally a no-op.
  // The react-debug-hooks package injects its own implementation
  // so that e.g. DevTools can display custom hook values.
}

const updateDebugValue = mountDebugValue;

function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

function updateCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;
  if (prevState !== null) {
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        return prevState[0];
      }
    }
  }
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

function mountMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const nextValue = nextCreate();
  hook.memoizedState = [nextValue, nextDeps];
  return nextValue;
}

function updateMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;
  if (prevState !== null) {
    // Assume these are defined. If they're not, areHookInputsEqual will warn.
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        return prevState[0];
      }
    }
  }
  const nextValue = nextCreate();
  hook.memoizedState = [nextValue, nextDeps];
  return nextValue;
}

// in a test-like environment, we want to warn if dispatchAction()
// is called outside of a batchedUpdates/TestUtils.act(...) call.
let shouldWarnForUnbatchedSetState = false;

if (__DEV__) {
  // jest isn't a 'global', it's just exposed to tests via a wrapped function
  // further, this isn't a test file, so flow doesn't recognize the symbol. So...
  // $FlowExpectedError - because requirements don't give a damn about your type sigs.
  if ('undefined' !== typeof jest) {
    shouldWarnForUnbatchedSetState = true;
  }
}

function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  invariant(
    numberOfReRenders < RE_RENDER_LIMIT,
    'Too many re-renders. React limits the number of renders to prevent ' +
      'an infinite loop.',
  );

  if (__DEV__) {
    warning(
      arguments.length <= 3,
      "State updates from the useState() and useReducer() Hooks don't support the " +
        'second callback argument. To execute a side effect after ' +
        'rendering, declare it in the component body with useEffect().',
    );
  }

  const alternate = fiber.alternate;
  if (
    fiber === currentlyRenderingFiber ||
    (alternate !== null && alternate === currentlyRenderingFiber)
  ) {
    // This is a render phase update. Stash it in a lazily-created map of
    // queue -> linked list of updates. After this render pass, we'll restart
    // and apply the stashed updates on top of the work-in-progress hook.
    didScheduleRenderPhaseUpdate = true;
    const update: Update<S, A> = {
      expirationTime: renderExpirationTime,
      action,
      eagerReducer: null,
      eagerState: null,
      next: null,
    };
    if (renderPhaseUpdates === null) {
      renderPhaseUpdates = new Map();
    }
    const firstRenderPhaseUpdate = renderPhaseUpdates.get(queue);
    if (firstRenderPhaseUpdate === undefined) {
      renderPhaseUpdates.set(queue, update);
    } else {
      // Append the update to the end of the list.
      let lastRenderPhaseUpdate = firstRenderPhaseUpdate;
      while (lastRenderPhaseUpdate.next !== null) {
        lastRenderPhaseUpdate = lastRenderPhaseUpdate.next;
      }
      lastRenderPhaseUpdate.next = update;
    }
  } else {
    flushPassiveEffects();

    const currentTime = requestCurrentTime();
    const expirationTime = computeExpirationForFiber(currentTime, fiber);

    const update: Update<S, A> = {
      expirationTime,
      action,
      eagerReducer: null,
      eagerState: null,
      next: null,
    };

    // Append the update to the end of the list.
    const last = queue.last;
    if (last === null) {
      // This is the first update. Create a circular list.
      update.next = update;
    } else {
      const first = last.next;
      if (first !== null) {
        // Still circular.
        update.next = first;
      }
      last.next = update;
    }
    queue.last = update;

    if (
      fiber.expirationTime === NoWork &&
      (alternate === null || alternate.expirationTime === NoWork)
    ) {
      // The queue is currently empty, which means we can eagerly compute the
      // next state before entering the render phase. If the new state is the
      // same as the current state, we may be able to bail out entirely.
      const eagerReducer = queue.eagerReducer;
      if (eagerReducer !== null) {
        let prevDispatcher;
        if (__DEV__) {
          prevDispatcher = ReactCurrentDispatcher.current;
          ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
        }
        try {
          const currentState: S = (queue.eagerState: any);
          const eagerState = eagerReducer(currentState, action);
          // Stash the eagerly computed state, and the reducer used to compute
          // it, on the update object. If the reducer hasn't changed by the
          // time we enter the render phase, then the eager state can be used
          // without calling the reducer again.
          update.eagerReducer = eagerReducer;
          update.eagerState = eagerState;
          if (is(eagerState, currentState)) {
            // Fast path. We can bail out without scheduling React to re-render.
            // It's still possible that we'll need to rebase this update later,
            // if the component re-renders for a different reason and by that
            // time the reducer has changed.
            return;
          }
        } catch (error) {
          // Suppress the error. It will throw again in the render phase.
        } finally {
          if (__DEV__) {
            ReactCurrentDispatcher.current = prevDispatcher;
          }
        }
      }
    }
    if (__DEV__) {
      if (shouldWarnForUnbatchedSetState === true) {
        warnIfNotCurrentlyBatchingInDev(fiber);
      }
    }
    scheduleWork(fiber, expirationTime);
  }
}

export const ContextOnlyDispatcher: Dispatcher = {
  readContext,

  useCallback: throwInvalidHookError,
  useContext: throwInvalidHookError,
  useEffect: throwInvalidHookError,
  useImperativeHandle: throwInvalidHookError,
  useLayoutEffect: throwInvalidHookError,
  useMemo: throwInvalidHookError,
  useReducer: throwInvalidHookError,
  useRef: throwInvalidHookError,
  useState: throwInvalidHookError,
  useDebugValue: throwInvalidHookError,
};

const HooksDispatcherOnMount: Dispatcher = {
  readContext,

  useCallback: mountCallback,
  useContext: mountContext,
  useEffect: mountEffect,
  useImperativeHandle: mountImperativeHandle,
  useLayoutEffect: mountLayoutEffect,
  useMemo: mountMemo,
  useReducer: mountReducer,
  useRef: mountRef,
  useState: mountState,
  useDebugValue: mountDebugValue,
};

const HooksDispatcherOnUpdate: Dispatcher = {
  readContext,

  useCallback: updateCallback,
  useContext: updateContext,
  useEffect: updateEffect,
  useImperativeHandle: updateImperativeHandle,
  useLayoutEffect: updateLayoutEffect,
  useMemo: updateMemo,
  useReducer: updateReducer,
  useRef: updateRef,
  useState: updateState,
  useDebugValue: updateDebugValue,
};

let HooksDispatcherOnMountInDEV: Dispatcher | null = null;
let HooksDispatcherOnUpdateInDEV: Dispatcher | null = null;
let InvalidNestedHooksDispatcherOnMountInDEV: Dispatcher | null = null;
let InvalidNestedHooksDispatcherOnUpdateInDEV: Dispatcher | null = null;

if (__DEV__) {
  const warnInvalidContextAccess = () => {
    warning(
      false,
      'Context can only be read while React is rendering. ' +
        'In classes, you can read it in the render method or getDerivedStateFromProps. ' +
        'In function components, you can read it directly in the function body, but not ' +
        'inside Hooks like useReducer() or useMemo().',
    );
  };

  const warnInvalidHookAccess = () => {
    warning(
      false,
      'Do not call Hooks inside useEffect(...), useMemo(...), or other built-in Hooks. ' +
        'You can only call Hooks at the top level of your React function. ' +
        'For more information, see ' +
        'https://fb.me/rules-of-hooks',
    );
  };

  HooksDispatcherOnMountInDEV = {
    readContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      return readContext(context, observedBits);
    },

    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      return mountCallback(callback, deps);
    },
    useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      currentHookNameInDev = 'useContext';
      return mountContext(context, observedBits);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      return mountEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      return mountImperativeHandle(ref, create, deps);
    },
    useLayoutEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useLayoutEffect';
      return mountLayoutEffect(create, deps);
    },
    useMemo<T>(create: () => T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useMemo';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountMemo(create, deps);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useReducer<S, I, A>(
      reducer: (S, A) => S,
      initialArg: I,
      init?: I => S,
    ): [S, Dispatch<A>] {
      currentHookNameInDev = 'useReducer';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountReducer(reducer, initialArg, init);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useRef<T>(initialValue: T): {current: T} {
      currentHookNameInDev = 'useRef';
      return mountRef(initialValue);
    },
    useState<S>(
      initialState: (() => S) | S,
    ): [S, Dispatch<BasicStateAction<S>>] {
      currentHookNameInDev = 'useState';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void {
      currentHookNameInDev = 'useDebugValue';
      return mountDebugValue(value, formatterFn);
    },
  };

  HooksDispatcherOnUpdateInDEV = {
    readContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      return readContext(context, observedBits);
    },

    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      return updateCallback(callback, deps);
    },
    useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      currentHookNameInDev = 'useContext';
      return updateContext(context, observedBits);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      return updateEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      return updateImperativeHandle(ref, create, deps);
    },
    useLayoutEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useLayoutEffect';
      return updateLayoutEffect(create, deps);
    },
    useMemo<T>(create: () => T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useMemo';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateMemo(create, deps);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useReducer<S, I, A>(
      reducer: (S, A) => S,
      initialArg: I,
      init?: I => S,
    ): [S, Dispatch<A>] {
      currentHookNameInDev = 'useReducer';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateReducer(reducer, initialArg, init);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useRef<T>(initialValue: T): {current: T} {
      currentHookNameInDev = 'useRef';
      return updateRef(initialValue);
    },
    useState<S>(
      initialState: (() => S) | S,
    ): [S, Dispatch<BasicStateAction<S>>] {
      currentHookNameInDev = 'useState';
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void {
      currentHookNameInDev = 'useDebugValue';
      return updateDebugValue(value, formatterFn);
    },
  };

  InvalidNestedHooksDispatcherOnMountInDEV = {
    readContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      warnInvalidContextAccess();
      return readContext(context, observedBits);
    },

    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      warnInvalidHookAccess();
      return mountCallback(callback, deps);
    },
    useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      currentHookNameInDev = 'useContext';
      warnInvalidHookAccess();
      return mountContext(context, observedBits);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      warnInvalidHookAccess();
      return mountEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      warnInvalidHookAccess();
      return mountImperativeHandle(ref, create, deps);
    },
    useLayoutEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useLayoutEffect';
      warnInvalidHookAccess();
      return mountLayoutEffect(create, deps);
    },
    useMemo<T>(create: () => T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useMemo';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountMemo(create, deps);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useReducer<S, I, A>(
      reducer: (S, A) => S,
      initialArg: I,
      init?: I => S,
    ): [S, Dispatch<A>] {
      currentHookNameInDev = 'useReducer';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountReducer(reducer, initialArg, init);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useRef<T>(initialValue: T): {current: T} {
      currentHookNameInDev = 'useRef';
      warnInvalidHookAccess();
      return mountRef(initialValue);
    },
    useState<S>(
      initialState: (() => S) | S,
    ): [S, Dispatch<BasicStateAction<S>>] {
      currentHookNameInDev = 'useState';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnMountInDEV;
      try {
        return mountState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void {
      currentHookNameInDev = 'useDebugValue';
      warnInvalidHookAccess();
      return mountDebugValue(value, formatterFn);
    },
  };

  InvalidNestedHooksDispatcherOnUpdateInDEV = {
    readContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      warnInvalidContextAccess();
      return readContext(context, observedBits);
    },

    useCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useCallback';
      warnInvalidHookAccess();
      return updateCallback(callback, deps);
    },
    useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      currentHookNameInDev = 'useContext';
      warnInvalidHookAccess();
      return updateContext(context, observedBits);
    },
    useEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useEffect';
      warnInvalidHookAccess();
      return updateEffect(create, deps);
    },
    useImperativeHandle<T>(
      ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
      create: () => T,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      warnInvalidHookAccess();
      return updateImperativeHandle(ref, create, deps);
    },
    useLayoutEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useLayoutEffect';
      warnInvalidHookAccess();
      return updateLayoutEffect(create, deps);
    },
    useMemo<T>(create: () => T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useMemo';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateMemo(create, deps);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useReducer<S, I, A>(
      reducer: (S, A) => S,
      initialArg: I,
      init?: I => S,
    ): [S, Dispatch<A>] {
      currentHookNameInDev = 'useReducer';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateReducer(reducer, initialArg, init);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useRef<T>(initialValue: T): {current: T} {
      currentHookNameInDev = 'useRef';
      warnInvalidHookAccess();
      return updateRef(initialValue);
    },
    useState<S>(
      initialState: (() => S) | S,
    ): [S, Dispatch<BasicStateAction<S>>] {
      currentHookNameInDev = 'useState';
      warnInvalidHookAccess();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateState(initialState);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useDebugValue<T>(value: T, formatterFn: ?(value: T) => mixed): void {
      currentHookNameInDev = 'useDebugValue';
      warnInvalidHookAccess();
      return updateDebugValue(value, formatterFn);
    },
  };
}