use-slate-selector.tsx

import {
  createContext,
  useCallback,
  useContext,
  useMemo,
  useReducer,
  useRef,
} from 'react'
import { Editor } from 'slate'
import { useIsomorphicLayoutEffect } from './use-isomorphic-layout-effect'

function isError(error: any): error is Error {
  return error instanceof Error
}

type EditorChangeHandler = (editor: Editor) => void
/**
 * A React context for sharing the editor selector context in a way to control rerenders
 */

export const SlateSelectorContext = createContext<{
  getSlate: () => Editor
  addEventListener: (callback: EditorChangeHandler) => () => void
}>({} as any)

const refEquality = (a: any, b: any) => a === b

/**
 * use redux style selectors to prevent rerendering on every keystroke.
 * Bear in mind rerendering can only prevented if the returned value is a value type or for reference types (e.g. objects and arrays) add a custom equality function.
 *
 * Example:
 * ```
 *  const isSelectionActive = useSlateSelector(editor => Boolean(editor.selection));
 * ```
 */
export function useSlateSelector<T>(
  selector: (editor: Editor) => T,
  equalityFn: (a: T, b: T) => boolean = refEquality
) {
  const [, forceRender] = useReducer(s => s + 1, 0)
  const context = useContext(SlateSelectorContext)
  if (!context) {
    throw new Error(
      `The \`useSlateSelector\` hook must be used inside the <Slate> component's context.`
    )
  }
  const { getSlate, addEventListener } = context

  const latestSubscriptionCallbackError = useRef<Error | undefined>()
  const latestSelector = useRef<(editor: Editor) => T>(() => null as any)
  const latestSelectedState = useRef<T>((null as any) as T)
  let selectedState: T

  try {
    if (
      selector !== latestSelector.current ||
      latestSubscriptionCallbackError.current
    ) {
      selectedState = selector(getSlate())
    } else {
      selectedState = latestSelectedState.current
    }
  } catch (err) {
    if (latestSubscriptionCallbackError.current && isError(err)) {
      err.message += `\nThe error may be correlated with this previous error:\n${latestSubscriptionCallbackError.current.stack}\n\n`
    }

    throw err
  }
  useIsomorphicLayoutEffect(() => {
    latestSelector.current = selector
    latestSelectedState.current = selectedState
    latestSubscriptionCallbackError.current = undefined
  })

  useIsomorphicLayoutEffect(
    () => {
      function checkForUpdates() {
        try {
          const newSelectedState = latestSelector.current(getSlate())

          if (equalityFn(newSelectedState, latestSelectedState.current)) {
            return
          }

          latestSelectedState.current = newSelectedState
        } catch (err) {
          // we ignore all errors here, since when the component
          // is re-rendered, the selectors are called again, and
          // will throw again, if neither props nor store state
          // changed
          latestSubscriptionCallbackError.current = err
        }

        forceRender()
      }

      const unsubscribe = addEventListener(checkForUpdates)

      checkForUpdates()

      return () => unsubscribe()
    },
    // don't rerender on equalityFn change since we want to be able to define it inline
    [addEventListener, getSlate]
  )

  return selectedState
}

/**
 * Create selector context with editor updating on every editor change
 */
export function getSelectorContext(editor: Editor) {
  const eventListeners = useRef<EditorChangeHandler[]>([]).current
  const slateRef = useRef<{
    editor: Editor
  }>({
    editor,
  }).current
  const onChange = useCallback((editor: Editor) => {
    slateRef.editor = editor
    eventListeners.forEach((listener: EditorChangeHandler) => listener(editor))
  }, [])

  const selectorContext = useMemo(() => {
    return {
      getSlate: () => slateRef.editor,
      addEventListener: (callback: EditorChangeHandler) => {
        eventListeners.push(callback)
        return () => {
          eventListeners.splice(eventListeners.indexOf(callback), 1)
        }
      },
    }
  }, [eventListeners, slateRef])
  return { selectorContext, onChange }
}