Fix reveal (#1563)

Fix race condition in slide show

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/components/markdown-renderer/hooks/use-extract-first-headline.ts

@@ -8,7 +8,7 @@ import React, { useCallback, useEffect, useRef } from 'react'
 
 export const useExtractFirstHeadline = (
   documentElement: React.RefObject<HTMLDivElement>,
-  content: string,
+  content: string | undefined,
   onFirstHeadingChange?: (firstHeading: string | undefined) => void
 ): void => {
   const extractInnerText = useCallback((node: ChildNode | null): string => {

src/components/markdown-renderer/hooks/use-reveal.ts

@@ -4,22 +4,39 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import Reveal from 'reveal.js'
 import { Logger } from '../../../utils/logger'
 import { SlideOptions } from '../../common/note-frontmatter/types'
 
 const log = new Logger('reveal.js')
 
-export const useReveal = (content: string, slideOptions?: SlideOptions): void => {
+export enum REVEAL_STATUS {
+  NOT_INITIALISED,
+  INITIALISING,
+  INITIALISED
+}
+
+export interface SlideState {
+  indexHorizontal: number
+  indexVertical: number
+}
+
+const initialSlideState: SlideState = {
+  indexHorizontal: 0,
+  indexVertical: 0
+}
+
+export const useReveal = (content: string, slideOptions?: SlideOptions): REVEAL_STATUS => {
   const [deck, setDeck] = useState<Reveal>()
-  const [isInitialized, setIsInitialized] = useState<boolean>(false)
+  const [revealStatus, setRevealStatus] = useState<REVEAL_STATUS>(REVEAL_STATUS.NOT_INITIALISED)
+  const currentSlideState = useRef<SlideState>(initialSlideState)
 
   useEffect(() => {
-    if (isInitialized) {
+    if (revealStatus !== REVEAL_STATUS.NOT_INITIALISED) {
       return
     }
-    setIsInitialized(true)
+    setRevealStatus(REVEAL_STATUS.INITIALISING)
     log.debug('Initialize with slide options', slideOptions)
     const reveal = new Reveal({})
     reveal
@@ -27,27 +44,43 @@ export const useReveal = (content: string, slideOptions?: SlideOptions): void =>
       .then(() => {
         reveal.layout()
         reveal.slide(0, 0, 0)
+        reveal.addEventListener('slidechanged', (event) => {
+          currentSlideState.current = {
+            indexHorizontal: event.indexh,
+            indexVertical: event.indexv ?? 0
+          } as SlideState
+        })
+
         setDeck(reveal)
+        setRevealStatus(REVEAL_STATUS.INITIALISED)
         log.debug('Initialisation finished')
       })
       .catch((error: Error) => {
         log.error('Error while initializing reveal.js', error)
       })
-  }, [isInitialized, slideOptions])
+  }, [revealStatus, slideOptions])
 
   useEffect(() => {
-    if (!deck) {
+    if (!deck || revealStatus !== REVEAL_STATUS.INITIALISED) {
       return
     }
     log.debug('Sync deck')
-    deck.layout()
-  }, [content, deck])
+    deck.sync()
+    deck.slide(currentSlideState.current.indexHorizontal, currentSlideState.current.indexVertical)
+  }, [content, deck, revealStatus])
 
   useEffect(() => {
-    if (!deck || slideOptions === undefined || Object.keys(slideOptions).length === 0) {
+    if (
+      !deck ||
+      slideOptions === undefined ||
+      Object.keys(slideOptions).length === 0 ||
+      revealStatus !== REVEAL_STATUS.INITIALISED
+    ) {
       return
     }
     log.debug('Apply config', slideOptions)
     deck.configure(slideOptions)
-  }, [deck, slideOptions])
+  }, [deck, revealStatus, slideOptions])
+
+  return revealStatus
 }

src/components/markdown-renderer/loading-slide.tsx

@@ -0,0 +1,22 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import React from 'react'
+import { Trans, useTranslation } from 'react-i18next'
+
+/**
+ * Shows a static text placeholder while reveal.js is loading.
+ */
+export const LoadingSlide: React.FC = () => {
+  useTranslation()
+  return (
+    <section>
+      <h1>
+        <Trans i18nKey={'common.loading'} />
+      </h1>
+    </section>
+  )
+}

src/components/markdown-renderer/slideshow-markdown-renderer.tsx

@@ -12,7 +12,7 @@ import { useExtractFirstHeadline } from './hooks/use-extract-first-headline'
 import { TocAst } from 'markdown-it-toc-done-right'
 import { useOnRefChange } from './hooks/use-on-ref-change'
 import { useTrimmedContent } from './hooks/use-trimmed-content'
-import { useReveal } from './hooks/use-reveal'
+import { REVEAL_STATUS, useReveal } from './hooks/use-reveal'
 import './slideshow.scss'
 import { ScrollProps } from '../editor-page/synced-scroll/scroll-props'
 import { DocumentLengthLimitReachedAlert } from './document-length-limit-reached-alert'
@@ -20,6 +20,7 @@ import { BasicMarkdownItConfigurator } from './markdown-it-configurator/basic-ma
 import { SlideOptions } from '../common/note-frontmatter/types'
 import { processRevealCommentNodes } from './process-reveal-comment-nodes'
 import { CommonMarkdownRendererProps } from './common-markdown-renderer-props'
+import { LoadingSlide } from './loading-slide'
 
 export interface SlideshowMarkdownRendererProps extends CommonMarkdownRendererProps {
   slideOptions: SlideOptions
@@ -59,17 +60,26 @@ export const SlideshowMarkdownRenderer: React.FC<SlideshowMarkdownRendererProps
     replacers,
     processRevealCommentNodes
   )
+  const revealStatus = useReveal(content, slideOptions)
 
-  useExtractFirstHeadline(markdownBodyRef, content, onFirstHeadingChange)
+  useExtractFirstHeadline(
+    markdownBodyRef,
+    revealStatus === REVEAL_STATUS.INITIALISED ? content : undefined,
+    onFirstHeadingChange
+  )
   useOnRefChange(tocAst, onTocChange)
-  useReveal(content, slideOptions)
+
+  const slideShowDOM = useMemo(
+    () => (revealStatus === REVEAL_STATUS.INITIALISED ? markdownReactDom : <LoadingSlide />),
+    [markdownReactDom, revealStatus]
+  )
 
   return (
     <Fragment>
       <DocumentLengthLimitReachedAlert show={contentExceedsLimit} />
       <div className={'reveal'}>
         <div ref={markdownBodyRef} className={`${className ?? ''} slides`}>
-          {markdownReactDom}
+          {slideShowDOM}
         </div>
       </div>
     </Fragment>

src/external-types/reveal.js/index.d.ts

@@ -184,9 +184,9 @@ declare module 'reveal.js' {
     public getPlugins(): { [name: string]: Plugin }
 
     // States
-    //  public addEventListener(type: string, listener: (event: any) => void, useCapture?: boolean): void
-
-    //  public removeEventListener(type: string, listener: (event: any) => void, useCapture?: boolean): void
+    // Added only the events we need
+    public addEventListener(type: 'slidechanged', listener: (event: SlideEvent) => void, useCapture?: boolean): void
+    public removeEventListener(type: 'slidechanged', listener: (event: SlideEvent) => void, useCapture?: boolean): void
 
     // State Checks
     public isFirstSlide(): boolean