Sync Split Editor scroll position

browser/components/CodeEditor.js

@@ -47,6 +47,7 @@ export default class CodeEditor extends React.Component {
     this.loadStyleHandler = (e) => {
       this.editor.refresh()
     }
+    this.scrollHandler = _.debounce(this.handleScroll.bind(this), 100, {leading: false, trailing: true})
   }
 
   componentDidMount () {
@@ -107,6 +108,7 @@ export default class CodeEditor extends React.Component {
     this.editor.on('blur', this.blurHandler)
     this.editor.on('change', this.changeHandler)
     this.editor.on('paste', this.pasteHandler)
+    this.editor.on('scroll', this.scrollHandler)
 
     const editorTheme = document.getElementById('editorTheme')
     editorTheme.addEventListener('load', this.loadStyleHandler)
@@ -126,6 +128,7 @@ export default class CodeEditor extends React.Component {
     this.editor.off('blur', this.blurHandler)
     this.editor.off('change', this.changeHandler)
     this.editor.off('paste', this.pasteHandler)
+    this.editor.off('scroll', this.scrollHandler)
     const editorTheme = document.getElementById('editorTheme')
     editorTheme.removeEventListener('load', this.loadStyleHandler)
   }
@@ -262,6 +265,12 @@ export default class CodeEditor extends React.Component {
     }
   }
 
+  handleScroll (e) {
+    if (this.props.onScroll) {
+      this.props.onScroll(e)
+    }
+  }
+
   handlePasteUrl (e, editor, pastedTxt) {
     e.preventDefault()
     const taggedUrl = `<${pastedTxt}>`

browser/components/MarkdownPreview.js

@@ -121,6 +121,7 @@ export default class MarkdownPreview extends React.Component {
     this.mouseDownHandler = (e) => this.handleMouseDown(e)
     this.mouseUpHandler = (e) => this.handleMouseUp(e)
     this.DoubleClickHandler = (e) => this.handleDoubleClick(e)
+    this.scrollHandler = _.debounce(this.handleScroll.bind(this), 100, {leading: false, trailing: true})
     this.anchorClickHandler = (e) => this.handlePreviewAnchorClick(e)
     this.checkboxClickHandler = (e) => this.handleCheckboxClick(e)
     this.saveAsTextHandler = () => this.handleSaveAsText()
@@ -151,6 +152,12 @@ export default class MarkdownPreview extends React.Component {
     this.props.onCheckboxClick(e)
   }
 
+  handleScroll (e) {
+    if (this.props.onScroll) {
+      this.props.onScroll(e)
+    }
+  }
+
   handleContextMenu (e) {
     this.props.onContextMenu(e)
   }
@@ -279,6 +286,7 @@ export default class MarkdownPreview extends React.Component {
     this.refs.root.contentWindow.document.addEventListener('dblclick', this.DoubleClickHandler)
     this.refs.root.contentWindow.document.addEventListener('drop', this.preventImageDroppedHandler)
     this.refs.root.contentWindow.document.addEventListener('dragover', this.preventImageDroppedHandler)
+    this.refs.root.contentWindow.document.addEventListener('scroll', this.scrollHandler)
     eventEmitter.on('export:save-text', this.saveAsTextHandler)
     eventEmitter.on('export:save-md', this.saveAsMdHandler)
     eventEmitter.on('export:save-html', this.saveAsHtmlHandler)
@@ -292,6 +300,7 @@ export default class MarkdownPreview extends React.Component {
     this.refs.root.contentWindow.document.removeEventListener('dblclick', this.DoubleClickHandler)
     this.refs.root.contentWindow.document.removeEventListener('drop', this.preventImageDroppedHandler)
     this.refs.root.contentWindow.document.removeEventListener('dragover', this.preventImageDroppedHandler)
+    this.refs.root.contentWindow.document.removeEventListener('scroll', this.scrollHandler)
     eventEmitter.off('export:save-text', this.saveAsTextHandler)
     eventEmitter.off('export:save-md', this.saveAsMdHandler)
     eventEmitter.off('export:save-html', this.saveAsHtmlHandler)

browser/components/MarkdownSplitEditor.js

@@ -2,6 +2,7 @@ import React from 'react'
 import CodeEditor from 'browser/components/CodeEditor'
 import MarkdownPreview from 'browser/components/MarkdownPreview'
 import { findStorage } from 'browser/lib/findStorage'
+import _ from 'lodash'
 
 import styles from './MarkdownSplitEditor.styl'
 import CSSModules from 'browser/lib/CSSModules'
@@ -12,13 +13,57 @@ class MarkdownSplitEditor extends React.Component {
     this.value = props.value
     this.focus = () => this.refs.code.focus()
     this.reload = () => this.refs.code.reload()
+    this.userScroll = true
   }
 
   handleOnChange () {
     this.value = this.refs.code.value
     this.props.onChange()
   }
 
+  handleScroll (e) {
+    const previewDoc = _.get(this, 'refs.preview.refs.root.contentWindow.document')
+    const codeDoc = _.get(this, 'refs.code.editor.doc')
+    let srcTop, srcHeight, targetTop, targetHeight
+
+    if (this.userScroll) {
+      if (e.doc) {
+        srcTop = _.get(e, 'doc.scrollTop')
+        srcHeight = _.get(e, 'doc.height')
+        targetTop = _.get(previewDoc, 'body.scrollTop')
+        targetHeight = _.get(previewDoc, 'body.scrollHeight')
+      } else {
+        srcTop = _.get(previewDoc, 'body.scrollTop')
+        srcHeight = _.get(previewDoc, 'body.scrollHeight')
+        targetTop = _.get(codeDoc, 'scrollTop')
+        targetHeight = _.get(codeDoc, 'height')
+      }
+
+      const distance = (targetHeight * srcTop / srcHeight) - targetTop
+      const framerate = 1000 / 60
+      const frames = 20
+      const refractory = frames * framerate
+
+      this.userScroll = false
+
+      let frame = 0
+      let scrollPos, time
+      const timer = setInterval(() => {
+        time = frame / frames
+        scrollPos = time < 0.5
+                  ? 2 * time * time // ease in
+                  : -1 + (4 - 2 * time) * time // ease out
+        if (e.doc) _.set(previewDoc, 'body.scrollTop', targetTop + scrollPos * distance)
+        else _.get(this, 'refs.code.editor').scrollTo(0, targetTop + scrollPos * distance)
+        if (frame >= frames) {
+          clearInterval(timer)
+          setTimeout(() => { this.userScroll = true }, refractory)
+        }
+        frame++
+      }, framerate)
+    }
+  }
+
   handleCheckboxClick (e) {
     e.preventDefault()
     e.stopPropagation()
@@ -69,6 +114,7 @@ class MarkdownSplitEditor extends React.Component {
           fetchUrlTitle={config.editor.fetchUrlTitle}
           storageKey={storageKey}
           onChange={this.handleOnChange.bind(this)}
+          onScroll={this.handleScroll.bind(this)}
        />
         <MarkdownPreview
           style={previewStyle}
@@ -85,6 +131,7 @@ class MarkdownSplitEditor extends React.Component {
           tabInde='0'
           value={value}
           onCheckboxClick={(e) => this.handleCheckboxClick(e)}
+          onScroll={this.handleScroll.bind(this)}
           showCopyNotification={config.ui.showCopyNotification}
           storagePath={storage.path}
        />