Tweak sidebar to update navigation highlight on scroll

content/tutorial/nav.yml

@@ -1,6 +1,6 @@
 - title: Tutorial
   items:
-    - id: tutorial
+    - id: before-we-start
       title: Before We Start
       href: /tutorial/tutorial.html#before-we-start
       forceInternal: true

src/components/MarkdownPage/MarkdownPage.js

@@ -26,6 +26,7 @@ const MarkdownPage = ({
   authors,
   createLink,
   date,
+  enableScrollSync,
   ogDescription,
   location,
   markdownRemark,
@@ -98,6 +99,7 @@ const MarkdownPage = ({
 
             <div css={sharedStyles.articleLayout.sidebar}>
               <StickyResponsiveSidebar
+                enableScrollSync={enableScrollSync}
                 createLink={createLink}
                 defaultActiveSection={findSectionForPath(
                   location.pathname,
@@ -132,6 +134,7 @@ MarkdownPage.propTypes = {
   authors: PropTypes.array.isRequired,
   createLink: PropTypes.func.isRequired,
   date: PropTypes.string,
+  enableScrollSync: PropTypes.bool,
   location: PropTypes.object.isRequired,
   markdownRemark: PropTypes.object.isRequired,
   sectionList: PropTypes.array.isRequired,

src/templates/components/Sidebar/ScrollSyncSection.js

@@ -0,0 +1,103 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ *
+ * This source code is licensed under the CC-BY-4.0 license found
+ * in the LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+import React, {Component} from 'react';
+import Section from './Section';
+
+class ScrollSyncSection extends Component {
+  constructor(props, context) {
+    super(props, context);
+
+    this.state = {
+      activeItemId: '',
+      itemTopOffsets: [],
+    };
+
+    this.calculateItemTopOffsets = this.calculateItemTopOffsets.bind(this);
+    this.handleResize = this.handleResize.bind(this);
+    this.handleScroll = this.handleScroll.bind(this);
+  }
+
+  componentDidMount() {
+    this.calculateItemTopOffsets();
+
+    window.addEventListener('resize', this.handleResize);
+    window.addEventListener('scroll', this.handleScroll);
+  }
+
+  componentWillUnmount() {
+    window.removeEventListener('resize', this.handleResize);
+    window.removeEventListener('scroll', this.handleScroll);
+  }
+
+  calculateItemTopOffsets() {
+    const {section} = this.props;
+
+    const itemIds = _getItemIds(section.items);
+    this.setState({
+      itemTopOffsets: _getElementTopOffsetsById(itemIds),
+    });
+  }
+
+  handleResize() {
+    this.calculateItemTopOffsets();
+    this.handleScroll();
+  }
+
+  handleScroll() {
+    const {itemTopOffsets} = this.state;
+    const item = itemTopOffsets.find((itemTopOffset, i) => {
+      const nextItemTopOffset = itemTopOffsets[i + 1];
+      if (nextItemTopOffset) {
+        return (
+          window.scrollY >= itemTopOffset.offsetTop &&
+          window.scrollY < nextItemTopOffset.offsetTop
+        );
+      }
+      return window.scrollY >= itemTopOffset.offsetTop;
+    });
+    this.setState({
+      activeItemId: item ? item.id : '',
+    });
+  }
+
+  render() {
+    const {activeItemId} = this.state;
+    return <Section isScrollSync activeItemId={activeItemId} {...this.props} />;
+  }
+}
+
+const _getItemIds = items =>
+  items
+    .map(item => {
+      let subItemIds = [];
+      if (item.subitems) {
+        subItemIds = item.subitems.map(subitem => subitem.id);
+      }
+      return [item.id, ...subItemIds];
+    })
+    .reduce((prev, current) => prev.concat(current));
+
+const _getElementTopOffsetsById = ids =>
+  ids
+    .map(id => {
+      const element = document.getElementById(id);
+      if (!element) {
+        return null;
+      }
+      return {
+        id,
+        offsetTop: element.offsetTop,
+      };
+    })
+    .filter(item => item);
+
+export default ScrollSyncSection;

src/templates/components/Sidebar/Section.js

@@ -9,17 +9,16 @@
 
 'use strict';
 
-import React from 'react';
 import {colors, media} from 'theme';
+import isItemActive from 'utils/isItemActive';
 import MetaTitle from '../MetaTitle';
 import ChevronSvg from '../ChevronSvg';
 
-// TODO Update isActive link as document scrolls past anchor tags
-// Maybe used 'hashchange' along with 'scroll' to set/update active links
-
 const Section = ({
+  activeItemId,
   createLink,
   isActive,
+  isScrollSync,
   location,
   onLinkClick,
   onSectionTitleClick,
@@ -67,6 +66,9 @@ const Section = ({
             marginTop: 5,
           }}>
           {createLink({
+            isActive: isScrollSync
+              ? activeItemId === item.id
+              : isItemActive(location, item),
             item,
             location,
             onLinkClick,
@@ -78,6 +80,9 @@ const Section = ({
               {item.subitems.map(subitem => (
                 <li key={subitem.id}>
                   {createLink({
+                    isActive: isScrollSync
+                      ? activeItemId === subitem.id
+                      : isItemActive(location, subitem),
                     item: subitem,
                     location,
                     onLinkClick,

src/templates/components/Sidebar/Sidebar.js

@@ -12,6 +12,7 @@
 import React, {Component} from 'react';
 import Flex from 'components/Flex';
 import Section from './Section';
+import ScrollSyncSection from './ScrollSyncSection';
 import {media} from 'theme';
 
 class Sidebar extends Component {
@@ -24,9 +25,17 @@ class Sidebar extends Component {
   }
 
   render() {
-    const {closeParentMenu, createLink, location, sectionList} = this.props;
+    const {
+      closeParentMenu,
+      createLink,
+      enableScrollSync,
+      location,
+      sectionList,
+    } = this.props;
     const {activeSection} = this.state;
 
+    const SectionComponent = enableScrollSync ? ScrollSyncSection : Section;
+
     return (
       <Flex
         type="nav"
@@ -46,7 +55,7 @@ class Sidebar extends Component {
           },
         }}>
         {sectionList.map((section, index) => (
-          <Section
+          <SectionComponent
             createLink={createLink}
             isActive={activeSection === section || sectionList.length === 1}
             key={index}

src/templates/tutorial.js

@@ -27,6 +27,7 @@ const Tutorial = ({data, location}) => {
 
   return (
     <MarkdownPage
+      enableScrollSync
       createLink={createLinkTutorial}
       location={location}
       markdownRemark={data.markdownRemark}

src/utils/createLink.js

@@ -14,13 +14,10 @@
 import Link from 'gatsby-link';
 import React from 'react';
 import ExternalLinkSvg from 'templates/components/ExternalLinkSvg';
-import isItemActive from 'utils/isItemActive';
 import slugify from 'utils/slugify';
 import {colors, media} from 'theme';
 
-const createLinkBlog = ({item, location, section}) => {
-  const isActive = isItemActive(location, item);
-
+const createLinkBlog = ({isActive, item, section}) => {
   return (
     <Link css={[linkCss, isActive && activeLinkCss]} to={item.id}>
       {isActive && <span css={activeLinkBefore} />}
@@ -29,7 +26,7 @@ const createLinkBlog = ({item, location, section}) => {
   );
 };
 
-const createLinkCommunity = ({item, location, section}) => {
+const createLinkCommunity = ({isActive, item, section}) => {
   if (item.href) {
     return (
       <a css={[linkCss]} href={item.href} target="_blank" rel="noopener">
@@ -46,15 +43,13 @@ const createLinkCommunity = ({item, location, section}) => {
     );
   }
   return createLinkDocs({
+    isActive,
     item,
-    location,
     section,
   });
 };
 
-const createLinkDocs = ({item, location, section}) => {
-  const isActive = isItemActive(location, item);
-
+const createLinkDocs = ({isActive, item, section}) => {
   return (
     <Link
       css={[linkCss, isActive && activeLinkCss]}
@@ -65,9 +60,7 @@ const createLinkDocs = ({item, location, section}) => {
   );
 };
 
-const createLinkTutorial = ({item, location, onLinkClick, section}) => {
-  const isActive = isItemActive(location, item);
-
+const createLinkTutorial = ({isActive, item, onLinkClick, section}) => {
   return (
     <Link
       css={[linkCss, isActive && activeLinkCss]}