From 94270569a5bc03b8e80c5ecb655f8a751cf0420d Mon Sep 17 00:00:00 2001
From: jxom <jakemoxey@gmail.com>
Date: Sat, 7 Oct 2017 18:33:53 +1100
Subject: [PATCH] Tweak sidebar to update navigation highlight on scroll

---
 content/tutorial/nav.yml                    |   2 +-
 src/templates/components/Sidebar/Section.js | 217 +++++++++++++-------
 src/utils/createLink.js                     |  15 +-
 3 files changed, 157 insertions(+), 77 deletions(-)

diff --git a/content/tutorial/nav.yml b/content/tutorial/nav.yml
index f3570cc98..adc5cf3a7 100644
--- a/content/tutorial/nav.yml
+++ b/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
diff --git a/src/templates/components/Sidebar/Section.js b/src/templates/components/Sidebar/Section.js
index e212f03e1..a264ebfdb 100644
--- a/src/templates/components/Sidebar/Section.js
+++ b/src/templates/components/Sidebar/Section.js
@@ -9,88 +9,161 @@
 
 'use strict';
 
-import React from 'react';
+import React, {Component} from 'react';
 import {colors, media} from 'theme';
 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
+class Section extends Component {
+  constructor(props, context) {
+    super(props, context);
 
-const Section = ({
-  createLink,
-  isActive,
-  location,
-  onLinkClick,
-  onSectionTitleClick,
-  section,
-}) => (
-  <div>
-    <MetaTitle
-      onClick={onSectionTitleClick}
-      cssProps={{
-        marginTop: 10,
+    this.state = {
+      activeItemId: null,
+      itemTopOffsets: [],
+    };
 
-        [media.greaterThan('small')]: {
-          color: isActive ? colors.text : colors.subtle,
+    this.handleScroll = this.handleScroll.bind(this);
+  }
 
-          ':hover': {
-            color: colors.text,
-          },
-        },
-      }}>
-      {section.title}
-      <ChevronSvg
-        cssProps={{
-          marginLeft: 7,
-          transform: isActive ? 'rotateX(180deg)' : 'rotateX(0deg)',
-          transition: 'transform 0.2s ease',
+  componentDidMount() {
+    const {section} = this.props;
 
-          [media.lessThan('small')]: {
-            display: 'none',
-          },
-        }}
-      />
-    </MetaTitle>
-    <ul
-      css={{
-        marginBottom: 10,
+    const itemIds = _getItemIds(section.items);
+    this.setState({
+      itemTopOffsets: _getElementTopOffsetsById(itemIds),
+    });
 
-        [media.greaterThan('small')]: {
-          display: isActive ? 'block' : 'none',
-        },
-      }}>
-      {section.items.map(item => (
-        <li
-          key={item.id}
-          css={{
-            marginTop: 5,
+    window.addEventListener('scroll', this.handleScroll);
+  }
+
+  componentWillUnmount() {
+    window.removeEventListener('scroll', 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 : null,
+    });
+  }
+
+  render() {
+    const {
+      createLink,
+      isActive,
+      location,
+      onLinkClick,
+      onSectionTitleClick,
+      section,
+    } = this.props;
+    const {activeItemId} = this.state;
+    return (
+      <div>
+        <MetaTitle
+          onClick={onSectionTitleClick}
+          cssProps={{
+            marginTop: 10,
+
+            [media.greaterThan('small')]: {
+              color: isActive ? colors.text : colors.subtle,
+
+              ':hover': {
+                color: colors.text,
+              },
+            },
           }}>
-          {createLink({
-            item,
-            location,
-            onLinkClick,
-            section,
-          })}
+          {section.title}
+          <ChevronSvg
+            cssProps={{
+              marginLeft: 7,
+              transform: isActive ? 'rotateX(180deg)' : 'rotateX(0deg)',
+              transition: 'transform 0.2s ease',
 
-          {item.subitems && (
-            <ul css={{marginLeft: 20}}>
-              {item.subitems.map(subitem => (
-                <li key={subitem.id}>
-                  {createLink({
-                    item: subitem,
-                    location,
-                    onLinkClick,
-                    section,
-                  })}
-                </li>
-              ))}
-            </ul>
-          )}
-        </li>
-      ))}
-    </ul>
-  </div>
-);
+              [media.lessThan('small')]: {
+                display: 'none',
+              },
+            }}
+          />
+        </MetaTitle>
+        <ul
+          css={{
+            marginBottom: 10,
+
+            [media.greaterThan('small')]: {
+              display: isActive ? 'block' : 'none',
+            },
+          }}>
+          {section.items.map(item => (
+            <li
+              key={item.id}
+              css={{
+                marginTop: 5,
+              }}>
+              {createLink({
+                item,
+                location,
+                onLinkClick,
+                section,
+                isActive: activeItemId === item.id,
+              })}
+
+              {item.subitems && (
+                <ul css={{marginLeft: 20}}>
+                  {item.subitems.map(subitem => (
+                    <li key={subitem.id}>
+                      {createLink({
+                        item: subitem,
+                        location,
+                        onLinkClick,
+                        section,
+                        isActive: activeItemId === subitem.id,
+                      })}
+                    </li>
+                  ))}
+                </ul>
+              )}
+            </li>
+          ))}
+        </ul>
+      </div>
+    );
+  }
+}
+
+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 Section;
diff --git a/src/utils/createLink.js b/src/utils/createLink.js
index 88bac8251..a2871d5fb 100644
--- a/src/utils/createLink.js
+++ b/src/utils/createLink.js
@@ -65,15 +65,22 @@ const createLinkDocs = ({item, location, section}) => {
   );
 };
 
-const createLinkTutorial = ({item, location, onLinkClick, section}) => {
-  const isActive = isItemActive(location, item);
+const createLinkTutorial = ({
+  item,
+  location,
+  onLinkClick,
+  section,
+  isActive,
+}) => {
+  const active =
+    typeof isActive === 'boolean' ? isActive : isItemActive(location, item);
 
   return (
     <Link
-      css={[linkCss, isActive && activeLinkCss]}
+      css={[linkCss, active && activeLinkCss]}
       onClick={onLinkClick}
       to={item.href}>
-      {isActive && <span css={activeLinkBefore} />}
+      {active && <span css={activeLinkBefore} />}
       {item.title}
     </Link>
   );