# Locomotive Scroll

Guide for implementing smooth scrolling, parallax effects, and scroll-driven animations using Locomotive Scroll.

## Overview

Locomotive Scroll is a JavaScript library that provides:
- **Smooth scrolling**: Hardware-accelerated smooth scroll with customizable easing
- **Parallax effects**: Element-level speed control for depth
- **Viewport detection**: Track when elements enter/exit viewport
- **Scroll events**: Monitor scroll progress for animation synchronization
- **Sticky elements**: Pin elements within defined boundaries
- **Horizontal scrolling**: Support for horizontal scroll layouts

**When to use Locomotive Scroll:**
- Building immersive landing pages with parallax
- Creating smooth, Apple-style scroll experiences
- Implementing scroll-triggered animations
- Developing narrative/storytelling websites
- Adding depth and motion to long-form content

**Trade-offs:**
- Scroll-hijacking can impact accessibility (provide disable option)
- Performance overhead on low-end devices (detect and disable)
- Mobile touch scrolling feels different (test extensively)
- Fixed positioning requires workarounds

## Installation

```bash
npm install locomotive-scroll
```

```javascript
// ES6
import LocomotiveScroll from 'locomotive-scroll';
import 'locomotive-scroll/dist/locomotive-scroll.css';

// Or via CDN
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/locomotive-scroll/dist/locomotive-scroll.min.css">
<script src="https://cdn.jsdelivr.net/npm/locomotive-scroll/dist/locomotive-scroll.min.js"></script>
```

## Core Concepts

### 1. HTML Structure

Every Locomotive Scroll implementation requires specific data attributes:

```html
<!-- Scroll container (required) -->
<div data-scroll-container>

  <!-- Scroll sections (optional, improves performance) -->
  <div data-scroll-section>

    <!-- Tracked elements -->
    <h1 data-scroll>Basic detection</h1>

    <!-- Parallax element -->
    <div data-scroll data-scroll-speed="2">
      Moves faster than scroll
    </div>

    <!-- Sticky element -->
    <div data-scroll data-scroll-sticky>
      Sticks within section
    </div>

    <!-- Element with ID for tracking -->
    <div data-scroll data-scroll-id="hero">
      Accessible via JavaScript
    </div>

    <!-- Call event trigger -->
    <div data-scroll data-scroll-call="fadeIn">
      Triggers custom event
    </div>

  </div>
</div>
```

### 2. Initialization

```javascript
const scroll = new LocomotiveScroll({
  el: document.querySelector('[data-scroll-container]'),
  smooth: true,
  lerp: 0.1,        // Smoothness (0-1, lower = smoother)
  multiplier: 1,    // Speed multiplier
  class: 'is-inview', // Class added to visible elements
  repeat: false,    // Repeat in-view detection
  offset: [0, 0]    // Global trigger offset [bottom, top]
});
```

### 3. Data Attributes

| Attribute | Purpose | Example |
|-----------|---------|---------|
| `data-scroll` | Enable detection | `data-scroll` |
| `data-scroll-speed` | Parallax speed | `data-scroll-speed="2"` |
| `data-scroll-direction` | Parallax axis | `data-scroll-direction="horizontal"` |
| `data-scroll-sticky` | Sticky positioning | `data-scroll-sticky` |
| `data-scroll-target` | Sticky boundary | `data-scroll-target="#section"` |
| `data-scroll-offset` | Trigger offset | `data-scroll-offset="20%"` |
| `data-scroll-repeat` | Repeat detection | `data-scroll-repeat` |
| `data-scroll-call` | Event trigger | `data-scroll-call="myFunction"` |
| `data-scroll-id` | Unique identifier | `data-scroll-id="hero"` |
| `data-scroll-class` | Custom class | `data-scroll-class="is-visible"` |

## Common Patterns

### 1. Basic Smooth Scrolling

```javascript
import LocomotiveScroll from 'locomotive-scroll';

const scroll = new LocomotiveScroll({
  el: document.querySelector('[data-scroll-container]'),
  smooth: true
});
```

```html
<div data-scroll-container>
  <div data-scroll-section>
    <h1>Smooth scrolling enabled</h1>
  </div>
</div>
```

### 2. Parallax Effects

```html
<!-- Slow parallax -->
<div data-scroll data-scroll-speed="0.5">
  Moves slower than scroll (background effect)
</div>

<!-- Fast parallax -->
<div data-scroll data-scroll-speed="3">
  Moves faster than scroll (foreground effect)
</div>

<!-- Reverse parallax -->
<div data-scroll data-scroll-speed="-2">
  Moves in opposite direction
</div>

<!-- Horizontal parallax -->
<div data-scroll data-scroll-speed="2" data-scroll-direction="horizontal">
  Moves horizontally
</div>
```

### 3. Viewport Detection and Callbacks

```javascript
// Track scroll progress
scroll.on('scroll', (args) => {
  console.log(args.scroll.y); // Current scroll position
  console.log(args.speed);    // Scroll speed
  console.log(args.direction); // Scroll direction

  // Access specific element progress
  if (args.currentElements['hero']) {
    const progress = args.currentElements['hero'].progress;
    console.log(`Hero progress: ${progress}`); // 0 to 1
  }
});

// Call events
scroll.on('call', (value, way, obj) => {
  console.log(`Event triggered: ${value}`);
  // value = data-scroll-call attribute value
  // way = 'enter' or 'exit'
  // obj = {id, el}
});
```

```html
<div data-scroll data-scroll-id="hero">Hero section</div>
<div data-scroll data-scroll-call="playVideo">Video section</div>
```

### 4. Sticky Elements

```html
<!-- Stick within parent section -->
<div data-scroll-section>
  <div data-scroll data-scroll-sticky>
    I stick while section is in view
  </div>
</div>

<!-- Stick with specific target -->
<div id="sticky-container">
  <div data-scroll data-scroll-sticky data-scroll-target="#sticky-container">
    I stick within #sticky-container
  </div>
</div>
```

### 5. Programmatic Scrolling

```javascript
// Scroll to element
scroll.scrollTo('#target-section');

// Scroll to top
scroll.scrollTo('top');

// Scroll to bottom
scroll.scrollTo('bottom');

// Scroll with options
scroll.scrollTo('#target', {
  offset: -100,      // Offset in pixels
  duration: 1000,    // Duration in ms
  easing: [0.25, 0.0, 0.35, 1.0], // Cubic bezier
  disableLerp: true, // Disable smooth lerp
  callback: () => console.log('Scrolled!')
});

// Scroll to pixel value
scroll.scrollTo(500);
```

### 6. Horizontal Scrolling

```javascript
const scroll = new LocomotiveScroll({
  el: document.querySelector('[data-scroll-container]'),
  smooth: true,
  direction: 'horizontal'
});
```

```html
<div data-scroll-container>
  <div data-scroll-section style="display: flex; width: 300vw;">
    <div>Section 1</div>
    <div>Section 2</div>
    <div>Section 3</div>
  </div>
</div>
```

### 7. Mobile Responsiveness

```javascript
const scroll = new LocomotiveScroll({
  el: document.querySelector('[data-scroll-container]'),
  smooth: true,

  // Tablet settings
  tablet: {
    smooth: true,
    breakpoint: 1024
  },

  // Smartphone settings
  smartphone: {
    smooth: false, // Disable on mobile for performance
    breakpoint: 768
  }
});
```

## Integration with GSAP ScrollTrigger

Locomotive Scroll and GSAP ScrollTrigger work together for advanced animations:

```javascript
import LocomotiveScroll from 'locomotive-scroll';
import { gsap } from 'gsap';
import { ScrollTrigger } from 'gsap/ScrollTrigger';

gsap.registerPlugin(ScrollTrigger);

const locoScroll = new LocomotiveScroll({
  el: document.querySelector('[data-scroll-container]'),
  smooth: true
});

// Sync Locomotive Scroll with ScrollTrigger
locoScroll.on('scroll', ScrollTrigger.update);

ScrollTrigger.scrollerProxy('[data-scroll-container]', {
  scrollTop(value) {
    return arguments.length
      ? locoScroll.scrollTo(value, 0, 0)
      : locoScroll.scroll.instance.scroll.y;
  },
  getBoundingClientRect() {
    return {
      top: 0,
      left: 0,
      width: window.innerWidth,
      height: window.innerHeight
    };
  },
  pinType: document.querySelector('[data-scroll-container]').style.transform
    ? 'transform'
    : 'fixed'
});

// GSAP animation with ScrollTrigger
gsap.to('.fade-in', {
  scrollTrigger: {
    trigger: '.fade-in',
    scroller: '[data-scroll-container]',
    start: 'top bottom',
    end: 'top center',
    scrub: true
  },
  opacity: 1,
  y: 0
});

// Update ScrollTrigger when Locomotive updates
ScrollTrigger.addEventListener('refresh', () => locoScroll.update());
ScrollTrigger.refresh();
```

## Instance Methods

```javascript
const scroll = new LocomotiveScroll();

// Lifecycle
scroll.init();     // Reinitialize
scroll.update();   // Refresh element positions
scroll.destroy();  // Clean up
scroll.start();    // Resume scrolling
scroll.stop();     // Pause scrolling

// Navigation
scroll.scrollTo(target, options);
scroll.setScroll(x, y);

// Events
scroll.on('scroll', callback);
scroll.on('call', callback);
scroll.off('scroll', callback);
```

## Performance Optimization

1. **Use `data-scroll-section`** to segment long pages:
```html
<div data-scroll-container>
  <div data-scroll-section>Section 1</div>
  <div data-scroll-section>Section 2</div>
  <div data-scroll-section>Section 3</div>
</div>
```

2. **Limit parallax elements** - Too many can impact performance

3. **Disable on mobile** if performance is poor:
```javascript
smartphone: { smooth: false }
```

4. **Update on resize**:
```javascript
window.addEventListener('resize', () => {
  scroll.update();
});
```

5. **Destroy when not needed**:
```javascript
scroll.destroy();
```

## Common Pitfalls

### 1. Fixed Positioning Issues

**Problem**: `position: fixed` elements break with smooth scroll

**Solution**: Use `data-scroll-sticky` instead or add fixed elements outside container:
```html
<!-- Fixed nav outside container -->
<nav style="position: fixed;">Navigation</nav>

<div data-scroll-container>
  <!-- Page content -->
</div>
```

### 2. Images Not Lazy Loading

**Problem**: All images load at once

**Solution**: Integrate with lazy loading:
```html
<img data-scroll data-src="image.jpg" class="lazy">
```

```javascript
scroll.on('call', (func) => {
  if (func === 'lazyLoad') {
    // Trigger lazy load
  }
});
```

### 3. Scroll Position Not Updating

**Problem**: Dynamic content doesn't update scroll positions

**Solution**: Call `update()` after DOM changes:
```javascript
// After adding content
addDynamicContent();
scroll.update();
```

### 4. Accessibility Concerns

**Problem**: Screen readers and keyboard navigation broken

**Solution**: Provide disable option:
```javascript
const prefersReducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

const scroll = new LocomotiveScroll({
  smooth: !prefersReducedMotion
});
```

### 5. Memory Leaks

**Problem**: Scroll instance not cleaned up on route changes (SPAs)

**Solution**: Always destroy on unmount:
```javascript
// React example
useEffect(() => {
  const scroll = new LocomotiveScroll();

  return () => scroll.destroy();
}, []);
```

### 6. Z-Index Fighting

**Problem**: Parallax elements overlap incorrectly

**Solution**: Set explicit z-index on parallax layers:
```css
[data-scroll-speed] {
  position: relative;
  z-index: var(--layer-depth);
}
```

## Related Skills

- **gsap-scrolltrigger**: Advanced scroll-driven animations (use together)
- **barba-js**: Page transitions with Locomotive Scroll integration
- **scroll-reveal-libraries**: Simpler alternative for basic fade-in effects
- **react-three-fiber**: Scroll-driven 3D scenes (sync with Locomotive events)
- **motion-framer**: Alternative scroll animations in React

## Resources

- **Scripts**: `generate_config.py` - Configuration generator, `integration_helper.py` - GSAP integration code
- **References**: `api_reference.md` - Complete API, `gsap_integration.md` - GSAP ScrollTrigger patterns
- **Assets**: `starter_locomotive/` - Complete starter template with examples