Optimize animations using the will-change property

What is the will-change Property

The will-change is a CSS property used to inform the browser in advance about upcoming changes to an element, allowing the browser to prepare for optimization. This property does not directly affect the rendering of the element but serves as a hinting mechanism to help the browser handle upcoming animations or transformations more efficiently.

.element {
  will-change: transform;
}

When the browser knows that an element is about to change, it can allocate resources in advance, create independent compositing layers, and avoid lag at the start of animations. This optimization is particularly important for mobile devices, as their computational resources are typically more limited than those of desktop devices.

How will-change Works

When a browser renders a page, it goes through multiple stages: style calculation, layout, painting, and compositing. When an element changes, the browser needs to recalculate these steps, a process known as "reflow" or "repaint."

will-change optimizes performance in the following ways:

1. Creates Independent Compositing Layers: The browser promotes elements marked with will-change to separate layers, so only that layer needs repainting during animations, without affecting other parts.

2. Pre-allocates Resources: The browser can allocate GPU resources in advance, reducing delays when animations start.

3. Optimizes Rendering Paths: The browser can choose more efficient rendering paths to handle changes to these elements.

.animated-element {
  will-change: transform, opacity;
  transition: transform 0.3s ease, opacity 0.3s ease;
}

.animated-element:hover {
  transform: scale(1.2);
  opacity: 0.8;
}

When to Use will-change

will-change is most suitable for the following scenarios:

1. Complex Animations: When elements need to perform complex transformations or animations.
2. Frequent Interactions: Elements that users frequently interact with, such as buttons or menus.
3. Parallax Scrolling: Elements with complex animations during page scrolling.
4. Sliders/Carousels: Elements that require smooth transitions frequently.

// Add will-change before animation starts
element.addEventListener('mouseenter', function() {
  this.style.willChange = 'transform';
});

// Remove will-change after animation ends
element.addEventListener('transitionend', function() {
  this.style.willChange = 'auto';
});

How to Use will-change Correctly

Although will-change can improve performance, misuse can have the opposite effect. Here are the correct ways to use it:

1. Enable Only When Needed: Do not apply will-change to many elements or keep it active for long periods.
2. Specify Specific Properties: Clearly inform the browser which properties will change.
3. Remove Timely: Remove will-change after animations complete.

/* Bad practice - too vague */
.element {
  will-change: all;
}

/* Good practice - specify explicitly */
.element {
  will-change: transform, opacity;
}

/* Better practice - add dynamically via JavaScript */
document.querySelector('.animate-me').style.willChange = 'transform';

will-change and Hardware Acceleration

will-change is often used with hardware acceleration, but the two are different:

1. Tricks like transform: translateZ(0) or backface-visibility: hidden can also trigger hardware acceleration.
2. will-change is a more modern, semantic approach.
3. Hardware acceleration forces the creation of new layers, while will-change lets the browser decide intelligently.

/* Traditional hardware acceleration method */
.old-school {
  transform: translateZ(0);
}

/* Modern method */
.modern {
  will-change: transform;
}

Practical Examples of will-change

Example 1: Smooth-Scrolling Navigation Bar

.navbar {
  position: fixed;
  top: 0;
  will-change: transform;
  transition: transform 0.2s ease-out;
}

.navbar.hidden {
  transform: translateY(-100%);
}

/* JavaScript to control show/hide */
window.addEventListener('scroll', function() {
  const navbar = document.querySelector('.navbar');
  if (window.scrollY > lastScrollY) {
    navbar.classList.add('hidden');
  } else {
    navbar.classList.remove('hidden');
  }
  lastScrollY = window.scrollY;
});

Example 2: High-Performance Carousel

.slider {
  will-change: transform;
}

.slide {
  display: inline-block;
  width: 100%;
  transition: transform 0.5s ease;
}

/* JavaScript to control sliding */
function goToSlide(index) {
  const slider = document.querySelector('.slider');
  slider.style.transform = `translateX(-${index * 100}%)`;
}

Limitations of will-change

1. Memory Usage: Each will-change element consumes additional memory.
2. Overuse Can Degrade Performance: The browser may create too many compositing layers.
3. Not All Properties Benefit: Only certain properties like transform and opacity show significant effects.
4. Browser Support: While modern browsers support it, older versions may ignore it.

/* These properties work well with will-change */
.good-candidates {
  will-change: transform, opacity;
}

/* These properties have limited benefits with will-change */
.not-so-good {
  will-change: width, height; /* May trigger reflow */
}

Best Practices for will-change

1. Test Performance Impact: Use DevTools' Performance panel to verify effects.
2. Combine with Other Optimization Techniques: Such as requestAnimationFrame and CSS animations.
3. Avoid Long-Term Retention: Use it only briefly around animations.
4. Consider User Devices: Use more cautiously on low-end devices.

// Best practice example
function animateElement(element) {
  // Inform the browser in advance
  element.style.willChange = 'transform';
  
  // Use requestAnimationFrame for smoothness
  requestAnimationFrame(() => {
    element.style.transform = 'translateX(100px)';
    
    // Clean up after animation
    element.addEventListener('transitionend', function handler() {
      element.style.willChange = 'auto';
      element.removeEventListener('transitionend', handler);
    });
  });
}

Comparing will-change with Other Performance Optimization Techniques

/* Combining multiple techniques */
.optimized-element {
  /* Declare properties that will change */
  will-change: transform, opacity;
  
  /* Hardware acceleration fallback */
  transform: translateZ(0);
  
  /* Use CSS animations */
  animation: fadeIn 0.5s ease-out;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(20px); }
  to { opacity: 1; transform: translateY(0); }
}

Debugging and Performance Analysis for will-change

Use Chrome DevTools to analyze will-change effects:

1. Open the "Rendering" panel and enable "Layer borders" to see which elements are promoted to layers.
2. Use the "Performance" panel to record animation processes and check frame rates.
3. Monitor "Memory" usage to ensure no excessive consumption.

// Debug example: Measure animation performance
function measureAnimation() {
  const element = document.getElementById('animated');
  element.style.willChange = 'transform';
  
  // Start performance recording
  console.time('animation');
  
  element.style.transform = 'translateX(200px)';
  
  element.addEventListener('transitionend', () => {
    console.timeEnd('animation');
    element.style.willChange = 'auto';
  });
}

Applying will-change in Responsive Design

Different devices may require different will-change strategies:

/* Base styles */
.card {
  transition: transform 0.3s ease;
}

/* Enable will-change only on devices capable of handling it */
@media (hover: hover) and (pointer: fine) {
  .card:hover {
    will-change: transform;
    transform: scale(1.05);
  }
}

/* Touch devices may need different optimizations */
@media (pointer: coarse) {
  .slider {
    will-change: transform;
  }
}

Combining will-change with the Web Animations API

The modern Web Animations API can work in tandem with will-change:

const element = document.getElementById('animated');
element.style.willChange = 'transform, opacity';

const animation = element.animate([
  { transform: 'translateX(0)', opacity: 1 },
  { transform: 'translateX(300px)', opacity: 0.5 }
], {
  duration: 1000,
  easing: 'ease-in-out'
});

animation.onfinish = () => {
  element.style.willChange = 'auto';
};