CSS `Runtime Computed Style` 分析与 `Style Recalculation` 成本

Alright, gather ’round, CSS aficionados! Welcome to my humble lecture hall. Today’s topic: the fascinating (and sometimes frustrating) world of CSS Runtime Computed Style analysis and Style Recalculation costs. Buckle up, because we’re about to dive deep into the browser’s rendering engine!

(A Dramatic Pause for Effect)

Let’s start with the basics. You write CSS, the browser reads it, and… magic happens, right? Well, not exactly magic. It’s more like a very complex series of calculations.

1. What’s the Deal with Computed Style?

Imagine you have a website with several CSS rules affecting the same element. Which rule wins? How does the browser figure out the actual style applied to that element? That, my friends, is where the Computed Style comes in.

The Computed Style is the final, resolved set of CSS properties that the browser applies to a particular element. It’s the result of all the CSS rules that affect that element, after cascading, specificity, and inheritance have done their thing.

Think of it like a final recipe. You might have several cookbooks (CSS files), each with its own version of a chocolate cake recipe (CSS rules). The Computed Style is the actual recipe you’re going to bake from, taking into account your personal preferences (user agent styles), what’s in your pantry (inherited styles), and the chef’s instructions (author styles).

How to See the Computed Style:

Every browser has developer tools that let you inspect the computed style of an element. In Chrome, for example, you can right-click on an element, select "Inspect," and then go to the "Computed" tab. You’ll see a list of all the CSS properties and their final, calculated values.

Example:

<!DOCTYPE html>
<html>
<head>
<title>Computed Style Example</title>
<style>
body {
  font-family: sans-serif;
  color: black;
}

div {
  font-size: 16px;
  color: blue;
}

.special {
  font-size: 20px;
  font-weight: bold;
}
</style>
</head>
<body>
  <div>This is a regular div.</div>
  <div class="special">This is a special div.</div>
</body>
</html>

If you inspect the first div, you’ll see a Computed Style that includes:

font-family: sans-serif (inherited from the body)
font-size: 16px (defined in the div rule)
color: blue (defined in the div rule)
font-weight: normal (default value)

For the second div (with class special), the Computed Style will be:

font-family: sans-serif (inherited from the body)
font-size: 20px (defined in the .special rule)
color: blue (defined in the div rule)
font-weight: bold (defined in the .special rule)

2. The Dreaded Style Recalculation

Now, here’s where things get interesting (and potentially slow). When anything changes that could affect the Computed Style of an element, the browser has to perform a Style Recalculation.

What kind of "anything"? Glad you asked!

CSS changes: Adding, removing, or modifying CSS rules.
DOM changes: Adding, removing, or modifying HTML elements.
Class changes: Adding or removing classes from elements.
JavaScript manipulations: Directly modifying element styles with JavaScript.
Pseudo-class changes: Hovering, focusing, or activating elements.
Window resizing: Especially if you use media queries.

Think of it like this: every time you add an ingredient to your cake batter, you have to re-mix everything to make sure it’s properly combined. That re-mixing is the Style Recalculation.

Why is Style Recalculation Costly?

Because it involves a lot of work! The browser has to:

Determine which elements are affected: This involves traversing the DOM tree and matching elements against CSS selectors.
Re-calculate the styles: This involves applying the CSS cascade, specificity rules, and inheritance to determine the new Computed Style for each affected element.
Update the rendering tree: The rendering tree is an internal representation of the DOM that the browser uses to paint the page. Changes to the Computed Style require the rendering tree to be updated.

All this takes time and processing power, which can lead to janky animations, slow scrolling, and an overall sluggish user experience.

3. Minimizing Style Recalculation Costs: Practical Tips

Alright, enough doom and gloom! Let’s talk about how to optimize your CSS and JavaScript to minimize the impact of Style Recalculation.

Tip 1: Avoid Unnecessary DOM Manipulations

DOM manipulations are expensive. Adding or removing elements, especially in large quantities, can trigger massive Style Recalculations.

Use documentFragment: When adding multiple elements, create them in a documentFragment first, and then append the fragment to the DOM. This reduces the number of reflows and repaints.

const fragment = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
  const div = document.createElement('div');
  div.textContent = `Div ${i}`;
  fragment.appendChild(div);
}
document.body.appendChild(fragment);

Batch updates: If you need to modify multiple properties of an element, do it all at once instead of in separate steps.

// Bad: Triggers multiple style recalculations
element.style.width = '200px';
element.style.height = '100px';
element.style.backgroundColor = 'red';

// Good: Triggers only one style recalculation
element.style.cssText = 'width: 200px; height: 100px; background-color: red;';

// Or even better:
element.classList.add('new-styles'); // Add a class with all the styles defined in CSS

Tip 2: Targeted CSS Changes

The more elements affected by a CSS change, the more expensive the Style Recalculation will be. Be as specific as possible with your CSS selectors to minimize the scope of the changes.

Avoid overly broad selectors: Don’t use selectors like * or html > body > div unless absolutely necessary.
Use classes: Classes are generally faster to match than complex selectors.
Avoid tag-qualified selectors: div.my-class is generally slower than .my-class.

Tip 3: Read Before You Write (Read Operations Cause Forced Synchronous Layout)

This is a big one! If you read a layout property (like offsetWidth, offsetHeight, scrollTop, etc.) after you’ve made a style change, the browser will often have to perform a forced synchronous layout to give you the correct value. This is extremely expensive.

// Bad: Forced synchronous layout
element.style.width = '200px';
const width = element.offsetWidth; // Forces the browser to recalculate layout *before* painting

// Good: Read first, then write
const width = element.offsetWidth;
element.style.width = '200px'; // This is better, but still can be optimized

//Best: Read them all first, then write them all
const width = element.offsetWidth;
const height = element.offsetHeight;
element.style.width = '200px';
element.style.height = '100px';

Tip 4: Use will-change Sparingly

The will-change CSS property tells the browser that an element is likely to change in the future. This allows the browser to optimize for those changes in advance, potentially improving performance.

However, will-change is a double-edged sword. Overusing it can actually hurt performance, as it forces the browser to allocate extra resources for elements that might not even change.

Use it only when necessary: Don’t apply will-change to every element on your page.
Remove it when done: Once the animation or transition is complete, remove the will-change property.

Tip 5: Debounce and Throttle

If you’re responding to events that fire frequently (like scroll or resize), consider using debouncing or throttling to limit the number of times you update the DOM or CSS.

Debouncing: Delays the execution of a function until after a certain amount of time has passed since the last time the event was fired.
Throttling: Executes a function at most once within a specified time period.

function debounce(func, delay) {
  let timeout;
  return function(...args) {
    const context = this;
    clearTimeout(timeout);
    timeout = setTimeout(() => func.apply(context, args), delay);
  };
}

function throttle(func, limit) {
  let inThrottle;
  return function(...args) {
    const context = this;
    if (!inThrottle) {
      func.apply(context, args);
      inThrottle = true;
      setTimeout(() => inThrottle = false, limit);
    }
  };
}

// Example: Debouncing a resize handler
const debouncedResizeHandler = debounce(() => {
  console.log('Resized!');
  // Perform expensive calculations here
}, 250);

window.addEventListener('resize', debouncedResizeHandler);

Tip 6: Offscreen Content Updates

Updating elements that are not currently visible on the screen (offscreen content) can still trigger style recalculations. If possible, defer updates to offscreen content until it becomes visible.

Tip 7: CSS Containment

The contain CSS property allows you to isolate parts of your website. It tells the browser that changes within a contained element should not affect the rest of the page. This can significantly reduce the scope of style recalculations.

There are several values for the contain property:

contain: none; (default): No containment.
contain: layout;: The element’s layout is independent of the rest of the page.
contain: paint;: The element’s painting is independent of the rest of the page.
contain: size;: The element’s size is independent of its content.
contain: content;: A shorthand for layout paint style.
contain: strict;: A shorthand for size layout paint.

Using contain: content or contain: strict on elements that are likely to change can be very effective.

Tip 8: Use a CSS Profiler

Modern browser developer tools include CSS profilers that can help you identify the most expensive CSS rules and DOM manipulations on your page. Use these tools to pinpoint performance bottlenecks and optimize your code.

Example of Style Recalculation Costs (Simplified):

Let’s say we have this HTML:

<div id="container">
  <div class="item">Item 1</div>
  <div class="item">Item 2</div>
  <div class="item">Item 3</div>
</div>

And this CSS:

#container {
  width: 300px;
}

.item {
  width: 100px;
  float: left;
}

If we change the width of the #container element using JavaScript:

const container = document.getElementById('container');
container.style.width = '400px';

This will trigger a Style Recalculation that affects:

The #container element itself.
All of the .item elements, because their layout depends on the width of the container.

Now, imagine you have hundreds or thousands of .item elements. That single CSS change could trigger a very expensive Style Recalculation.

Simplified Table of Optimization Techniques:

Technique	Description	Benefit
Avoid Unnecessary DOM Manipulations	Minimize adding, removing, or modifying DOM elements.	Reduces the frequency of reflows and repaints.
Targeted CSS Changes	Use specific CSS selectors to minimize the scope of style changes.	Reduces the number of elements that need to be re-styled.
Read Before You Write	Avoid reading layout properties immediately after making style changes.	Prevents forced synchronous layouts.
Use `will-change` Sparingly	Indicate to the browser which elements are likely to change.	Allows the browser to optimize for those changes, but overuse can be detrimental.
Debounce and Throttle	Limit the frequency of DOM updates in response to frequent events.	Prevents excessive style recalculations.
Offscreen Content Updates	Defer updates to elements that are not currently visible on the screen.	Reduces the number of style recalculations performed on elements that are not immediately visible.
CSS Containment	Isolate parts of your website to prevent changes from affecting the entire page.	Limits the scope of style recalculations.
CSS Profilers	Use browser developer tools to identify performance bottlenecks.	Helps pinpoint expensive CSS rules and DOM manipulations.

Conclusion:

Understanding how the browser calculates styles and the costs associated with Style Recalculation is crucial for building performant web applications. By following these tips, you can write more efficient CSS and JavaScript, resulting in a smoother and more responsive user experience.

Remember, profiling is your friend. Don’t guess where the bottlenecks are; use the tools to find them and optimize accordingly.

Now, go forth and write performant code! And may your websites always render smoothly. That concludes our lecture. Questions? (Just kidding, I can’t actually hear you.)

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