渲染主线程的工作原理 (How the Rendering Main Thread Works): Explanation of How the Main Thread Processes Tasks

渲染主线程的工作原理：解释主线程如何处理任务

The rendering main thread is a critical component of modern web browsers, responsible for processing the tasks necessary to render web pages. Understanding how the main thread works is essential for optimizing web performance and ensuring smooth user experiences. The main thread handles various tasks, including parsing HTML, CSS, and JavaScript, rendering the page, and responding to user interactions.

渲染主线程是现代网页浏览器的重要组成部分，负责处理渲染网页所需的任务。理解主线程的工作原理对于优化网页性能和确保流畅的用户体验至关重要。主线程处理的任务包括解析 HTML、CSS 和 JavaScript，渲染页面，并响应用户交互。

1. Overview of the Main Thread’s Role

主线程角色概述

Single-Threaded Nature

单线程特性

The main thread in a web browser operates in a single-threaded environment. This means that only one task can be executed at a time on the main thread. Despite this limitation, the browser efficiently manages various tasks by queuing them and processing them in a specific order, ensuring that the web page is rendered and interactive as quickly as possible.

网页浏览器中的主线程在单线程环境中运行。这意味着在主线程上一次只能执行一个任务。尽管有这一限制，浏览器通过将各种任务排队并按特定顺序处理来高效地管理它们，确保网页尽快渲染并具有交互性。

Critical Tasks Handled by the Main Thread

主线程处理的关键任务

The main thread is responsible for several critical tasks that directly impact the rendering and interactivity of web pages. These tasks include:

Parsing HTML: Converting the HTML code into a Document Object Model (DOM) tree.
Parsing CSS: Creating the CSS Object Model (CSSOM) tree from CSS code.
JavaScript Execution: Running JavaScript code, including manipulating the DOM and CSSOM.
Style Calculation: Calculating the final styles for each DOM element based on CSS rules.
Layout: Determining the size and position of each element on the page.
Paint: Filling in pixels based on the layout and style information.
Compositing: Combining layers to produce the final visual representation on the screen.

主线程负责多个关键任务，这些任务直接影响网页的渲染和交互性。这些任务包括：

解析 HTML：将 HTML 代码转换为文档对象模型 (DOM) 树。
解析 CSS：从 CSS 代码创建 CSS 对象模型 (CSSOM) 树。
执行 JavaScript：运行 JavaScript 代码，包括操作 DOM 和 CSSOM。
样式计算：根据 CSS 规则计算每个 DOM 元素的最终样式。
布局：确定页面上每个元素的大小和位置。
绘制：根据布局和样式信息填充像素。
合成：组合图层以在屏幕上生成最终的视觉效果。

2. Detailed Breakdown of Main Thread Operations

主线程操作的详细分解

1. HTML Parsing and DOM Construction

HTML 解析与 DOM 构建

When a web page is loaded, the main thread begins by parsing the HTML document. As it parses the HTML, it constructs the DOM tree, which represents the structure and content of the page. Each HTML tag corresponds to a node in the DOM tree. This process is synchronous, meaning that the browser cannot render the page or process user interactions until the DOM tree is fully constructed.

当网页加载时，主线程首先解析 HTML 文档。在解析 HTML 的同时，它构建了 DOM 树，该树表示页面的结构和内容。每个 HTML 标签对应于 DOM 树中的一个节点。这个过程是同步的，这意味着在 DOM 树完全构建之前，浏览器不能渲染页面或处理用户交互。

<!-- Example HTML -->
<!DOCTYPE html>
<html>
<head>
    <title>Sample Page</title>
</head>
<body>
    <h1>Hello, World!</h1>
</body>
</html>

Example: In this example, the browser will create a DOM tree with nodes for <html>, <head>, <title>, <body>, and <h1>.

示例：在此示例中，浏览器将创建一个 DOM 树，其中包含 <html>、<head>、<title>、<body> 和 <h1> 的节点。

2. CSS Parsing and CSSOM Construction

CSS 解析与 CSSOM 构建

Simultaneously with HTML parsing, the main thread parses any linked CSS files or style tags. The CSS rules are then used to create the CSSOM tree, which maps the styles that need to be applied to each element in the DOM tree. The CSSOM tree is built in parallel with the DOM tree, but both must be completed before the browser can proceed to layout and rendering.

与 HTML 解析同时进行的还有任何链接的 CSS 文件或样式标签的解析。然后使用 CSS 规则创建 CSSOM 树，该树映射需要应用于 DOM 树中每个元素的样式。CSSOM 树与 DOM 树并行构建，但在浏览器进行布局和渲染之前，必须完成两者。

/* Example CSS */
body {
    font-family: Arial, sans-serif;
}

h1 {
    color: blue;
}

Example: The browser will create a CSSOM tree with styles for the <body> and <h1> elements.

示例：浏览器将为 <body> 和 <h1> 元素创建一个 CSSOM 树。

3. JavaScript Execution

JavaScript 执行

JavaScript can manipulate both the DOM and CSSOM, so it has a significant impact on the rendering process. When the browser encounters a <script> tag, it pauses the construction of the DOM and CSSOM trees to execute the JavaScript code. This is why JavaScript that manipulates the DOM or CSSOM directly can block rendering, leading to potential performance issues.

JavaScript 可以操作 DOM 和 CSSOM，因此它对渲染过程有重大影响。当浏览器遇到 <script> 标签时，它会暂停 DOM 和 CSSOM 树的构建以执行 JavaScript 代码。这就是为什么直接操作 DOM 或 CSSOM 的 JavaScript 可能会阻塞渲染，从而导致潜在的性能问题。

// Example JavaScript
document.querySelector('h1').style.color = 'red';

Example: This script changes the color of the <h1> element to red, which affects how the page is rendered.

示例：此脚本将 <h1> 元素的颜色更改为红色，这会影响页面的渲染方式。

4. Style Calculation

样式计算

Once the DOM and CSSOM trees are complete, the main thread calculates the final styles for each element. This process involves resolving all the CSS rules that apply to each element, including inherited and overridden styles. The result is a set of computed styles that determine the appearance of each element on the page.

一旦 DOM 和 CSSOM 树完成，主线程就会计算每个元素的最终样式。这个过程涉及解析应用于每个元素的所有 CSS 规则，包括继承和覆盖的样式。结果是一组计算样式，决定了页面上每个元素的外观。

5. Layout

布局

After calculating styles, the main thread performs the layout phase. In this phase, the browser calculates the position and size of each element on the page. The layout process considers factors such as the element’s dimensions, margins, padding, and the content it contains. The result is a layout tree that represents the geometric information of the elements.

在计算样式之后，主线程执行布局阶段。在此阶段，浏览器计算页面上每个元素的位置和大小。布局过程考虑了元素的尺寸、边距、填充和包含的内容等因素。结果是一个布局树，表示元素的几何信息。

6. Paint

绘制

The paint phase involves filling in the pixels for each element based on the layout tree and the computed styles. The main thread sends draw commands to the GPU, which will ultimately handle the pixel rendering. This phase determines the visual appearance of the elements, such as colors, borders, text, and images.

绘制阶段涉及根据布局树和计算样式填充每个元素的像素。主线程将绘制命令发送到 GPU，后者最终将处理像素渲染。此阶段决定了元素的视觉外观，例如颜色、边框、文本和图像。

7. Compositing

合成

In the compositing phase, the browser combines all the layers that have been painted to produce the final image that is displayed on the screen. Each layer may be rendered independently and then composited together. Compositing is crucial for rendering complex web pages efficiently, especially those with overlapping elements, animations, or transformations.

在合成阶段，浏览器将已绘制的所有图层组合在一起，以生成最终显示在屏幕上的图像。每个图层可能独立渲染，然后合成在

一起。合成对于高效渲染复杂网页（尤其是包含重叠元素、动画或转换的网页）至关重要。

3. Performance Considerations for the Main Thread

主线程的性能考虑

Avoid Long-Running JavaScript

避免长时间运行的 JavaScript

Long-running JavaScript code can block the main thread, preventing the browser from rendering the page or responding to user interactions. To avoid this, break up large tasks into smaller ones or use web workers to offload tasks to background threads.

长时间运行的 JavaScript 代码可能会阻塞主线程，阻止浏览器渲染页面或响应用户交互。为避免这种情况，可以将大任务分解为较小的任务，或使用 Web Worker 将任务卸载到后台线程。

Optimize CSS for Faster Rendering

优化 CSS 以加快渲染速度

Complex or deeply nested CSS rules can slow down the style calculation and layout phases. Simplify CSS rules, avoid excessive specificity, and minimize reflows and repaints to improve rendering performance.

复杂或深层嵌套的 CSS 规则可能会减慢样式计算和布局阶段的速度。简化 CSS 规则，避免过度特异性，并最小化回流和重绘，以提高渲染性能。

Use Asynchronous JavaScript Loading

使用异步 JavaScript 加载

Loading JavaScript asynchronously using async or defer attributes allows the browser to continue parsing and rendering the page while the script is being loaded. This reduces the likelihood of blocking the main thread.

使用 async 或 defer 属性异步加载 JavaScript，允许浏览器在加载脚本时继续解析和渲染页面。这减少了阻塞主线程的可能性。

4. Conclusion

结论

The rendering main thread in a web browser is responsible for a wide range of tasks that are critical to displaying and interacting with web pages. Understanding how the main thread processes these tasks—from parsing HTML and CSS to executing JavaScript and rendering the final page—can help developers optimize their code for better performance and responsiveness. By minimizing blocking tasks and optimizing resource loading, web developers can ensure that their pages load quickly and provide a smooth user experience.

网页浏览器中的渲染主线程负责广泛的任务，这些任务对于显示和与网页的交互至关重要。了解主线程如何处理这些任务——从解析 HTML 和 CSS 到执行 JavaScript 以及最终渲染页面——可以帮助开发人员优化代码，以提高性能和响应能力。通过最小化阻塞任务和优化资源加载，网页开发人员可以确保他们的页面加载迅速并提供流畅的用户体验。

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