Deep understanding of the mechanics of CSS layout recalculation and rendering

CSS reflow and repaint are very important concepts in web page performance optimization. When developing web pages, understanding how these two concepts work can help us improve the response speed and user experience of the web page. This article will delve into the mechanics of CSS reflow and repaint, and provide specific code examples.

1. What is CSS reflow?
When the visibility, size, or position of elements in the DOM structure changes, the browser needs to recalculate and apply CSS styles, and then relayout the page. This process is called reflow. Reflow will affect the rendering of relevant nodes in the entire DOM tree, which will have a greater impact on performance.

Common operations that trigger reflow include:

1. Change the window size
2. Change the position or size of the element
3. Change the content of the element
4. Add or delete DOM elements
5. Change the browser's default font size

2. What is CSS repaint?
When the style of an element changes, but does not affect its layout, the browser will redraw, that is, update the visible appearance of the element. Redrawing does not require relayout, so the overhead is less than reflowing. But frequent redrawing will also affect web page performance.

Common operations that trigger redrawing include:

1. Change the background color, font color, etc. of elements
2. Change certain CSS properties, such as border and box-shadow Wait

3. How to optimize reflow and redraw?

1. Use CSS3 animation instead of JavaScript animation. The advantage of CSS3 animation is that it can be optimized at the GPU level, reducing the cost of reflow and redrawing.
2. Avoid frequent DOM operations, try to change multiple attributes at once, or use document fragments (DocumentFragment) for operations.
3. To set elements that need to be redrawn multiple times as a layer, you can use the CSS will-change attribute or use transform: translateZ(0).
4. Avoid using table layout because table layout requires a lot of reflow operations.
5. Use CSS transform to replace traditional animation effects, such as using translate to replace attributes such as top and left.
6. Avoid using JavaScript to obtain layout information. If you need to obtain the position, size and other information of elements, you can obtain it through the API provided by CSSOM.

Here are some specific code examples:

// Operations that trigger reflow
element.style.width = '100px';
element.style.height = '200px';

// Trigger redraw operation
element.style.color = 'red';

// Use CSS3 animation
.element {
transition: transform 1s;
}

.element:hover {
transform: scale(1.2);
}

// Use layers to improve performance
.element {
will-change: transform;
}

.element {
transform: translateZ(0);
}

By understanding CSS reflow With the redrawing mechanism, we can better optimize web page performance, reduce user waiting time, and improve user experience. During the development process, we should try to reduce the number of reflows and redraws as much as possible, and try to use appropriate methods and techniques to achieve page effects.

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