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Revealing the principles of CSS reflow and redrawing

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Release： 2024-01-26 09:59:06

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Revealing the principles of CSS reflow and redrawing

Decrypt the working principle of CSS reflow and redraw

Introduction:
In the process of web development, we often hear about CSS reflow (reflow) and redraw. The two concepts of repaint. Understanding how they work is crucial to optimizing web page performance and improving user experience. This article will delve into how CSS reflow and repaint work, and provide specific code examples to help readers better understand these two concepts.

1. How CSS reflow works
1.1 What is CSS reflow
CSS reflow refers to the process in which the browser recalculates the position and size of elements and updates the page layout. When the layout properties of an element on the page change, CSS reflow is triggered.

1.2 Trigger conditions for CSS reflow
The following situations will trigger CSS reflow:

When DOM nodes are added, deleted, or modified;
When the page is changed When the position, size or style of an element;
When the window size is changed;
When the user scrolls the page;
When the browser window changes.

1.3 The process of CSS reflow
The process of CSS reflow is as follows:

When CSS reflow is triggered, the browser will traverse the DOM tree starting from the root node at the top , calculate the position and size of each node;
If the position or size of a node depends on the attributes of its parent node or sibling node, the position and size of these nodes need to be recalculated;
When the positions and sizes of all nodes are calculated, the browser will update the layout of the page.

1.4 How to avoid unnecessary CSS reflow
In order to improve web page performance, we can avoid some unnecessary CSS reflow. The following are several common optimization methods:

Avoid using table layout and try to use CSS layout model;
Avoid frequent DOM operations and try to modify multiple elements at one time;
Use batch style modification to apply style changes to multiple elements at once;
Reduce operations to obtain layout information during reflow, such as offsetLeft, offsetTop, etc.

2. How CSS redrawing works
2.1 What is CSS redrawing
CSS redrawing refers to the process in which the browser redraws the page according to changes in style. When the style attribute of an element on the page changes, a CSS redraw is triggered.

2.2 Trigger conditions for CSS redrawing
The following situations will trigger CSS redrawing:

When the background color, font color, border color and other style attributes of the element are changed;
When adding, deleting, and modifying style sheets;
When changing the visibility of elements.

2.3 The process of CSS redrawing
The process of CSS redrawing is as follows:

When CSS redrawing is triggered, the browser will redraw according to the new style of the element. Draw elements;
The browser will create a bitmap based on the drawn elements and then display it on the screen.

2.4 How to avoid unnecessary CSS redrawing
In order to improve web page performance, we can avoid some unnecessary CSS redrawing. The following are several common optimization methods:

Use class selectors instead of modifying element styles individually;
Merge frequently changing style attributes together and modify them at once;
Use CSS animation instead of JavaScript animation (JavaScript animation will frequently change the style attributes of elements, causing redrawing);
Avoid using CSS expressions.

3. Code Example
The following is a simple code example that demonstrates how to avoid unnecessary CSS reflow and redrawing.

<!DOCTYPE html>
<html>
<head>
  <style>
  .box {
    width: 100px;
    height: 100px;
    background-color: red;
    transition: width 1s;
  }
  </style>
</head>
<body>
  <div class="box"></div>
  <button onclick="changeWidth()">改变宽度</button>
  <script>
    function changeWidth() {
      var box = document.querySelector('.box');
      // 触发一次CSS回流和重绘
      box.style.width = '200px';
    }
  </script>
</body>
</html>

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In the above code, when the button is clicked to change the width of the box, due to the use of the transition attribute, the browser will use CSS animation to transition the width change, thereby triggering CSS only once Reflow and redraw, improved performance.

Conclusion:
This article decrypts the working principles of CSS reflow and redraw in depth and provides specific code examples. By understanding how these two concepts work, we can optimize web page performance and improve user experience. I hope readers can use this knowledge to better develop high-performance web pages.

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