What is the use of reflow and redraw?

Reflow and redrawing are the basis for web page rendering, realizing the dynamic effects of web pages, optimizing the performance of web pages, etc. Detailed introduction: 1. The basis of web page rendering. When a user visits a web page, the browser will build a DOM tree and CSSOM tree based on HTML and CSS codes, then merge them into a rendering tree, and finally reflow and redraw the rendering tree. Convert to pixels on the screen; 2. Achieve dynamic effects on web pages. By modifying the style attributes of elements, you can achieve effects such as animation, interaction, and responsive layout of elements; 3. Optimize the performance of web pages, etc.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

Reflow and redraw are very important concepts in front-end development, and they play a key role in the web page rendering process. This article will introduce the definition, principles and uses of reflow and repaint.

Reflow refers to the process by which the browser calculates the geometric properties (such as position and size) of elements based on the DOM structure and style. When the page layout changes, the browser triggers a reflow operation to recalculate the position and size of elements. Reflow is a relatively slow operation because it requires traversing the entire DOM tree and calculating the geometric properties of each element.

Repaint refers to the process in which the browser draws elements based on their style attributes. When an element's style properties change, the browser triggers a redraw operation to redraw the element's appearance. Redrawing is relatively fast because it only requires updating the element's style properties.

The difference between reflow and redraw is that reflow will cause changes to the page layout, while redraw will only change the appearance of the element. Therefore, reflow is more expensive and consumes more computing resources and time.

So what is the use of reflow and redraw?

First of all, reflow and redrawing are the basis of web page rendering. When a user visits a web page, the browser builds the DOM tree and CSSOM tree based on the HTML and CSS code, then merges them into a rendering tree, and finally converts the rendering tree into pixels on the screen through reflow and redraw operations.

Secondly, reflow and redrawing can achieve dynamic effects on web pages. By modifying the style attributes of elements, we can achieve effects such as animation, interaction, and responsive layout of elements. For example, when a user clicks a button, we can change the button's appearance by modifying the button's style properties to provide feedback to the user.

In addition, reflow and redraw can also optimize the performance of web pages. Since reflow is costly, frequent reflow operations will lead to page performance degradation. Therefore, we should try to reduce the number of reflows as much as possible. A common optimization method is to use the CSS3 transform property to achieve animation effects, because the transform property does not trigger a reflow operation.

In addition, we can also use some tools to monitor and optimize reflow and redraw operations. For example, the Chrome browser provides Performance and Rendering panels that can help us analyze the performance bottlenecks of the page and provide optimization suggestions.

To sum up, reflow and redrawing are very important concepts in the web page rendering process. They are not only the basis for web page rendering, but also can achieve dynamic effects of web pages and optimize the performance of web pages. Therefore, when developing web pages, we should pay attention to reducing the number of reflows and improving the performance of web pages. At the same time, we can also use tools to monitor and optimize reflow and redraw operations to improve the user experience.

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