Learn more about the principles of page reflow and redraw

In-depth understanding of the mechanism of reflow and redraw requires specific code examples

Reflow and redraw are very important concepts in front-end development. Understanding its mechanism is essential for optimizing the page. Performance and improving user experience are crucial. This article will delve into the mechanics of reflow and redraw and provide corresponding code examples.

Reflow and redraw refer to the process by which the browser updates the layout and style of a web page. When we change the layout or style of an element, the browser recalculates the entire page and redraws the corresponding parts. This process is completed by the browser's rendering engine and consumes a certain amount of computing resources.

Let’s first look at a simple sample code:

HTML:

<div id="box" style="width: 100px; height: 100px;"></div>

JavaScript:

const box = document.getElementById('box');

box.style.width = '200px';
box.style.height = '200px';

In the above code, we get a A div element with a specific style and its width and height changed via JavaScript. This will trigger the browser to reflow and redraw.

When we change the style of an element, the browser will follow the following steps:

1. Generate DOM tree: The browser will parse the HTML code and generate the corresponding DOM tree .
2. Generate Render tree: The browser will generate the corresponding Render tree based on the DOM tree and style information.
3. Reflow: When changing the layout of an element, the browser needs to recalculate and determine the geometric properties of the element. This process is called reflow. The reflow operation will start from the root node, calculate the position and size of each element step by step, and re-determine the structure of the Render tree.
4. Redraw: After determining the geometric properties of the element, the browser needs to redraw based on the information of the Render tree to generate the final page content.

In the above example, when we change the width and height of the div element, the browser will perform reflow and redraw operations. During the reflow process, the browser needs to recalculate and determine the position and size of the box element, and then perform a redraw operation to apply the new style to the element.

Reflow and redraw operations will bring certain performance overhead, especially for complex pages, the cost of reflow and redraw is higher. Therefore, during the development process, we need to minimize the number of reflows and redraws to improve page performance. Here are some tips to reduce reflow and redraw:

1. Use CSS3 animations instead of JavaScript animations: CSS3 animations are more efficient and can be hardware accelerated through the GPU, reducing the overhead of reflow and redraw.
2. Reasonable use of document fragments: When adding a large number of DOM elements to the page, you can first add them to the document fragments, and then add the document fragments to the page at once, which can reduce the page size. Number of reflows.
3. Use cached layout information: When you need to access the layout information of a node multiple times, you can cache the layout information to avoid triggering reflow multiple times.

To summarize, reflow and redraw are key steps for browsers to update web page layout and style. Understanding its mechanics is crucial to optimizing page performance. By properly using techniques such as CSS3 animation, document fragmentation, and caching layout information, we can reduce the number of reflows and redraws and improve page performance. In actual development, we should try to avoid frequently changing the layout and style of elements to reduce the burden on the browser and improve the user experience.

Note: The above code is for example only. In the actual optimization process, you need to choose the appropriate optimization strategy based on the situation of the specific page.

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