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Browser rendering process analysis: the impact of redrawing and reflowing

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Release： 2024-01-26 09:32:13

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Browser rendering process analysis: the impact of redrawing and reflowing

What happens after redraw and reflow? In-depth analysis of the browser rendering process,
Requires specific code examples

In web development, it is very important to understand the browser rendering process. The browser rendering process includes two important processes: repaint and reflow. This article will analyze these two processes in detail and provide specific code examples.

First, let’s understand the concepts of redraw and reflow.

Redrawing refers to changing the appearance style of an element, such as modifying the color, background, etc. of the element. Redrawing does not necessarily result in a page layout or recalculation of element positions and sizes, so it is less expensive.

Reflow means that when the layout of elements on the page changes, the position and size of the elements need to be recalculated, such as modifying the width, height, margins, etc. of the elements. Reflow will cause page re-layout, which is relatively expensive.

Next, let us use specific code examples to demonstrate the redraw and reflow process in the browser rendering process.

First, we create a simple web page structure, including a button and a text box:

<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            width: 400px;
            height: 400px;
        }

        .button {
            width: 100px;
            height: 50px;
            background-color: green;
            color: white;
        }

        .input {
            width: 200px;
            height: 30px;
            margin-top: 20px;
        }
    </style>
</head>
<body>
    <div class="container">
        <button class="button" onclick="changeColor()">点击我改变按钮颜色</button>
        <input class="input" placeholder="输入文本内容">
    </div>

    <script>
        function changeColor() {
            document.querySelector('.button').style.backgroundColor = 'red';
        }
    </script>
</body>
</html>

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In the above code, we define the style of a button and a text box, click the button The background color of the button will be modified. Now let us analyze the redraw and reflow processes in the browser rendering process in detail.

After the page is loaded, the browser will parse the HTML, build the DOM tree, build the CSSOM tree in sequence, then merge the two trees into one rendering tree (Render Tree), and finally perform layout and Paint.

When we click the button, the changeColor function is triggered, which triggers the redraw process by modifying the background color of the button. The browser will update the corresponding pixels to display the new color, but it will not relayout the page.

If we modify the changeColor function as follows:

function changeColor() {
    document.querySelector('.button').style.width = '200px';
}

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This time we modify the width of the button, not the background color. At this time, the browser will trigger the reflow process. In addition to the redraw operation, it also needs to recalculate the position and size of the button, as well as the position of the corresponding text box.

In actual web development, we should try to reduce the number of reflows as much as possible, because reflow is a relatively performance-consuming operation. Unnecessary reflows can be avoided through some optimization techniques, such as using the transform attribute instead of modifying the width and height of the element.

To sum up, redrawing and reflow are two very important processes in the browser rendering process. Redrawing is used to change the appearance style of elements, and the overhead is relatively small; reflow requires recalculating the position and size of elements, and causes page re-layout, which is relatively expensive. In web development, we should understand their characteristics and minimize the number of reflows to improve page performance.

(The above code examples are for reference only and may need to be adjusted according to specific circumstances during actual development)

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