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Understanding redraw and reflow: which rendering stage is more affected?

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Release： 2024-01-26 09:43:45

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Understanding redraw and reflow: which rendering stage is more affected?

In front-end development, performance optimization is an important task. When improving web page performance, we often encounter two related concepts: redraw and reflow. Both concepts are related to the rendering phase of web pages, but their impact on performance is different. This article will introduce redraw and reflow in terms of theory and code examples, and discuss in depth which rendering stage is affected more.

First, let’s understand the definitions of redraw and reflow. Redrawing means that when an element's style changes without affecting its layout, the browser will apply the new style to the element and redraw it. Reflow means that when the size, layout or style of an element changes, the browser will recalculate the geometric properties of the element and re-layout the page. Redraw happens after reflow, so reflow triggers redraw.

So, which one has a greater impact on rendering performance, redrawing or reflowing? The answer is reflow. Reflow is more complex than a redraw operation because it requires recalculation of layout information and may cause other related elements to be re-layout. This means that the overhead of reflow is greater and the impact on performance is more obvious.

Below we use specific code examples to illustrate redrawing and reflow and their impact differences.

First, we create a simple HTML structure containing a button and a text box.

<!DOCTYPE html>
<html>
  <head>
    <style>
      .button {
        width: 100px;
        height: 30px;
        background-color: blue;
        color: white;
      }
      .text-field {
        width: 200px;
        height: 30px;
        border: 1px solid black;
        padding: 5px;
      }
    </style>
  </head>
  <body>
    <button class="button">按钮</button>
    <input class="text-field" type="text" placeholder="请输入文本">
  </body>
</html>

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Next, we use JavaScript to change the color of the button. We write two pieces of code respectively, one that only changes the color of the button, and one that changes the color of both the button and the text box.

The code that only changes the color of the button is as follows:

var button = document.querySelector('.button');
button.style.backgroundColor = 'red';

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The code that changes the color of the button and text box at the same time is as follows:

var button = document.querySelector('.button');
var textField = document.querySelector('.text-field');
button.style.backgroundColor = 'red';
textField.style.backgroundColor = 'green';

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Run these two pieces of code, and Use your browser's developer tools to view redraws and reflows.

It can be observed that the code that only changes the color of the button only triggers the redraw operation, while the code that changes the color of the button and text box at the same time not only triggers the redraw, but also triggers the reflow operation. This is because changing the color of both the button and the text box causes their layout to change, so the browser needs to do reflow calculations.

It is obvious from this example that the reflow operation is more expensive than the redraw operation. Therefore, in performance optimization, we should try to reduce the number of reflows as much as possible. A common practice is to use CSS for batch operations, such as changing the CSS class name to modify the styles of multiple elements at once, thereby reducing the number of reflows.

To sum up, redrawing and reflow are both important concepts in the rendering phase, but reflow has a greater impact on performance. In the actual development process, we should try to reduce the number of reflows to improve the rendering performance of the web page.

Summary:

Redrawing means that when the style of an element changes, the browser will redraw the element.
Reflow means that when the size, layout or style of an element changes, the browser will recalculate the geometric properties of the element and re-layout the page.
Reflow is more complex than redrawing and has a more obvious impact on performance.
In performance optimization, the number of reflows should be minimized. You can use CSS to perform batch operations to reduce reflows.

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