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Performance consumption comparison: Which one consumes more resources, reflow or redraw?

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Release： 2024-01-26 09:31:05

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Performance consumption comparison: Which one consumes more resources, reflow or redraw?

Reflow and redraw: Which one consumes more performance?

In front-end development, performance optimization is an important issue. One of the performance bottlenecks is the browser's reflow and repaint operations. In this article, we'll explore the definitions of reflow and redraw and compare their performance penalties with concrete code examples.

Reflow refers to the process by which the browser recalculates the position and geometric properties of page elements. When the layout changes or the style attributes change, the browser needs to recalculate the position and size of the element. This process is called reflow. Reflow will cause the entire rendering tree to be rebuilt, which is very performance consuming.

Redrawing refers to the process in which the browser redraws elements based on the latest style calculations without affecting the layout. Redrawing does not cause layout changes, it only affects the style of the element. However, the redrawing process still requires traversing and redrawing the elements, so there will also be a certain performance loss.

To better understand the performance difference between reflow and redraw, we will test it with the following code example. Suppose we have a page containing 1000 div elements, each div has a style with class name "box".

<!DOCTYPE html>
<html>
<head>
  <style>
    .box {
      width: 100px;
      height: 100px;
      background-color: red;
      margin: 10px;
    }
  </style>
</head>
<body>
  <div id="container">
    <!-- 1000个div -->
  </div>

  <script>
    // 创建1000个div元素
    for (let i = 0; i < 1000; i++) {
      const div = document.createElement('div');
      div.classList.add('box');
      document.getElementById('container').appendChild(div);
    }

    // 添加一个scroll事件监听器
    window.addEventListener('scroll', function() {
      // 修改样式属性
      document.getElementById('container').style.backgroundColor = 'blue';
    });
  </script>
</body>
</html>

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In the above code, we created 1000 div elements with class "box" and added a scroll event listener. As the page scrolls, we will modify the background color of the container containing these div elements.

Let us observe the performance loss of reflow and redraw through the browser's developer tools.

First, we click on the scroll bar in the page to scroll. On each scroll event, the browser needs to perform a reflow operation to recalculate the position and layout of the elements. Through the browser's renderer tool, we can see how long the reflow operation takes.

Next, we continue to click on the scroll bar to scroll. At this time, we only perform redraw operations. Through the browser's renderer tool, we can see how long the redraw operation takes.

After testing, we can conclude that the reflow operation is more performance-intensive than the redraw operation. Because the reflow operation requires recalculating the position and layout of the element, while the redraw operation only requires redrawing the element's style.

In the actual development process, we should try to avoid frequent reflow operations because it will lead to performance degradation. An optimization method is to use the transform attribute of CSS to replace the change of style attributes, which can reduce the frequency of reflow.

To sum up, reflow and redraw are two important concepts in the browser rendering process. Reflow is more performance-intensive than redrawing because reflow requires recalculating the position and layout of elements. In actual development, we should try to reduce the frequency of reflows to improve page performance.

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