Performance analysis: Comparison of consumption between reflow and redraw

Performance consumption: Comparative analysis of reflow and redraw, specific code examples are required

Foreword:
In web development, performance optimization has always been an important topic. During the web page rendering process, the most common performance consumption is reflow and repaint. This article will conduct a detailed comparative analysis of reflow and redraw, and give specific code examples to help readers better understand and optimize performance.

1. Explanation of the concepts of reflow and redraw
Reflow and redraw refer to two important processes when the browser renders a web page.

1. Reflow:
   Reflow refers to the process by which the browser recalculates the layout of the web page when the DOM changes (such as element position, size, content, etc.). Reflow is a very performance-intensive operation because it causes the entire page to be re-rendered.
2. Repaint:
   Repaint refers to the process in which the browser redraws this part of the content when part of the web page (such as color, background, etc.) changes. Compared to reflow, redrawing has a smaller performance cost because it only affects the re-rendering of part of the page.

2. The difference between reflow and redraw
Reflow and redraw have the following differences:

1. Scope of influence:
   Reflow will cause the entire Re-rendering of the page, and re-rendering will only affect the re-rendering of part of the page.
2. Overhead size:
   Reflow is a very performance-intensive operation because it requires recalculation of the layout of the entire page, and the performance consumption of redrawing is small.
3. Trigger conditions:
   The trigger conditions for reflow are more complicated than redrawing, including changes in the position, size, content and other factors of the element, while redrawing only requires changes in the appearance attributes of the element (such as color, background) wait).

3. Comparison of examples of reflow and redraw
In order to better understand reflow and redraw, two specific code examples are given below.

Example 1:

<div id="box" style="width: 100px; height: 100px; background-color: red;"></div>
<script>
    var box = document.getElementById('box');
    box.style.width = '200px';
    box.style.height = '200px';
</script>

In the above example, when the JavaScript code changes the width and height of the box element, the browser will trigger a reflow operation because the position and size of the element have changed. This will cause the entire page to be re-rendered, including all parts related to the box element.

Example 2:

<div id="box" style="width: 100px; height: 100px; background-color: red;"></div>
<script>
    var box = document.getElementById('box');
    box.style.backgroundColor = 'blue';
</script>

In the above example, when the JavaScript code changes the background color of the box element, the browser will trigger a redraw operation because only the appearance attribute of the element has changed. And the layout has not changed. This will only cause the box element to be re-rendered and will not affect the re-rendering of the entire page.

It can be seen from the comparison of the above two examples that the performance consumption of reflow is greater than the performance consumption of redrawing. Therefore, in actual work, the number of reflows should be reduced as much as possible to improve the performance of web pages.

4. How to reduce the number of reflows and redraws
In order to improve the performance of web pages, we can take the following measures to reduce the number of reflows and redraws:

1. Batch DOM operations:
   Combine multiple operations into one operation to reduce the number of reflows. For example, use document fragments to reduce multiple reflows caused by the addition and deletion of DOM nodes.
2. Use CSS animations instead of JavaScript animations:
   CSS animations generally perform better than JavaScript animations because it only triggers redraws and not reflows. Try to use CSS animations to achieve dynamic effects on the page.
3. Use transform and opacity attributes:
   Changes to transform and opacity attributes will only trigger redrawing, not reflow. Try to use these two properties to change the appearance of the element.
4. Avoid attributes that trigger layout changes:
   Avoid using attributes that trigger reflow, such as offsetTop, offsetLeft, etc. You can use the offsetHeight and offsetWidth properties to get the dimensions of an element without triggering reflow.

Conclusion:
Reflow and redraw are common performance optimization issues in web development. A deep understanding of the difference between reflow and redraw, and taking corresponding optimization measures, can significantly improve the performance of web pages. Through reasonable code writing and optimization methods, we can minimize the number of reflows and improve the rendering efficiency of web pages.

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