Understand web page performance optimization methods of reflow and redraw and their application scenarios

Webpage performance optimization: differences and application scenarios between reflow and redraw

With the rapid development of the Internet, webpage performance optimization has become an important link that cannot be ignored. Improving the performance of web pages can not only improve user experience, but also reduce server load and reduce maintenance and operating costs. In web page performance optimization, reflow and repaint are two common and key concepts. In this article, we’ll dive into the differences between reflow and repaint and their use cases.

Reflow and redrawing are the two main processes of web page rendering. When the elements in the web page change, the browser will recalculate the layout of the elements and redraw the page. This is reflow and redraw. However, there is a difference between reflow and repaint, and understanding the difference is crucial to improving the performance of your web pages.

First of all, reflow refers to the layout change of an element, which affects the position and size of other elements. Reflow is an expensive operation because it requires the browser to recalculate the entire render tree and then adjust the position and size of elements. The cost of reflow is that it requires the browser to redraw part or all of the page, which consumes a lot of computing resources. Therefore, reducing the number of reflows is crucial to improving web page performance.

In contrast, redrawing means that the appearance of an element changes, but it does not affect the layout of other elements. Redraw only requires the browser to redraw the affected portion without recalculating the layout of the element. Redrawing is much less expensive than reflowing because it does not require calculations on the entire render tree. Therefore, when you need to modify the style of an element, try to avoid unnecessary layout changes to the element, which can effectively reduce the number of reflows and improve web page performance.

After understanding the difference between reflow and redrawing, we can apply corresponding optimization strategies according to different scenarios to improve the performance of web pages.

First of all, when we need to make layout changes to multiple elements, try to merge these operations together. Because the cost of reflow is high, if we make separate layout changes to multiple elements, the browser will perform the reflow operation multiple times, thus increasing performance consumption. Combining multiple operations into one operation can reduce the number of reflows and improve performance.

Secondly, when modifying element styles, you can use CSS3 animations instead of JavaScript operations. CSS3 animations can improve the performance of animations through GPU acceleration because it avoids reflow operations and only requires redrawing the affected elements. In contrast, using JavaScript operations to modify element styles causes frequent reflows and redraws, which degrades performance.

In addition, when rendering large amounts of data, you can consider using virtual lists or lazy loading to optimize performance. Virtual lists are a technique that only renders part of the data that is currently visible, rather than all of it, thus reducing the number of reflows and redraws. Lazy loading means loading images or other resources when the page scrolls to a specific position. This method can reduce the number of reflows and redraws during the first load and improve the loading speed of the page.

To sum up, reflow and redraw are two key concepts in web page performance optimization. Understanding their differences and applying corresponding optimization strategies according to different scenarios can effectively improve the performance of web pages. By reducing the number of reflows and redraws, properly optimizing layout and style modification operations, and using appropriate techniques to render large amounts of data, we can improve the loading speed of web pages and provide a better user experience.

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