Triggering reflows and repaints: why are they important?

Reflow and repaint: why are they important?

With the development of the Internet, more and more people are browsing the web and using mobile applications online. For developers, how to improve the performance of web pages and applications has become one of the important topics. In the process of optimizing these applications, reflow and redrawing are two aspects that must be focused on. This article will detail the concepts of reflow and redraw and why they are so important for performance optimization.

Reflow and redrawing are key steps for the browser rendering engine to display the page. Reflow refers to the process when the rendering engine discovers that the size, position, or layout of a certain part has changed, causing the entire page or part of the page to be recalculated and drawn. Redrawing means that when the style of a certain part (such as color, background, etc.) changes, the rendering engine only needs to redraw that part without recalculating the position and layout.

Reflow and redraw are relatively performance-consuming operations, so the number of times they occur should be minimized during the development process. Frequent reflows and redraws will cause page freezes and delays, thereby affecting user experience. The following will introduce some common situations that easily cause reflow and redraw.

1. Modify the layout of the page: When the page layout changes, the rendering engine needs to recalculate the position and size of all elements in the page, which will cause reflow. For example, modifying CSS property values ​​such as width, height, margin, padding will cause reflow. In order to reduce the number of reflows, you can use the transform and opacity properties for animation effects, which will not cause reflows.

const element = document.getElementById("example");
element.style.width = "200px";
element.style.height = "200px";
element.style.margin = "10px";

2. Modify style attributes: When modifying the style attributes of an element, such as color, font, etc., a redraw operation will be triggered. For example, modifying CSS properties such as background-color, color, font-size, etc. will cause redrawing to occur. In order to reduce the number of redraws, you can use the transition and animation properties of CSS3 to make the style change smoother.

const element = document.getElementById("example");
element.style.backgroundColor = "red";
element.style.color = "white";
element.style.fontSize = "20px";

In addition to the above situations, there are some other operations that will also cause reflow and redrawing, such as modifying or obtaining the geometric properties of the element (such as offsetLeft, offsetWidth, etc.), change the window size, scroll the page, etc. Therefore, during the development process, we should try to avoid performing these operations frequently, or reduce the number of reflows and redraws by optimizing algorithms and designs.

In order to better optimize page performance, we can use some tools to detect the occurrence of reflow and redraw, such as Performance and Paint Profiler in the Chrome browser's developer tools. Through these tools, we can observe the impact of each operation, find out which code causes reflow and redrawing, and then make targeted optimizations.

Reflow and redraw are key steps in the browser rendering engine and play an important role in page performance optimization. Properly handling reflow and redraw issues can increase page rendering speed and improve user experience. Therefore, developers should try to avoid frequently triggering reflow and redraw when writing code, and reasonably optimize layout and style to improve application performance and user satisfaction.

The above is the detailed content of Triggering reflows and repaints: why are they important?. For more information, please follow other related articles on the PHP Chinese website!