Experience sharing: What is the difference between reflow and redraw for web page performance optimization?

Reflow vs. redraw: Which one is better for optimizing web page performance?

When designing and developing web pages, optimizing web page performance is a crucial issue. Web page performance refers to the loading speed and response speed of web pages, which directly affects user experience and website usability. Reflow and repaint are two common operations that have an important impact on web page performance. This article will explore the concepts of reflow and repaint, their differences, and how to optimize web page performance.

First of all, reflow and redraw are two concepts related to web page rendering. Reflow refers to the browser calculating the size and position of each element based on the DOM tree and style, and calculating the layout of the entire page to determine the exact position and size of each element. Redrawing draws elements to the screen based on the latest layout information, including drawing the element's background color, text, borders, etc. Reflow and redraw often occur when the DOM structure or style changes, such as adding, deleting, or modifying the style of elements.

However, reflow and redrawing are expensive and consume a lot of computing resources and time. Reflow is generally more performance-intensive than redrawing because reflow operates on a wider scope and requires recalculation of the entire page layout. Redrawing only requires redrawing the changed elements, which is relatively fast. Therefore, if we want to optimize web page performance, we should minimize the number of reflows.

So how to reduce the number of reflows? The following are some practical suggestions for optimizing web page performance:

1. Avoid frequent DOM operations: DOM operations are one of the main causes of backflow, so frequent DOM operations should be avoided as much as possible. You can reduce the number of reflows by removing the elements that need to be operated from the document flow first, and then re-inserting them into the document flow after the operation is completed.
2. Use CSS3 animations instead of JavaScript animations: CSS3 animations are handled by the browser itself and are usually more efficient than JavaScript animations. You can use CSS3's transform and opacity properties to achieve animation effects and avoid frequent reflow and redrawing.
3. Use virtual DOM technology: Virtual DOM is a lightweight data structure that is similar to the real DOM structure in memory. By comparing the difference between the virtual DOM and the real DOM, DOM operations are minimized, thereby Reduce the number of reflows and redraws. Frameworks such as React provide the implementation of virtual DOM, which can effectively improve web page performance.
4. Reasonable use of CSS3 hardware acceleration: CSS3 hardware acceleration can use the GPU to accelerate the rendering of the page, thereby reducing the cost of reflow and redrawing. Hardware acceleration can be triggered by setting the transform attribute of an element.
5. Use debounce and throttle to optimize event processing: When binding events, you can use the debounce and throttle functions to reduce the frequency of event triggers and reduce the number of reflows. The debounce function can be executed only the last time when events are triggered continuously, while the throttle function can be limited to execution only once within a certain time interval.

In short, reflow and redraw are important reasons for the degradation of web page performance. Optimizing web page performance requires minimizing the number of their occurrences. By reducing DOM operations, using CSS3 animations, using virtual DOM technology, and rationally utilizing hardware acceleration, the loading speed and response speed of web pages can be effectively improved, and a better user experience can be provided.

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