Redraw vs. reflow: which one has more critical impact on the rendering phase?

The impact of redraw and reflow on the rendering phase: who is more important?

When a web page is rendered, the browser will perform a series of operations in a certain order to display the page content. Among them, redrawing and reflow are two important steps in the rendering process. This article will explore the impact of redraw and reflow on the rendering phase and analyze their importance.

1. The meaning and difference of redraw and reflow
   Before understanding the impact of redraw and reflow on rendering, let's first understand their meaning and difference.

Repaint means that when the style of an element changes but does not affect its layout, the browser redraws the element. Redrawing does not cause the layout of the page to change, it just redraws the appearance of the elements.

Reflow means that when the layout properties of an element change, the browser recalculates the geometric properties of the element and then layouts it. Reflow will cause the entire rendering tree to be rebuilt, affecting page layout.

2. Redraw and reflow performance considerations
   During the web development process, we should try to reduce the number of redraws and reflows to improve performance and user experience. While both have an impact on the rendering stage, their importance is not the same.

Redrawing has less impact on rendering than reflowing. Because redrawing is just a redrawing of the element's appearance and does not require recalculation of the layout, its overhead is relatively small. When an element's style changes, the browser can quickly redraw it.

Reflow has a greater impact on rendering because it triggers the rebuilding and layout calculation of the entire rendering tree. When a page reflows, the browser needs to recalculate the layout attributes of the elements and re-execute the rendering process, which consumes more time and resources.

So, from a performance perspective, we should try to avoid triggering too many reflow operations, while relatively few redraw operations will have less impact on performance.

3. Code Example
   The following uses some specific code examples to demonstrate the impact of redrawing and reflow on the rendering phase.

Example 1: Frequently changing element styles

const box = document.querySelector('.box');

// 频繁改变元素样式
for (let i = 0; i < 1000; i++) {
  box.style.color = 'red';
  box.style.backgroundColor = 'blue';
}

In this example, we frequently change the style of an element. Since the change in style only triggers a redraw operation and does not involve changes in layout, the entire rendering process is relatively fast.

Example 2: Triggering a large number of reflows

const container = document.querySelector('.container');

// 触发大量回流
for (let i = 0; i < 100; i++) {
  container.style.width = i + 'px';
  container.style.height = i + 'px';
}

In this example, we continuously change the width and height of a container element. Since this involves layout changes, the browser needs to perform a large number of reflow operations, which affects rendering performance.

To sum up, both redraw and reflow have an impact on the rendering phase, but from a performance perspective, reflow has a greater impact. Therefore, during the development process, we should try to reduce frequent reflow operations and avoid triggering too many layout changes to improve the rendering performance and user experience of the page.

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