Analyzing reflow and repaint: exploring their differences and functions

Reflow and redraw: analyze the difference and role of the two

In front-end development, optimizing web page performance is often an important task. Reflow and repaint are two key factors that affect web page performance. This article will analyze the difference between reflow and redraw in detail, and explore their role in optimizing web page performance.

The difference between reflow and redraw
Reflow and redraw both refer to the operation when the browser renders the page, but their difference lies in the scope and impact of the operation.

Reflow, also known as layout calculation, is a process in which the browser needs to recalculate the geometric properties of the element and rearrange the page when the size, position, or layout of the element changes. This can involve a rearrangement of the entire page or part of the page, causing the size and position of other elements to change.

Redrawing, also known as style calculation, is the process by which the browser needs to redraw the element according to the new style when the style of the element changes. This only involves the visual representation of the element, but does not involve the calculation of the element's size, position and other geometric properties.

The difference between reflow and redraw can be briefly summarized as follows:

1. Reflow involves the calculation of page layout, while redraw only involves the calculation of style;
2. Reflow may affect the size and position of other elements, while redrawing only affects the visual performance of the element;
3. The cost of reflow is higher and the layout of the page needs to be recalculated, while the cost of redrawing is relatively low. Only the elements need to be redrawn.

Methods to optimize reflow and redraw
Because the cost of reflow is high, it will lead to a decrease in web page performance, so we need to optimize reflow, and there is relatively little optimization for redraw. The following are several common optimization methods:

1. Batch modification of styles: To avoid frequently changing the style of elements, you can add a CSS class name and then modify the style of the element at once to reduce multiple reflows. number of times.

Sample code:

// 不优化的写法
element.style.width = '100px';
element.style.height = '100px';
element.style.background = 'red';

// 优化的写法
element.className = 'optimized-style';

2. Use document fragments: When you need to create multiple DOM elements, you can use document fragments (DocumentFragment) to add elements in batches to avoid frequent Involves reflow and repaint.

Sample code:

// 不优化的写法
for (let i = 0; i < 1000; i++) {
  const element = document.createElement('div');
  document.body.appendChild(element);
}

// 优化的写法
const fragment = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
  const element = document.createElement('div');
  fragment.appendChild(element);
}
document.body.appendChild(fragment);

3. Use offline DOM: When performing frequent operations on the DOM, you can remove elements from the document, modify them and then put them back into the document middle. This avoids the effects of reflow and redraw.

Sample code:

// 不优化的写法
const list = document.getElementById('list');
for (let i = 0; i < 1000; i++) {
  const item = document.createElement('li');
  list.appendChild(item);
}

// 优化的写法
const list = document.getElementById('list');
const wrapper = document.createElement('div');
for (let i = 0; i < 1000; i++) {
  const item = document.createElement('li');
  wrapper.appendChild(item);
}
list.appendChild(wrapper);

Summary
Reflow and redraw are two key factors that affect the performance of web pages. Understanding their differences and learning to optimize reflow operations can improve the performance of web pages. performance. By modifying styles in batches, using document fragments, and offline DOM optimization methods, you can reduce the number of reflows and improve the performance and response speed of web pages. In actual development, we need to choose appropriate optimization methods based on specific scenarios and needs to ensure the smooth and efficient operation of web pages.

The above is the detailed content of Analyzing reflow and repaint: exploring their differences and functions. For more information, please follow other related articles on the PHP Chinese website!