Differences and interrelationships in implementation between reflow and redraw

Reflow and redraw are two concepts that are often mentioned in front-end development, and they are closely related to web page performance. This article will provide a detailed introduction in terms of definitions, differences and connections, as well as specific code examples.

1. Definition
Reflow refers to the process in which the browser determines the size and position of each element based on the style information after obtaining the rendering tree, and finally generates the web page layout. When the size or position of an element changes or an element is added or deleted, the browser will perform a reflow operation.

Repaint refers to the process of rendering a new layout to the screen based on reflow. When the style of an element is modified without affecting its layout, the browser will redraw it.

2. Differences and connections
The difference between reflow and redraw is mainly reflected in the following aspects:

1. Trigger conditions: The reflow operation is triggered by changes in layout , and the redraw operation is triggered by changes in style.
2. Scope of impact: The reflow operation involves the recalculation and layout of the element and its sub-elements, while the redraw operation only involves the visual update of the element.
3. Cost: Since the reflow operation involves changes to the entire layout, its cost is higher than the redraw operation, so excessive reflow operations should be avoided as much as possible.

Although there are obvious differences between reflow and redraw, they also have a certain connection:

1. Trigger sequence: During the page rendering process, the reflow operation will definitely occur during the redraw process. Occurs before the drawing operation. Because the layout of the elements needs to be determined before they can be redrawn.
2. Combined operation: Since reflow and redraw have different costs, in some cases, the browser will combine multiple reflows into one, or combine reflow and redraw operations into one, thereby improving performance. .

3. Specific code examples

1. Operation that triggers reflow:

// 修改元素的样式属性
element.style.width = '100px';

// 修改元素的尺寸
element.style.height = '200px';

2. Operation that triggers redraw:

// 修改元素的颜色属性
element.style.color = 'red';

// 修改元素的文本内容
element.innerText = 'Hello World';

It should be noted that in order to avoid excessive reflow and redraw operations, we can use the following optimization techniques:

1. Use the transform attribute of CSS3 to realize the displacement of elements, Rotation and other operations, because transform will not trigger the reflow operation.
2. Combine operations that require multiple style modifications into one to avoid triggering frequent reflow and redrawing.
3. Avoid using JavaScript to operate DOM elements during the layout process. You can modify the style by modifying the CSS class name.

Summary: Reflow and redrawing are performance issues that need to be paid attention to in front-end development. Understanding their differences, connections, and practical applications can help us optimize the rendering performance of web pages and improve user experience. By reducing the number of reflow and redraw operations, the re-rendering of the page can be effectively reduced and the response speed of the web page can be improved.

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