What is the difference between redrawing and reformatting

Redraw and reflow are two concepts often encountered in front-end development. They are very important for understanding performance optimization and page rendering process. This article explains the difference between redrawing and reflowing, and provides some concrete code examples.

1. Repaint

Repaint refers to the process of redrawing an element when its appearance changes without affecting its layout. For example, change the background color, font color, etc. of an element. Redrawing does not cause page layout or recalculation of the position and size of elements, so the performance overhead is small.

Code example:

// 改变元素的背景色
element.style.backgroundColor = 'blue';

// 改变元素的字体颜色
element.style.color = 'red';

The above code will only trigger the redrawing of the element and will not cause the relayout or calculation of other parts of the page.

2. Reflow

Reflow refers to the process triggered when the page layout changes and the position and size of elements need to be recalculated. For example, operations such as adding, deleting, modifying the structure of elements, and changing the size of elements will trigger reflow. Reflow is much more expensive than redrawing because reflow causes the rearrangement and calculation of other elements.

Code example:

// 修改元素的宽度和高度
element.style.width = '200px';
element.style.height = '200px';

// 添加一个新的元素
var newElement = document.createElement('div');
document.body.appendChild(newElement);

The above code will cause the page to reflow because the size of the element is changed and new elements are added.

3. The relationship between redrawing and reflow

Redrawing and reflow are related to each other. Reflow will inevitably cause redrawing, because after the position, size, etc. of the element change, its appearance also changes. Changes will occur. But redrawing does not necessarily trigger reflow because redrawing does not involve the layout of the element.

In order to optimize page performance and reduce unnecessary reflows and redraws, the following strategies can be adopted:

1. Avoid frequent changes to element styles: If you need to modify multiple style attributes at one time, It's better to use CSS class names to control styles rather than modifying an element's style properties directly.
2. Use document fragment (DocumentFragment): In DOM operation, first use the document.createDocumentFragment() method to create a document fragment, add the elements that need to be added to the document fragment, and finally add the document fragment at once into the DOM, reducing the number of reflows.
3. Use local scope and reasonable style structure: Avoid using global styles or overly complex selectors, which can reduce style cascading and calculations and improve rendering performance.

Summary:

Redraw and reflow are very important concepts in page rendering and are crucial for optimizing page performance. Understand the difference between redraw and reflow, and avoid unnecessary reflow and redraw operations, which can effectively improve the rendering speed and user experience of the page. During the development process, you need to make a reasonable choice to use redraw or reflow based on specific scenarios.

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