Redraw and reflow: parsing which rendering phase is more critical?

Redraw and reflow: parsing which rendering phase is more critical?

In web development, the rendering stage is a process that cannot be ignored. In the rendering stage, the two concepts of "redraw" and "reflow" are also very important. Understanding their differences and impacts is critical to optimizing web page performance and user experience. This article will analyze in detail the concepts of redraw and reflow and their differences in the rendering process, and combine them with specific code examples to illustrate their principles and impact.

1. The concept of redrawing and reflow

1. Repaint: When the appearance of an element is changed but the layout is not changed, the browser will redraw the element. Appearance, this process is called repainting. Redrawing does not affect the layout of other elements.
2. Reflow: When the layout attributes of an element change and affect the calculation of its geometric size, the browser needs to recalculate the layout of the element and re-render. This process is called reflow. Reflow affects the layout and rendering of other elements.

2. The difference between redrawing and reflow

1. Scope of influence: Redrawing will only redraw the appearance of the element and will not affect the layout of other elements; reflowing The layout of elements will be recalculated and re-rendered, which will affect the layout and rendering of other elements.
2. Performance consumption: The performance consumption of redrawing is relatively small, because it only changes the appearance of the element; the performance consumption of reflow is large, because it needs to calculate the layout and re-rendering of the element.

3. Influencing factors and sample code

1. Modify the style attributes of the element: Modifying the color, background, font and other style attributes of the element will only trigger redrawing. For example:

var element = document.getElementById('demo');
element.style.color = 'red'; // 只触发重绘，不会触发回流

2. Modify the size of the element: Modifying the width, height, position and other layout properties of the element will trigger reflow. For example:

var element = document.getElementById('demo');
element.style.width = '200px'; // 会触发回流

3. Get some attributes of the element: getting the offset, size and other attributes of the element will also trigger reflow. For example:

var element = document.getElementById('demo');
var width = element.offsetWidth; // 获取元素宽度，会触发回流

4. Optimization strategy

1. Avoid frequent modification of style attributes: merge multiple modifications of style attributes into one operation to reduce the number of redraws and reflows.
2. Use class instead of inline style: Concentrate the style in the class, and change the element style by modifying the class instead of directly modifying the inline style.
3. Use document fragments for batch insertion: By using document fragments, multiple element insertion operations are combined into one operation, reducing the number of reflows.
4. Use transform to replace attributes such as top and left: The transform attribute will not trigger reflow and can be used to replace attributes such as top and left of elements that require animation effects.

Summary:

In web development, we need to pay attention to the impact of redrawing and reflow on performance. Although the impact of redrawing is small, reflow is expensive, especially when layout properties are frequently modified or a large number of DOM elements are manipulated. Understanding the concepts, differences, and related optimization strategies of redrawing and reflowing can help us better improve web page performance and user experience.

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