The effects and precautions of redrawing and reflowing fluid layout in responsive design

The role and precautions of reflow and redraw in responsive design

In modern web design, responsive design is a very important concept. It optimizes the display of web pages on different devices, thereby providing a better user experience. In the process of implementing responsive design, reflow and redraw are two very key concepts. They have a direct impact on web page performance and user experience. This article will discuss the role and considerations of reflow and redraw in responsive design, and give specific code examples.

Reflow means that the browser needs to recalculate the layout and geometric position of the web page to ensure that web page elements are displayed according to the latest styles and attributes. Reflow will cause the entire page to be re-rendered, which is relatively expensive, so we should try to reduce the number of reflows as much as possible. Reflow generally occurs under the following circumstances:

1. Adding, deleting, and modifying DOM elements: When we operate on DOM elements, the browser needs to recalculate the layout of the web page.
2. Modify the style of an element: When we modify the style of an element, such as changing the size, position and other attributes of the element, the browser needs to recalculate the layout of the web page.
3. Change window size: When we change the window size, the browser needs to recalculate the layout of the web page.

The impact of reflow is very large. It will cause the web page to be redrawn. The browser needs to recalculate the position of each element and then redraw it to the screen. This process is time-consuming and can cause page freezes and reduce user experience.

When implementing responsive design, we should try to reduce the number of reflows to improve page performance and user experience. The following are some notes on reducing reflow:

1. Use CSS animations instead of JavaScript animations: CSS animations are drawn by the browser. The browser is better optimized for drawing animations, while JavaScript animations require scripting. Calculating the position for each frame will cause reflow.
2. Use transform to change the position of the element: The transform attribute is executed on the GPU and does not cause reflow, so we can use transform to change the position of the element instead of changing the left and top attributes of the element.
3. Use class to centrally modify the style: If we need to modify multiple style attributes of an element, it is best to use class to centrally modify the style instead of directly modifying the element's style. This can reduce the number of reflows.

The following is a specific code example that demonstrates how to use class to centrally modify styles:

<div id="box" class="red"></div>
<button onclick="changeColor()">Change Color</button>

<style>
    .red {
        background-color: red;
    }

    .blue {
        background-color: blue;
    }
</style>

<script>
    function changeColor() {
        var box = document.getElementById('box');
        box.className = 'blue';  // 使用class来修改样式
    }
</script>

In this example, when the user clicks the button, the changeColor() function will be called , changes the element's style from red to blue. Although using classes to modify styles will also cause reflows, it will reduce the number of reflows, thereby improving page performance and user experience.

Repaint refers to the browser redrawing the style of page elements to the screen without involving layout changes. Redrawing generally occurs under the following circumstances:

1. Modify the style of an element: When we modify the style of an element, such as changing the color, background and other attributes of the element, the browser will redraw the style of the element. to the screen.

When implementing responsive design, the performance impact of redrawing is relatively small, but you also need to pay attention to the following matters:

1. Try to reduce unnecessary redrawing: try to Avoid modifying the styles of elements, especially when the styles of a large number of elements change at the same time, which will cause a large number of redraws and affect the performance of the page.
2. Use CSS3 hardware acceleration: CSS3 hardware acceleration can use the GPU to draw and improve redrawing performance. For example, you can use the transform and opacity properties to implement hardware acceleration.

To summarize, reflow and redraw play a very important role in responsive design. We should try to reduce the number of reflows and use classes to focus on modifying styles to avoid unnecessary redrawing. By optimizing reflow and redraw, you can improve the performance and user experience of your pages.

References:

• https://developers.google.com/web/fundamentals/performance/rendering/avoid-large-complex-layouts-and-layout-thrashing? hl=zh-cn
• https://csstriggers.com/

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