A deep dive into the importance of reflow and redraw in front-end development

[Title] Exploring the key roles of reflow and repaint in front-end development

[Introduction] Reflow and repaint are key roles in front-end development A very important concept that plays a vital role in optimizing web page performance and improving user experience. This article will delve into the definition and reasons of reflow and redraw, combined with specific code examples, so that readers can better understand their key role in front-end development.

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1. Definition of Reflow and Repaint

1. Reflow: When the browser renders the web page, Calculate the position and geometric properties of elements and determine the size and positional relationship of elements in web pages. When the layout of a web page or the size of elements and other information are modified, the browser needs to recalculate the size and position of all elements. This process is called reflow or rearrangement. Reflow causes the browser to redraw the affected portion or the entire page.
2. Repaint: When the style of an element (such as color, background, etc.) changes without affecting its layout or geometric properties, the browser only needs to repaint the element. without the need for reflow.

2. Reasons for reflow and redraw

1. Addition, deletion and modification operations of elements: When we modify DOM elements through JavaScript, such as adding, deleting or modifying elements Style, size, etc., the browser will trigger reflow or redraw.
2. Change of browser window: When the size of the browser window changes, the browser needs to recalculate and layout the elements in the page, so reflow is triggered.
3. Change of text: When the text content changes, the browser needs to recalculate the size of the text elements involved, so reflow will also be triggered.

3. How to avoid excessive reflow and redraw

1. Use document fragments: When you need to frequently modify the DOM tree, you can use document fragments (DocumentFragment) to operate , and then insert the document fragments into the document after the operation is completed to reduce the number of reflows.

// 使用文档碎片示例
var fragment = document.createDocumentFragment();
for (var i = 0; i < 100; i++) {
  var li = document.createElement("li");
  li.innerHTML = "列表项" + i;
  fragment.appendChild(li);
}
document.getElementById("list").appendChild(fragment);

2. Avoid frequent modification of styles:

// 避免频繁修改样式示例
var box = document.getElementById("box");
box.style.width = "200px";
box.style.height = "300px";
box.style.backgroundColor = "red";

// 替代方法，通过修改 class 名称一次性修改多个样式
.box {
  width: 200px;
  height: 300px;
  background-color: red;
}
box.className = "box";

3. Use CSS3 animation instead of JavaScript animation: CSS3 animation uses GPU acceleration, the effect is smoother and reduces backflow and the number of redraws.

/* 使用 CSS3 动画示例 */
@keyframes fade {
  0% { opacity: 0; }
  100% { opacity: 1; }
}

.fade-in {
  animation: fade 1s ease-in;
}

4. Optimize layout structure and style: avoid using unnecessary nesting and complex selectors to reduce the time and resources required for rendering.

4. The impact of reflow and redraw on performance

1. Reflow consumes more performance than redraw: Reflow requires recalculation and layout of the elements in the page, which consumes more of CPU resources.
2. Batch processing of DOM modifications: Merge modifications to the DOM to reduce the number of reflows and redraws.
3. Use the transform attribute of CSS3: The transform attribute will not trigger reflow and redrawing, which can improve the performance of the page.

// 使用 transform 示例
var box = document.getElementById("box");
box.style.transform = "translateX(100px)";

[Summary]

Reflow and redrawing are two concepts that cannot be ignored in front-end development. They have an important impact on web page performance and user experience. By properly optimizing the layout structure, modifying styles, and manipulating the DOM, we can reduce unnecessary reflows and redraws and improve the performance of web pages. I believe that through the introduction and examples of this article, readers will have a deeper understanding of reflow and redrawing, and can use them in actual development to improve the efficiency and performance of front-end development.

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