Optimizing Web Page Performance: A Guide to Choosing and Practicing Reflow, Repaint, and Reflow

Web page performance optimization guide: selection and practice of reflow, redraw and reflow

With the rapid development and popularity of the Internet, web page performance optimization has become more and more important. an increasingly important issue. A high-performance web page can improve user experience, reduce loading time, and help improve web page rankings. When optimizing web page performance, we often need to face the three concepts of reflow, repaint and layout. This article will provide an in-depth discussion of these three concepts and give specific code examples to help developers choose the appropriate optimization solution.

1. What are reflow, redraw and reflow?

Reflow refers to the process of the browser recalculating the layout of the web page. When the position, size, or style of a web page element changes, the browser triggers a reflow operation. Reflow is a very expensive operation because it involves recalculating the layout of the entire web page. Therefore, frequent reflows can lead to reduced web page performance.

Redrawing refers to the process of the browser redrawing the web page. When the style of a web page element changes, the browser triggers a redraw operation. Redrawing is less expensive than reflowing because it only involves redrawing part of the web page.

Reflow is a combined operation of reflow and redraw. When the position, size, or style of a web page element changes, the browser triggers a reflow operation. Reflow includes the process of reflowing and redrawing, so its overhead is the largest.

2. How to avoid frequent reflow, redraw and reflow?

In order to optimize web page performance, we need to avoid frequent reflows, redraws and reflows. The following are some commonly used optimization tips:

• Use absolute positioning or fixed positioning: Absolutely positioned or fixedly positioned elements will not affect other elements, so their changes will not trigger reflow and reflow. operate.
• Avoid using table layout: Table layout will cause the structure of the web page to be complex, thereby increasing the number of reflows and reflows.
• Batch operation of DOM elements: Combine multiple operations on DOM elements into one, which can reduce the number of rearrangements and reflows. For example, use DocumentFragment to batch insert multiple DOM elements.
• Use transform for animation effects: Use transform or opacity to achieve animation effects, which can reduce the number of rearrangements and redraws.

3. How to accurately locate the code that caused the reflow?

The code that causes rearrangement usually includes the following aspects:

• Modify the position, size or style attributes of the element: for example, modify the left, top, width, Attributes such as height and margin.
• Modify the document flow: such as adding or deleting elements, changing the display state of elements.
• Resize and scroll events of the browser window: When the user resizes the window or scrolls the page, the browser will trigger the reflow operation.

In order to accurately locate the code that caused the reflow, we can use the browser's developer tools to detect the number of reflows and the time taken. In the Chrome browser, you can view performance metrics through the Performance panel.

4. Specific code examples

The following are some common code examples that may cause reflow, redraw and reflow:

• Modify the position, size or style attributes of the element:

var element = document.getElementById("element");
element.style.left = "100px";
element.style.width = "200px";

• Modify the document flow:

var container = document.getElementById("container");
var element = document.createElement("div");
container.appendChild(element);

• Resize and scroll events of the browser window:

window.addEventListener("resize", function() {
    // do something
});

window.addEventListener("scroll", function() {
    // do something
});

For the above code example, we can perform the following optimizations:

• Cache references of DOM elements: To avoid multiple queries of DOM elements, the query results can be cached .
• Use CSS animation: Use CSS transition or animation attributes to achieve animation effects and avoid frequently modifying the position and style attributes of elements.
• Avoid frequent DOM operations: merge multiple operations on DOM elements into one.

Summary:

Reflow, redraw and reflow are important concepts in web page performance optimization. Understanding these concepts and following the corresponding optimization techniques can greatly improve the performance of your web pages. This article discusses the meaning of reflow, redraw and reflow, and gives specific optimization solutions and code examples, hoping to be helpful to developers in optimizing web page performance. In practice, we can also use some tools and technologies to help us further optimize the performance of web pages, such as using CDN acceleration, compressing and merging static files, lazy loading, etc. Optimizing web page performance is a continuous process, and we need to make adjustments and improvements based on actual conditions.

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