How to reduce redraw and reflow in virtual DOM

Methods for virtual DOM to reduce redrawing and reflowing: 1. Batch update, virtual DOM will record all changes and update the real DOM tree, which can avoid multiple redrawing and reflowing; 2. Diff algorithm , compare the nodes of the old and new virtual DOM trees, find out the differences, and update the real DOM tree; 3. Batch insertion and deletion, the virtual DOM will record these operations, and then update the real DOM tree at once; 4. Avoid Forced synchronization of layout, changes in layout attributes will trigger reflow, avoiding synchronized layout can reduce the number of reflows, etc.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

Virtual DOM (Virtual DOM) is a technology used to improve the performance of web applications. It creates a virtual DOM tree in memory instead of directly operating the real DOM tree, thereby reducing the number of redraws and reflows and improving the efficiency of page rendering. This article will introduce how virtual DOM can reduce redrawing and reflow.

First of all, it is important to understand the concepts of redraw and reflow. Redrawing means that when the element's style changes, the browser redraws the appearance of the element, but does not affect its layout. Reflow means that when the layout properties of an element change, the browser recalculates the position and size of the element and redraws the appearance of the element. Both redraws and reflows consume significant computing resources, so reducing their frequency is critical to improving page performance.

The core idea of ​​virtual DOM is to abstract the real DOM tree into a lightweight JavaScript object tree, called a virtual DOM tree. When the application state changes, the virtual DOM tree will be compared with the previous virtual DOM tree to find the differences and update the real DOM tree. In this way, you can avoid directly operating the real DOM tree, thereby reducing the number of redraws and reflows.

The methods of virtual DOM to reduce redrawing and reflow are as follows:

1. Batch update: Virtual DOM will combine multiple state changes into one update operation. When the application state changes, the virtual DOM will record all changes and then update the real DOM tree at once. This avoids multiple redraws and reflows and improves performance.

2. Diff algorithm: Virtual DOM uses the Diff algorithm to compare the differences between the old and new virtual DOM trees. The Diff algorithm will compare the nodes of the old and new virtual DOM trees, find the differences, and update the real DOM tree. The core idea of ​​the Diff algorithm is to reuse existing DOM nodes as much as possible instead of re-creating nodes. This reduces the number of redraws and reflows.

3. Batch insertion and deletion: Virtual DOM will combine multiple insertion and deletion operations into one operation. When multiple nodes need to be inserted or deleted, the virtual DOM will record these operations and then update the real DOM tree at once. This avoids multiple redraws and reflows and improves performance.

4. Avoid forced synchronization layout: Virtual DOM will try to avoid forced synchronization layout. Changes to layout properties will trigger reflow, and reflow is a very performance-consuming operation. The virtual DOM will record the changes to the layout attributes, and then perform layout calculations during the next update. This can reduce the number of reflows and improve performance.

To sum up, virtual DOM reduces the number of redraws and reflows and improves the efficiency of page rendering through batch updates, Diff algorithms, batch insertion and deletion, and avoiding forced synchronization of layout. Virtual DOM is a very effective performance optimization technology that can be widely used in web applications.

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