Rearrange, Redraw, and Relayout: Which is Better?

Reflow, Redraw, and Reflow: Which is Better?

When developing web pages, performance optimization is a key issue. When a user visits a web page, the browser needs to parse HTML, CSS, and JavaScript codes, and use these codes to create a DOM tree, a rendering tree, and the final page presented to the user. Throughout the process, three main concepts are involved: rearrangement, redrawing, and reflow. Understanding their differences and how to optimize them can help us improve web page performance.

First, let’s understand what reflow is. When the size, position, or other attributes that affect layout of a DOM element change, the browser needs to recalculate and update the element's geometric properties. This process is called reflow. Reflow will trigger the browser to relayout, consuming larger computing resources. Therefore, frequent reflows can lead to degraded page performance.

Next, repaint means that when the style of a DOM element changes but does not affect its geometric properties, the browser only needs to redraw the element without recalculating the layout properties. This process is called redrawing. The performance cost of redrawing is relatively low, but it may still affect the performance of the web page. In a page, if a large number of elements are redrawn, performance will decrease.

Finally, reflow (layout), sometimes called invalid reflow (useless reflow), refers to the situation when the browser needs to recalculate the layout of the element, but the layout result does not change. This situation occurs when the layout is repeatedly calculated. For example, caching is not used when obtaining the size and position of an element, but is recalculated each time. Reflow is very inefficient because recalculating the same layout properties wastes computational resources.

So, which one is better among reflow, redraw and reflow? Generally speaking, redrawing has the lowest performance cost and reflow has the highest performance cost. Therefore, in performance optimization, we should try to reduce the number of rearrangements and reflows, and try to use redrawing to achieve optimization purposes.

The following are some optimization tips that can help us reduce reflows and reflows in the page:

1. Use CSS3 animations and transitions: CSS3 animations and transitions use hardware acceleration, which greatly reduces Reduces the number of page reflows and improves performance.
2. Avoid frequent DOM operations: Frequent DOM operations will lead to rearrangement and reflow. By merging multiple operations, the number of DOM operations can be reduced.
3. Use event delegation: Event delegation can reduce the number of event handlers, thereby reducing the number of rescheduling and reflows.
4. Avoid using tables layout: tables layout will cause frequent reflows, try to use other layout methods instead.
5. Use cached calculation results: When obtaining the size and position of elements, try to use cached calculation results to avoid repeated calculations.

In short, reflow, redraw and reflow are aspects that cannot be ignored in web page performance optimization. Understanding their differences and how to optimize them can help us improve the loading speed and responsiveness of web pages. Through reasonable layout and reducing unnecessary operations, we can reduce the number of reflows and reflows, thereby improving the performance of web pages.

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