Resource Timing API

这篇翻译不完整。请帮忙从英语翻译这篇文章

 
通过Resource Timing API可以获取和分析应用资源加载的详细网络计时数据, 应用程序可以
使用时间度量标准来确定加载特定资源所需要的时间, 比如 XMLHttpRequest, <SVG>, 图片, 或者脚本.
 

Resource Timing API为网络事件(如重定向的开始和结束事件, DNS查找的开始和结束事件, 请求开始, 响应开始和结束时间等)生成有高分辨率时间戳( high-resolution timestamps )的资源加载时间线, 并提供了资源大小和资源类型.

本文档是Resource Timing API的概述. 更多详细信息, 请查阅每个接口的参考说明, Using Resource Timing 和 附录 的参考链接. 有关资源时序处理模型的图示,请参阅 resource timing phases .

PerformanceResourceTiming 接口只统计performance entriesentryType 为"resource"类型的 PerformanceEntry

高分辨率时间戳

一些 Resource Timing 属性返回高分辨率时间戳, 顾名思义, 代表了高分辨率的时间点. 高分辨率时间戳类型是DOMHighResTimeStamp, 用双精度数字(double)表示, 它的值是一个离散的时间点或者两个离散时间点之间的时间差.

DOMHighResTimeStamp的单位是毫秒(ms), 并且应该可以准确到 5 微秒(µs). 但是, 如果浏览器无法以提供精确到5微秒的时间值(如软硬件限制), 则可以将该值表示为以毫秒为单位的精确到毫秒的时间。

资源载入时间戳

应用程序可以获得用于加载资源的各个阶段的时间戳. 处理模型中的第一个属性是 startTime , 它在资源加载过程开始之前立即返回时间. fetchStart 跟踪和重定向处理(如果适用),并在DNS查找之前进行. 下个阶段是connectStart 和 connectEnd 分别是开始连接到服务器和连接建立完成的时间戳. 最后三个按顺序分别是: requestStart - 在浏览器开始向服务器请求资源时; responseStart - 资源请求首包返回时; and responseEnd - 资源全部接收完成时. 如果资源是通过安全连接加载的 secureConnectionStart 的值将会在connectStart和connectEnd之间.

CORS 生效时, 除非服务器的访问策略允许共享这些值,否则这些值中的许多将返回为零. 这需要提供资源的服务器发送 Timing-Allow-Origin HTTP 响应头并且指定origin[s]来源才能允许获取这些被限制的时间戳 .

在非web页面本身的域名下,这些属性在默认都会返回0值 : redirectStart, redirectEnd, domainLookupStart, domainLookupEnd, connectStart, connectEnd, secureConnectionStart, requestStart, 和 responseStart.

The PerformanceResourceTiming interface also includes several network timing properties. The redirectStart and redirectEnd properties return timestamps for redirect start and end times, respectively. Likewise, the The domainLookupStart and domainLookupEnd properties return timestamps for DNS lookup start and end times, respectively.

This would be a nice place to have a diagram showing the relationships between these segments of the resource loading time.

Resource size

The PerformanceResourceTiming interface has three properties that can be used to obtain size data about a resource. The transferSize property returns the size (in octets) of the fetched resource including the response header fields plus the response payload body.

The encodedBodySize property returns the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings. decodedBodySize returns the size (in octets) received from the fetch (HTTP or cache) of the message body, after removing any applied content-codings.

Other properties

The nextHopProtocol property returns the network protocol used to fetch the resource.

The initiatorType property returns the type of resource that initiated the performance entry such as "css" for a CSS resource, "xmlhttprequest" for an XMLHttpRequest and "img" for an image (such as a JPEG).

If the current context is a worker, the workerStart property can be used to obtain a DOMHighResTimeStamp when the worker was started.

Methods

The Resource Timing API includes two methods that extend the Performance interface. The clearResourceTimings() method removes all "resource" type performance entries from the browser's resource performance entry buffer. The setResourceTimingBufferSize() method sets the resource performance entry buffer size to the specified number of resource performance entries.

The PerformanceResourceTiming interface's toJSON() method returns a JSON serialization of a "resource" type performance entry.

Implementation status

As shown in the PerformanceResourceTiming interface's Browser Compatibility table, most of these interfaces are broadly implemented by desktop browsers. However, note that some properties have little to no implementation so see each property's "Browser compatibility" section for more specific interoperability data.

To test your browser's support for these interfaces, run the perf-api-support application.

附录

文档标签和贡献者

此页面的贡献者: suchasplus
最后编辑者: suchasplus,