如何检测 XHR 何时返回缓存的资源?

2022-01-15 00:00:00 http-headers http xmlhttprequest ajax http-caching

我想知道是否有一种方法可以检测何时从本地缓存返回响应?是否可以?

I'm wondering if there is a way how to detect when a response is returned from a local cache? Is it possible?

解决方案应该是通用的并且适用于无条件请求.在这种情况下，响应代码总是 200 OK，但是 XHR 为第二个请求返回一个缓存的资源(例如，第一个响应包含 Expires 标头，因此无需在到期日期之前向服务器请求新资源).

The solution should be general and work for unconditional requests. In this case, the response code is always 200 OK, but XHR returns a cached resource for the second request (e.g. the first response contains Expires header, so there is no need to ask a server for a new resource before the expiration date).

推荐答案

答案是Date header

The answer is Date header

  • 如果日期标头在发送日期之前，则响应来自缓存.
  • 如果日期标头晚于发送请求的日期，则响应是新的.

例如

  • 来自缓存:请求于 11:00 发送，响应日期为 10:59
  • 无缓存:请求在 11:00 发送，响应日期为 11:01

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