'reload' vs 'reloadFromOrigin' in WKWebView

Question

What's the difference between reload and reloadFromOrigin in WKWebView? Apple's documentation says that reloadFromOrigin:

Reloads the current page, performing end-to-end revalidation using cache-validating conditionals if possible.

But I am not sure what that really means.

Answer 1

I was interested in this too. Looking at WebKit's source (Source/WebCore/loader/FrameLoader.cpp, FrameLoader::addExtraFieldsToRequest(...) around the if (loadType == FrameLoadType::Reload) conditional) it looks like the key difference is in what extra HTTP header request fields are specified.

reloadFromOrigin() sets the Cache-Control and Pragma fields to no-cache, while a simple reload() only results in in the Cache-Control header field having max-age=0 set.

To work out what this means I looked at the Header Field Definitions section of the HTTP 1.1 spec. Section 14.9.4 'Cache Revalidation and Reload Controls' states:

The client can specify these three kinds of action using Cache- Control request directives:

End-to-end reload The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field names MUST NOT be included with the no-cache directive in a request. The server MUST NOT use a cached copy when responding to such a request.

Specific end-to-end revalidation The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional with the client's current validator.

Unspecified end-to-end revalidation The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator.

From my reading of the spec, it looks like reload() uses only max-age=0 so may result in you getting a cached, but validated, copy of the requested data, whereas reloadFromOrigin() will force a fresh copy to be obtained from the origin server.

(This seems to contradict Apple's header/Class Reference documentation for the two functions in WKWebView. I think the descriptions for the two should be swapped over - I have lodged Bug Report/Radar 27020398 with Apple and will update this answer either way if I hear back from them...)