Wraps its inner route with support for Conditional Requests as defined by http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-26.
def conditional(eTag: EntityTag, lastModified: DateTime): Directive0
Depending on the given eTag and lastModified values this directive immediately responds with 304 Not Modified or 412 Precondition Failed (without calling its inner route) if the request comes with the respective conditional headers. Otherwise the requests is simply passed on to its inner route.
All responses (the ones produces by this directive itself as well as the ones coming back from the inner route) are augmented with respective ETag and Last-Modified response headers.
Since this directive requires the EntityTag and lastModified time stamp for the resource as concrete arguments it is usually used quite deep down in the route structure (i.e. close to the leaf-level), where the exact resource targeted by the request has already been established and the respective ETag/Last-Modified values can be determined.
The FileAndResourceDirectives internally use the conditional directive for ETag and Last-Modified support (if the spray.routing.file-get-conditional setting is enabled).