Description: An ESSource specific ESProductResolver which is type safe and can run concurrently with other DataProxies from the same ESSource.
Usage: Inherit from this class and override void prefetch(edm::eventsetup::DataKey const& iKey)
and DataT const* fetch() const
prefetch is guaranteed to be called before fetch where fetch should return the value obtained during the call to prefetch. The inheriting class must maintain the lifetime of the object returned from fetch until invalidateCache() is called.