Logged out from dropbox and login again. It is allowing me to access files without asking credential

As you mentioned, when processing the OAuth app authorization flow in an app using ObjectiveDropboxOfficial, SFSafariViewController is used. The web session in SFSafariViewController (such as whether the user is already signed in to www.dropbox.com) is separate from whether or not the ObjectiveDropboxOfficial SDK has any locally stored access/refresh tokens. The clearAllTokens and unlinkAndResetClients methods only affect the locally stored access/refresh tokens, not the web session, so calling those won't sign the user out of the Dropbox web site.

Also, ODBoxHandler isn't made by Dropbox so I can't help with that, but if you're referring to this third party library, it looks like that clientRequestedLogout method just calls unlinkAndResetClients too.

Likewise, while I can't provide support for NSURLCache as that's not made by Dropbox, it sounds like it's not possible to clear the SFSafariViewController session using NSURLCache. For instance, I found this post that indicates it's not possible to interact with or clear SFSafariViewController credentials.

So, it looks like it's not possible to proactively clear out the entire SFSafariViewController session directly. You could clear the cookies for www.dropbox.com in SFSafariViewController however by presenting a SFSafariViewController for the URL "https://www.dropbox.com/logout", though that's not part of the API/SDK, so ObjectiveDropboxOfficial doesn't offer a method for that. Doing so from your own code should work, though it would involve showing the SFSafariViewController UI again.