Saving and restoring a short-term token

Hi -- still trying to un-mothball and update an old SwiftyDropbox-based app, and migrate it to short-life tokens.  The standard auth process is basically working now, but I'm wondering if I can preserve more of its old behaviour.

 

Previously, I saved the long-life token to a plist file, and if it was still valid on app re-start it would simply create the client from the token:

 

client = DropboxClient(accessToken : token)

 

What's the equivalent with short-life tokens?  Presumably the access token would still work if they quit and relaunch the app within four hours... but is there any way to preserve the refresh token as well, so that they don't then have to re-authorise when that token expires?  Would that require implementing a new provider derived from ShortLivedAccessTokenProvider?  And if so how would I get the refresh token out of the client to save it in the first place?