Kevin Leong thanks so much for a very helpful article. Saved me quite a bit of time.

Small update: Since the tutorial declares:

let googleDriveService = GTLRDriveService()
var googleUser: GIDGoogleUser?

should the call to getFolderID inside of

func populateFolderID() {

accept parameters:
getFolderID(name: filesFolderName, service: googleDriveService, user: googleUser!)

?

Currently the text lists:

getFolderID(name: myFolderName, service: service, user: user)

I made this change after getting an error & things began working — just in case this helps. Sorry if I missed something obvious in your wonderful tutorial. Again, much thanks!