API v2 - Long polling

Hi Frank, I'll be happy to help here. 

"Question 1: Is this really the most efficient way that this can be done?"

Reading through your description (thank you for the detailed write up, it really helps), I believe there are a few optimizations to make:

a) "does a files.listFolder" ... "files.listFolderContinue" ... "then for each file it does a files.getMetaData and compares the rev":

If you've already done files.listFolder/files.listFolderContinue, you don't also need to call files.getMetadata for each file. That just returns the same Metadata objects as were already returned in ListFolderResult.entries from files.listFolder/files.listFolderContinue.

Part of the confusion may be that you need to cast the Metadata to e.g., FileMetadata to access FileMetadata.rev though, as shown here.

b) "if an updates occurs it restarts the entire sync process":

You don't need to start totally over. When files.listFolderLongPoll indicates something changed, you should just call back to files.listFolderContinue with the latest cursor you recieved, and it will tell you only about what has changed since then. (You should just continue applying the same steps from the files.listFolder documentation to apply those additional changes to your local state to get up to date.)

"Question 2: Is there are way of polling only for changes to a specific directory?"

No, unfortunately files.listFolderLongPoll will notify of changes in the account even if the changes only occurred outside of the path originally specified in the path set in the original files.listFolder call. We'll consider that a feature request, but I don't know if/when that would be implemented. (It would be non-trivial due to the way these pieces of infrastructure work on the backend.)

Hope this helps, and thanks for the kind words about the support!