Download from OneDrive: how to know if the Online Editor is done

Question

I have a workflow where I upload an Office file to OneDrive, open it in an Office Online Editor, then after closing the editor the file is downloaded again. (This is an educational scenario with students working on a tablet, who have to 'hand in' their documents.)

My question: how can I know that the Online Editor is closed, and/or that all updates are available in OneDrive?

I have tried the Sharepoint API, MS Graph API, root folder deltas, Webhooks: they all tell me about the file changes and TimeLastModified.
Unfortunately, if you open a large Office document in OneDrive in the Office Online Editor, make a change, then close the editor, then it can take up to a minute before OneDrive is updated. Usually the first update is within 6 seconds, but sometimes a second update happens (seems related to the 30s 'autosave' timer in the Online Editors).
I have a feeling that Sharepoint 'knows' more than it is telling. Online Editors use WOPI, and WOPI supposedly calls lock/unlock when opening and closing an editor. But how to access this info..

Did anyone try this before: download your file after editing online?
Mentioning Ryan Gregg in this post because of his invitation to ask questions in the Ignite session on OneDrive APIs ;-)

Many thanks for any help in this,
Casper

edit

As dalibor suggested, I did try the Graph change notifications (I think they used to be called webhooks), and polling for TimeLastModified. Both work, but my main problem is that I cannot see if the 'file changed' notification is the last one, or if the 'close my Online Editor' action will be causing more saves.

In the mean time, I tried the LockedByUser property on a Sharepoint file (https://msdn.microsoft.com/en-us/library/office/jj247073.aspx). This property is a bit hard to read (not yet available in MSGraph, and the Sharepoint API seems to require direct user/password login). But it does signal if an Online Editor is open. However, if you close your Online Editor, then the LockedByUser property is cleared immediately, but the final save(s) can still occur later. Or there will be no final save at all, if nothing was changed. So we still do not know for sure when the files is updated, after Online Editing stopped.

NB: Implementing WOPI ourselves does work, we tried that as well. But as suggested below, that requires a different workflow as storage partner.

Answer 1

I don't think you can get WOPI lock/unlock event unless you have actually registered as microsoft storage partner and developed your own WOPI host where you actually implemented Lock endpoint with bunch of other WOPI REST endpoints that give you edit online capability like CheckFileInfo, GetFile and PutFile... In that case from your own WOPI host app you could potentially listen to unlock events and fire some other event somewhere else. Although this is possible I wouldn't recommend it. Also if you have WOPI up & running it already does this workflow of upload and download for you and writing files to your own storage.

Few other alternatives:

1. Use the Microsoft Graph API to get change notifications not sure if this is what you are referring as webhooks but it seems you can subscribe to different events like email and OneDrive and receive notifications that you can track and process as you like.
2. If 1. does not work for some reason than some kind of custom scheduled app that runs every 30 sec (since everything is async and save persistence does not happen instantly) tracks OneDrive fileId and TimeLastModified and compares previous state that you have stored somewhere (database or similar) with current response from OneDrive REST API. Not the architecture for production but it would work as a proof of concept.