Contents change notifier

[davidbrochart]

Problem

In JupyterLab we are transitioning to automatically saving after each change to a document, à la Google Docs. When a file change occurs in the back-end, for instance a version-controlled file after a git checkout my_branch, we want to notify the user that the file was changed "on disk", and ask if they want to keep their content and overwrite the file, or revert to the saved file and loose their changes.
I am thinking that we could have a new service in the contents manager, to notify for changes in directories/files.

Proposed Solution

I don't have a solution yet 😄
This issue is mainly to discuss if that would be helpful, and how we would specify it. Maybe it doesn't have a place in core jupyter-server.
But since we already provide file managers, I think that a change notifier could be part of it. We could use watchfiles for that.

Additional context

See jupyterlab/jupyterlab#12306.

[blink1073]

We talked about adopting Cal Poly's Notifications extension in a team meeting

[davidbrochart]

Thanks for mentioning that @blink1073, I think a contents change notifier could use this notifications extension.
Right now I don't even need the file change notification to be sent to the front-end, since it would be handled in the back-end. But I am more and more convinced that it should be part of the contents manager, because it depends on the underlying file system. We could ship a default implementation in our file managers, and custom content managers could decide to implement it or not.