Handle currentWidget in KernelWidgetTracker

[jtpio]

FIxes #204

The tracker now keeps track of the currentWidget. This is useful in the case of Notebook 7 where connecting to currentChanged might not be enough. Notebook 7 adds the notebook widget to the page very early, and the kernel usage component might be connecting to tracker.currentChanged after the tracker.currentChanged is emitted.

notebook-7-kernel-usage.mp4

[mahendrapaipuri]

@jtpio Just to get my head around this, you are saying that when we start a notebook server, the widget has been created and emitted. But only when the user opens a notebook file from the interface, tracker.currentChanged is attempting to connect but it is too late? So, we should ensure that we connect to the signal before it emits data?

[jtpio]

I would say it's more that the notebook tracker (which keeps track of all the notebook widgets currently opened on the page - on the frontend) fires currentChanged quickly in the case of Notebook 7, and only once. And that the consumers of the currentChanged signal register their callbacks after the first currentChanged signal is emitted, which would explain why we noticed notebookChangeCallback was never called.

This was likely not an issue in JupyterLab because the UI takes more time to initialize and the signal might be emitted multiple times.

[mahendrapaipuri]

Ok, I see. What I noticed from my tests is that the notebookChangeCallback was indeed fired but on the tab of the notebook interface (which shows list of files and directories) but there is no widget on that tab and so, it returns immediately. But this notebookChangeCallback was never fired on the new tab where actual notebook was opened.

What you told is coherent with the behavior I observed. Thanks a lot for the explanation @jtpio

[mahendrapaipuri]

@jtpio Btw, I have noticed that IStatusBar does not exist for notebook interface. I guess we should make it optional for resourceStatusPlugin extension?

[jtpio]

I guess we should make it optional for resourceStatusPlugin extension?

yes that would make sense. Would you like to open a PR?

[mahendrapaipuri]

Sure, will do that!

[jtpio]

Thanks!