Added automatic cell evaluation

[gpopides]

this refs #614
hope this time i got it right.

notes:
i wanted to enqueue the cell evaluation using push_event/3 but it didn't work. My phoenix experience is not big, especially live view. If there is a way to do it, please let me know.

[jonatanklosko]

As I mentioned this is the kind of core logic that we handle on "data level", so it shouldn't be visible on LiveView side :)

If you look at Session.Data, there's apply_operation that takes data (essentially the session state) and an operation, it updates the data and handles all the implications of the operation. For example when the user clicks "Evaluate" we end up applying {:queue_cell_evaluation, client_pid, cell_id} to the data, which adds the cell to the queue, and then if nothing is evaluating we immediately mark it as evaluating. LiveView only gets the new state and renders accordingly.

Changes to Data.apply_operation need to consider all operations/scenarios, but I think in this case it may be a simple change. Whenever an operation affects cells validity (evaluated/stale/aborted/fresh), we call this function:

livebook/lib/livebook/session/data.ex

Lines 1313 to 1318 in da8b55b

	# Computes cell snapshots and updates validity based on the new values.
	defp compute_snapshots_and_validity(data_actions) do
	data_actions
	|> compute_snapshots()
	|> update_validity()
	end

In update_validity a cell may transition into the :stale state. So I think what we need is to pipe this into another function like maybe_queue_reevaluating_cells, which looks up all automatically reevaluating cells that are :stale and queues these cells (similarly to this).

Feel free to poke around and let me know if you have any questions :)

[jonatanklosko]

There's a number of things we also need to do (mostly a note to myself :)):

• add relevant tests for the data transitions
• persist the reevaluation setting as we do with formatting
• remove reactive input

[gpopides]

Hey, i made the changes and have some questions about the second part.

• I could not find the place that we persist the formatting place so i can do the same for reevaluation setting.
• By removing the reactive input, does it mean removing the phx-click from the input when the input has the reevaluation setting on?

Thanks

[jonatanklosko]

Sweet!

I could not find the place that we persist the formatting place so i can do the same for reevaluation setting.

We handle this in Livebook.LiveMarkdown.Export and Livebook.LiveMarkdown.Import (parsing and storing the notebook content respectively).

By removing the reactive input, does it mean removing the phx-click from the input when the input has the reevaluation setting on?

Nah, with the automatically reevaluating cells there is no need for reactive inputs at all, so we will just remove that feature. There's a number of places involved, so it will be easier if I do it myself :)

[jonatanklosko]

@gpopides thanks! I've just pushed some more tests and adjusted the UI a bit :)

[jonatanklosko]

Here's how it looks:

autoeval.mp4

@josevalim let me know what you think.

I will remove reactive inputs in a separate PR :)