Add notebook option for persisting outputs

[jonatanklosko]

Closes #206.

Adds a new notebook :persist_outputs attribute (persisted as metadata). The attribute can be set in the persistence modal. Once set, outputs are persisted the same way as in export (#483).

livemd_with_output.mp4

[jonatanklosko]

@josevalim I think we need to keep the notebook metadata comment at the top rather than bottom. One point is consistency, since all other meta comments are said to refer to the elements they precede. Also, a meta comment at the end may refer to an empty Markdown cell (currently we don't have any metadata for such, but we may have), or it could be livebook:{"break_markdown":true}, so the subject of a trailing meta would be ambiguous. Given it's just a single line, I don't think it's particularly problematic to keep it in the first line, especially that notebooks don't have any metadata by default in the first place.

[josevalim]

What does the UI do in this case? Just fails to import it? Perhaps we should print an error message?

[jonatanklosko]

This isn't used for importing, it's just an operation similar to :set_cell_attributes that updates the session data we have. These attributes are usually updated via UI in one way or another and we control what these are. Technically we could raise an error, though returning :error this is more in line with all other errors.

[josevalim]

Beautiful!

Quick question: are we already importing them?

also, what do you think about moving the checkbox closer to the bottom just above the save button on the save screen? I think you can also increase the vertical spacing there, the buttons and controls are very close to each other. :)

[jonatanklosko]

Quick question: are we already importing them?

Correct, we import each output snippet as {:text, snippet_content}.

what do you think about moving the checkbox closer to the bottom just above the save button on the save screen?

It feels more natural to place global options above, since clicking the buttons shows conditional content and the checkbox would jump unnecessarily.

[josevalim]

Sounds good. Last question: what happens when there is an output cell not after an Elixir cell? Do we discard it? Treat it as markdown? If this is handled, ship it!

[jonatanklosko]

what happens when there is an output cell not after an Elixir cell?

When importing, once we encounter Elixir cell, we then take all subsequent output snippets as output, so snippets anywhere else are treated as Markdown (they use output language so it's rather unlikely anyway).

[jonatanklosko]

I think you can also increase the vertical spacing there, the buttons and controls are very close to each other. :)

WDYT?

[josevalim]

We need some spacing between the file text and the button, as it looks inconsistent with the rest, otherwise looks good!

[josevalim]

You can also look at the size we are using in other similar screens, such as the runtime one, so we make it all consistent. :)

[jonatanklosko]

We need some spacing between the file text and the button, as it looks inconsistent with the rest, otherwise looks good!

Done!

You can also look at the size we are using in other similar screens, such as the runtime one, so we make it all consistent. :)

That's what I usually do, although a single spacing doesn't fit all types of content. For example in the runtime modal we have quite a lot of text or similar controls, in which case large spacing looks weird :)