Allow acessing :pre snapshot in item itself

[denisdefreyne]

Fix for #537.

The :pre snapshot can now be accessed within the item itself, provided that the :post snapshot is present (because at that point, the :pre snapshot stops moving).

The implementation of the fix in this PR is conceptually the same as in the aforementioned bug, but refactored.

CC @nanoc/contributors @jethrogb

[denisdefreyne]

Can I get a code review?

[mpapis]

👍 looks reasonable - although I do not have full understanding of it so I might be missing something

[bobthecow]

Is the :pre snapshot usable after other (explicit) snapshots have been created but before :post is present?

[denisdefreyne]

Yeah, :pre is always usable.

Edit: :pre is usable except in the item itself. See this comment.

[bobthecow]

Then maybe I missed something? I thought this was setting it up to allow accessing :pre once :post is present?

[denisdefreyne]

@bobthecow This allows accessing :pre from the item itself, once :post is set. Previously, accessing :pre was only possible from other items.

[bobthecow]

Right. So I was asking if you should be able to access :pre from the item itself, once, e.g., :mysnapshot is set.

[denisdefreyne]

@bobthecow Hm, re-reading the original question, I think my answer is misleading.

From within the item itself, using :pre when there is no :post snapshot will lead to an exception (since you’d recursively let compiled content depend on itself). When there is a :post snapshot, that works from within the item itself (since :pre cannot move anymore).

When accessing the compiled content from another item, accessing :pre is possible even if there is no :post: nanoc will compile the accessed item first, guaranteeing that :pre is in its final position.

These moving pre-snapshots and post-snapshots are difficult to reason about and one of the reasons why they’ll be gone in nanoc 4!

[gpakosz]

Does it make :pre accessible at the time you're laying out an item? A while ago, when I tried to extract a TOC from @item.compiled_content(:snapshot => :pre) inside my default layout.haml layout, I think nanoc bailed out. So I used yield instead:

%main#content.container
  - toc = extract_toc(yield, :depth => @item[:toc_depth])

[denisdefreyne]

@gpakosz That’s still not possible, but yield is intended to be used in that case anyway.