Add behaviors to #content_for

[denisdefreyne]

Fixes #704.

#content_for, when given a block, now gains a second parameter, :existing, which determines what to do when existing content already exists. The three possible values are:

• :error (current behavior): when content already exists and is not identical, raise an error.
• :overwrite: overwrite the previously captured content with the newly captured content
• :append: append the newly captured content to the previously captured content

For example:

<% content_for(:stuff) do %>banana<% end %>
<% content_for(:stuff, existing: :append) do %>donkey<% end %>

will result in content_for(:stuff) containing "bananadonkey"

<% content_for(:stuff) do %>banana<% end %>
<% content_for(:stuff, existing: :error) do %>donkey<% end %>

will raise an exception

<% content_for(:stuff) do %>banana<% end %>
<% content_for(:stuff, existing: :overwrite) do %>donkey<% end %>

will result in content_for(:stuff) containing "donkey"

[denisdefreyne]

CC @Fjan — this is the behavior you’re looking for.

[Fjan]

Yes, thank you. I would probably still be a good idea to make :append the default at some point to make it's behaviour similar to the content_for in Rails.

[denisdefreyne]

That’ll have to be for Nanoc 5 (since it’s a breaking change).

[Fjan]

We can be pretty confident that the current default behaviour is not used by anyone, because it would raise an exception, so I wouldn't call it a breaking change. But I'm fine with waiting until the next version.