Question

Intro: I’m working on a gem that, by default, will pull information out of some XML data and do some sort of processing on the document. I’m using nokogiri to parse the XML. However, I wish to allow the user to parse the XML themselves and pass in the necessary information for my data processing methods to run, in case they don’t want to install nokogiri or have already parsed the XML.

Question: Is there any way to allow the user to specify, during gem installation, that they don’t wish to install the nokogiri dependency? For example (very hand-wavey here),

gem install crazy_gem --no-nokogiri

and in the gemspec perhaps something like

Gem::Specification.new do |s|
  ...
  s.add_dependency 'nokogiri' unless Proc.new { install_flags('no-nokogiri') }
  ...
end

[edit] I don’t want to focus too much on the gemspec code above, as I know it doesn’t work–it’s just an example of the kind of thing I want to do. [/edit]

gem install crazy_gem --ignore-dependencies works great until there are additional dependencies.

Answer 1

I don’t think you can do exactly what you’re after, but there’s apossible solution if you reframe what your gem does. Rather than a gem that by default parses some XML and processes the data, but optionally allows you to pass in the pre-parsed data, what about a gem that is mainly concerned with the processing, but optionally will parse the XML for you (if you have Nokogiri).

To do this just leave Nokogiri out of your gemspec dependencies (you could add it as a development dependency or a requirement).

Inside your code, make sure you only call require 'nokogiri' in a begin..end block with a rescue LoadError and deal with it as appropriate.