I am working on a gem that has needs to set dependencies conditionally when the gem is installed. I've done some digging around
and it looks like i'm not alone in this need.
# this is a long thread
The only way I can see to add dependencies to a gem is to use add_dependency method within a Gem::Specifiction block in a .gemspec file
Gem::Specification.new do |s|
# ... standard setup stuff
# conditionally set dependencies s.add_dependency "rb-inotify", "~> 0.8.8" if RUBY_PLATFORM =~ /linux/ i s.add_dependency "rb-fsevent", "~> 0.4.3.1" if RUBY_PLATFORM =~ / darwin/i s.add_dependency "rb-fchange", "~> 0.0.5" if RUBY_PLATFORM =~ /mswin| mingw/i
end
Based on all of the docs and threads I found on the net, I would have expected that if you install the gem on
- Linux, then, rb-inotify would be a dependency and auto-installed - Mac - rb-fsevent would be installed - Windows - rb-fchange would be installed
However, it seems that is not the case. The "if" statements within the block are evaluated at the time the gem is built and packaged. Therefore, if you build and package the gem on Linux, then, rb-inotify is added as a dependency, Mac, then, rb-fsevent, Windows - rb-fchange.
Still needing a solution, I dug around in the rubygems code and it seems the following is a broad stoke of what happens.
- build all of your code for your gem: foo.gem - create a foo.gemspec file - build, package, and release the gem to a gem server such as rubygems.org - let everyone know - developers install it locally via: gem install foo - the foo.gem file is downloaded, unpacked, and installed. all dependencies are installed as well. - everything should be set and we can beging using the gem.
It seems that when the gem is built and released the foo.gemspec file is loaded and the Gem::Specification block is evaluated and converted to YAML, compressed as metadata.gz, and included in foo.gem. The ruby code is compressed into data.tar.gz and included as well. When the gem is installed on the local developer machine, the YAML is extracted from metadata.gz and converted back into a Gem::Specification block, however, it is not converted back to the original block.
instead, you will see something like the following:
Gem::Specification.new do |s|
if s.respond_to? :specification_version then s.specification_version = 3
if Gem::Version.new(Gem::VERSION) >= Gem::Version.new('1.2.0') then s.add_runtime_dependency(%q<rb-inotify>, ["~> 0.8.8"]) else s.add_dependency(%q<rb-inotify>, ["~> 0.8.8"]) end else s.add_dependency(%q<rb-inotify>, ["~> 0.8.8"]) end
end
Ok. So, I have a bird's eye view of the process, however, that does not change my desire to build a single gem and conditionally specify dependencies for a range of OS targets.
If anyone has a solution other than building multiple .gemspec files for each target OS... I'm all ears!!
