proper way to specify optional dependencies of a gem in ruby Gemfile

Question

a gem intends to support gems a or b as alternatives for a functionality.

In code I check with defined?(A) if I fall back to b that's fine.

But as a gem developer how to specify these dependencies?

1) what do I put in the Gemfile.

group :development, :test do
  gem 'a', :require => false
  gem 'b', :require => false
end

This allows Bundle.require(:test) not to auto-require a,b?

2) How can explicitly require a and b separately to mimic (or mock) the scenario when we fall back to b in my tests?

3) Also how do I specify that either a or b is prerequisite for the gem.

thanks

Matheus Moreira · Answer 1 · 2012-11-13T22:32:37.013

Don't include the a gem in your dependencies, but require it anyway. If that fails, it will raise LoadError, from which you can rescue.

begin
  require 'a'
rescue LoadError
  # The 'a' gem is not installed
  require 'b'
end

I believe this is the best way to use and test this setup:

Define an interface for your backend and allow a custom implementation to be easily plugged in.

module YourGem
  class << self
    attr_accessor :backend

    def do_something_awesome
      backend.do_something_awesome
    end
  end
end

Implement the a and b backends.

# your_gem/backends/a.rb
require 'a'

module YourGem::Backends::A
  def self.do_something_awesome
    # Do it
  end
end

# your_gem/backends/b.rb
require 'b'

module YourGem::Backends::B
  def self.do_something_awesome
    # Do it
  end
end

Set the one you want to use.

begin
  require 'your_gem/backends/a'
  Gem.backend = YourGem::Backends::A
rescue LoadError
  require 'your_gem/backends/b'
  Gem.backend = YourGem::Backends::B
end

This will use YourGem::Backend::A even if b is installed.

Before testing, make sure both a and b gems are installed, require both backends in the test code, run the tests with one backend, then run the tests again with the other backend.

this is ok for implementation code, but in tests what is best practice to mock require to throw this LoadError? — Viktor Trón, Nov 13 '12 at 21:25
@viktortron, I've updated my answer. This is a rather unusual setup, but that's how I'd test it. — Matheus Moreira, Nov 13 '12 at 22:33

score 0 · Answer 2 · edited May 18 '15 at 06:18

0

I got this same question a while ago. My solution was to think that the developer should specify this behavior. I would not specify it on the gem, but on the wiki. I would recommend you to document it clearly that the developer need to define one of the dependencies.

To make it better, you can make a check on initialization of the gem, looking for the dependencies, if none can be found, just raise a runtime exception or if you prefer, your own exception. =)

edited May 18 '15 at 06:18

lulalala

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answered Nov 13 '12 at 13:50

Paulo Henrique

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thx, you have a view on 3) then, maybe 1): you do not mention these gems correct? any thoughts on 2)? – Viktor Trón Nov 13 '12 at 14:00
I didnt understand your comment. =/ – Paulo Henrique Nov 13 '12 at 14:01

proper way to specify optional dependencies of a gem in ruby Gemfile

2 Answers2