Question

I have a Rails app that repeatedly talks to another Web server through a wrapper, and I’d like to stick the wrapper in a Singleton class so it’s not recreated for every request. Easy enough, I thought:

class AppWrapper < Wrapper
  include Singleton
end
...
wrapper = AppWrapper.instance "url"

Only it doesn’t work:

wrong number of arguments (0 for 1)
/usr/lib/ruby/1.8/singleton.rb:94:in `initialize'
/usr/lib/ruby/1.8/singleton.rb:94:in `new'
/usr/lib/ruby/1.8/singleton.rb:94:in `instance'

Wrapper.initialize needs an argument, and apparently it’s not getting passed through, since line 94 in question says

@__instance__ = new # look Ma, no argument

How do I work around this? Redefining initialize in AppWrapper doesn’t seem to help, and
mucking around with Wrapper to separate “set URL” from “initialize” seems suboptimal.

Answer 1

I asked this question while I was still getting my head around Ruby, and it seems so naive now. The easy solution is to just store the Wrapper object in a member variable and use ||= to initialize it only if it hasn’t been set yet:

class WrapperUserClass
  def initialize
     @wrapper = nil # Strictly speaking unnecessary, but it's a bit clearer this way
  end

  def wrapper
    @wrapper ||= Wrapper.new(foobar)
  end

  def do_something
    wrapper.booyakasha
  end
end