inject_dependency.rb

#
# starting point
#
class DeployCLI
  def run
    template, args = ...
    TemplateRenderer.new.render(template, args)
  end
end

#
# intermediate step 1 - create dependencies in constructor
#
# I normally do this as an intermediate step just to make the dependencies more
# explicit, which I find useful if there are many of them.
#
class DeployCLI
  def initialize
    @renderer = TemplateRenderer.new
  end

  def run
    template, args = ...
    @renderer.render(template, args)
  end
end

#
# intermediate step 2 - create default dependency but also allow injection
#
# This is a further intermediate step I sometimes do when I have a lot of code
# that depends on the API I'm about to change. Benefit: I can change the
# dependent code one by one without breaking anything. Drawback: i) can't be
# done in every language. ii) the coupling to the actual implementation still
# exists.
#
class DeployCLI
  def initialize(renderer = TemplateRenderer.new)
    @renderer = renderer
  end

  ...
end

#
# final step. inject dependency and break coupling
#
class DeployCLI
  def initialize(renderer)
    @renderer = renderer
  end

  def run
    template, args = ...
    @renderer.render(template, args)
  end
end