Short hand for an attribute writer with a default value

gistfile1.rb

# When using dependency injection, I often find myself storing the dependency as
# an attribute on the object. This is fine, except when you squint at the class there
# looks to be more methods then what's really needed to understand the class.
#
# A typical class with dependency injection might look like this
class TimeParam
  attr_writer :parser

  def initialize value
    @value = value
  end

  def date_time
    parser.parser value if value
  end

  def parser
    @parser ||= DateTime
  end
end

# Wouldn't it be nice if it looked like this instead?
class TimeParam
  include Dependency

  dependency :parser, DateTime

  def initialize value
    @value = value
  end

  def date_time
    parser.parser value if value
  end
end

# And just like the first implementation, we can swap it out like this.
class TimeParamTest < MiniTest::Unit::TestCase
  def test_date_time
    parser    = MiniTest::Mock.new
    parser.expect :parse, [String], DateTime.new('2012-12-12')
    time_param = TimeParam.new date_time
    time_param.parser = parser
    assert_equal DateTime.new('2012-12-12'), time_param.date_time
  end
end

I suppose you could also set this up in the initialise, removing the extra noise of memoized readers.

def initialize value
  @parser = DateTime
  @value = value
end

def initialize value, parser = DateTime
  @parser = parser
  @value = value
end

Yeah, I sometimes use that approach, but my preference is to only use it on instance methods, not the constructor. The reason being is when you change the order of arguments everything that instantiates this object breaks. By exposing it as an attribute then you at least get a consistent interface to it. The only time this would be useful if if you needed to access the dependancy from inside the constructor.