Question

How do you access an instance variable within a mixin method? I can think of 2 ways, but both seem problematic.

1. Have the mixin method access the instance variable directly as any class method would, e.g self.text. Problem with this is that it places restrictions on where the mixin method can be used, and forces the class doing the mixing to have a particular instance method named in a particular way.

2. Pass the instance variable as a parameter to the mixin method, which would result in code like this:

example

self.do_something(self.text)

or

@thing.do_something(@thing.text)

which looks nasty to me, and doesn’t conform to the principles of object orientation.

Is there any other way to do it?, am I right to be concerned?

Answer 1

In general, avoid having mixins access member variables: It’s a very tight form of coupling that can make future refactoring unnecessarily difficult.

One useful strategy is for the Mixin to always access variables via accessors. So, instead of:

#!/usr/bin/ruby1.8

module Mixin

  def do_something
    p @text
  end

end

class Foo

  include Mixin

  def initialize
    @text = 'foo'
  end

end

Foo.new.do_something     # => "foo"

the mixin accesses the “text” accessor, which is defined by the including class:

module Mixin

  def do_something
    p text
  end

end

class Foo

  attr_accessor :text

  include Mixin

  def initialize
    @text = 'foo'
  end

end

Foo.new.do_something     # => "foo"

What if you need to include the Mixin in this class?

class Foo

def initialize
  @text = "Text that has nothing to do with the mixin"
end

end

Using generic and common data names in mixins can lead to conflicts when the including class uses the same name. In that case, have the mixin look for data with a less common name:

module Mixin

  def do_something
    p mixin_text
  end

end

and let the including class define the appropriate accessor:

class Foo

  include Mixin

  def initialize
    @text = 'text that has nothing to do with the mixin'
    @something = 'text for the mixin'
  end

  def mixin_text
    @something
  end

end

Foo.new.do_something     # => "text for the mixin"

In this way, the accessor acts as sort of “impedance matcher” or “translator” between the mix-in’s data and the including class’s data.