The Strategy Pattern really resolves to “composition” in practice. This OO technique says to take components that could change at runtime and encapsulate them in their own objects. The master-object is then “composed” of instances of the dependent clasess.

Example:

class Duck
  def fly
    puts ‘Flap Flap’
  end

  def quack
    puts ‘Quack!’
  end
end

d = <span class="constant">Duck.new
d.flap
d.quack

Produces:

Flap Flap
Quack!

So now handle the en-user request to have ducks with different flap :

class FlapWings
  def fly
    puts ‘Flap Flap’
  end
end

class FlapSoaring
  def fly
    puts ‘Soaring in the wind’
  end
end

class Duck
  def initialize (flapInstance)
    @flapDelegate = flapInstance
  end
  def changeFlap(flapInstance)
    @flapDelegate = flapInstance
  end
  def fly
    @flapDelegate.fly
  end

  def quack
    puts ‘Quack!’
  end
end

mallard = Duck.new(FlapWings.new)
mallard.fly
mallard.quack
mallard.changeFlap(FlapSoaring.new)
mallard.fly

Now produces :

Flap Flap
Quack!
Soaring in the wind

This example looks trivial. The technique is powerful. Behold that in Java/C#, this requires a lot of effort including Inheritance and Interfaces.

Nice that Ruby allows this run-time change through duck-typing!!