1 – Strategy Pattern

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:

[text]
Flap Flap
Quack!
[/text]

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 :
[text]
Flap Flap
Quack!
Soaring in the wind
[/text]

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!!

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