From: Peter Fitzgibbons <Peter.Fitzgibbons@...>
Date: 2005-08-12T03:20:18+09:00
Subject: Re: Head First Design Patterns - Strategy Pattern [was: Java/C# "interface" in Ruby ?]

Hello all.

Ok, ok, I'll give more detail on what I'm up to.

In an effort to learn much more Ruby (and more software technique in
general), I'm working through the O'Reilly book "Head First Design
Patterns" (Gang-of-four rewritten).

So, the first "pattern" (not really a pattern officially) is the
Strategy Pattern.
Duck class is copied below.

My question is this: The :quackDelegate and :flyDelegate are written in
Java/C# as object instances of an interface.  Clearly I've ditched the
interface itself, but the rest is pretty much as described in the book.

So, my initial question is: Since ruby has no interfaces, and obeys
duck-typing, is there a more-terse implementation of the below that is
more "The Ruby Way" ?

Much thanks for your sage advice.


---------- duck.rb ----------------
class Duck
  attr :quackDelegate, :flyDelegate

  def performQuack
    @quackDelegate.quack
  end
  
  def performFly
    @flyDelegate.fly
  end
  
  def swim
    "All ducks float, even decoys!"
  end
  
  def setFlyDelegate(fd)
    @flyDelegate = fd
  end
  
end

class FlyWithWings
  def fly
    "I'm flying!"
  end
end

class FlyNoWay
  def fly
    "I can't fly"
  end
end

class FlyRocketPowered
  def fly
    "I'm flying Rocket Powered!!"
  end
end

class Quack
  def quack
    "Quack"
  end
end

class MuteQuack
  def quack
    "<< Silence >>"
  end
end

class Squeak
  def quack
    "Squeak"
  end
end

class MallardDuck < Duck
  def initialize
    @quackDelegate = Quack.new
    @flyDelegate = FlyWithWings.new
  end
end

class ModelDuck < Duck
  def initialize
    @flyDelegate = FlyNoWay.new
    @quackDelegate = Quack.new
  end
End
----------- end duck.rb -----------------


------------ duck_test.rb ---------------
require 'test/unit'
require 'duck'

class DuckTest < Test::Unit::TestCase
  def testMallard
    mallardDuck = MallardDuck.new
    assert_equal "Quack", mallardDuck.performQuack
    assert_equal "I'm flying!", mallardDuck.performFly
  end 

  def testModelDuck
    modelDuck = ModelDuck.new
    assert_equal "Quack", modelDuck.performQuack
    assert_equal "I can't fly", modelDuck.performFly
    modelDuck.setFlyDelegate FlyRocketPowered.new
    assert_equal "I'm flying Rocket Powered!!", modelDuck.performFly
  end  
  
end
-------------- end duck_test.rb ----------



Peter J. Fitzgibbons
Applications Manager
Lakewood Homes - "The American Dream Builder"(r)
Peter.Fitzgibbons@Lakewoodhomes.net
(847) 884-8800