CSC/ECE 517 Fall 2009/wiki2 11 zv

Overview

Before starting off with design patterns for Ruby we need to define what a design pattern is, Design patterns can be described as "a general reusable solution to a commonly occurring problem in software design." [1] The idea of design patterns is to not to reinvent the wheel but to solve the current problems by using solutions that have worked in the past. A design pattern names, abstracts, and identifies the key aspects of a common design structure that make it useful for creating a reusable object-oriented design. It helps to identify the classes and instances and the way they collaborate with each other to form a solution to a problem. Design patterns c an be classified into 3 parts Creational, Structural, Behavioral (See if we can give links for these.)

Factory

Factories The factory design pattern is an object oriented design pattern. It is a creational design pattern and deals with the issues faced in creating objects. The main goal of this implementation is to isolate teh code that creates the class form the concete implementation of that class. Ruby example for the same is given below.

Factory Example in Ruby

   class GearFactory
     def new() 
       if ( ... some condition )
          return Sprocket.new()
       else
          return Cog().new()
       end
     end
   end

Our client class now becomes:

   class GearUser 
     def doSomething(factory )
       ...
     my_gear = factory.new()
       ...
     end
   end

The above code does not have to distinguish between a factory and an ordinary class. We can call the class using the followijng code.

   client.doSomething(GearFactory.new)          #Use the factory
   client.doSomething(Cog)                      #Use the Cog class
   client.doSomething(Sprocket)                 #Use the Sprocket class

Factory Example in Java

In java the Factory implementation is done using interfaces. Declaring them as interfaces helps to maintain a general overview and not depending on the type of factory object that needs to be used. All of these can be placed in a huge factory in a client application. A well known example for Java Factory is the UI toolkits that are designed to run on different windowing systems.

   interface ScrollBar { ... }
   interface MenuBar   { ... }
   ...

And associated classes implementing them on different windowing systems:

   class MotifScrollBar implements ScrollBar { ... }
   class Win95ScrollBar implements ScrollBar { ... }
   ...

And a factory interface that also doesn't commit to representation:

   interface Factory {
     public abstract ScrollBar newScrollBar();
     public abstract MenuBar   newMenuBar();
     ... 
   }

But implementation classes that do:

   class MotifFactory implements Factory {
     public ScrollBar newScrollBar() { return new MotifScrollBar(...); }
     ...
   }

AbstractFactory Model

Abstract Factory Design Pattern encapsulates a group of objects that have common theme. It implements a generic interface to create these objects that are part of the theme. It does not care about the details of the implementation of these objects. It is a type of Creational pattern [2] “Provide an interface for creating families of related or dependent objects without specifying their concrete classes” [3]

Ruby automatically implements the Abstract Factory pattern as Class Objects. All Class objects have the same interface: the new method of each class object creates new instances of the class. Thus the code can pass references to class objects around and they can be used to call new method without knowing the exact type of object that the class creates.

   Class Foo; end
   Class Bar, end

Here is the use of Abstract Factory Pattern

   def create_something(factory)

new_object = factory.new puts "created a new #{new_object.class} with a factory"

   end

Here we select a factory to use

   Create_something(Foo)
   Create_something(Bar)

Output of the code:
Created a Foo with a factory
Created a Bar with a factory

The create_something method is creating objects through an abstract interface. It does not have details about implementation used to create these objects. Thus the use of create_something() is used to shield the rest of the code from that knowledge.

But in Java for implementation of Abstract Factory design pattern we need to create a class which has method who defers creation of product objects to its concrete class. This class then needs to be ”extended” by the client class which uses only these interfaces to create objects of concrete class. [4]. Thus Java implementation needs a well defined interface to do so but in Ruby it is directly implemented because of private class object property.

Iterator Design Pattern

An Iterator object encapsulates the internal structure of how the iteration occurs. It is a type of Behavioral design pattern. [5]. “Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.” [6]

Ruby implements iterators with blocks and the ‘each’ method, and with ‘for..in’ statements. For example consider the following example;

   def print_element(container)
   	Container.each {|o| puts o.inspect }
   end

   list = [1,2,2,3]
   hash = {“a”=>1, “b”=>2,”c”=>3, “d”=>4 }
   print_elements list
   print_elements hash

The output of the code is,

   1
   2
   3
   4
   [“a”,1]
   [“b”,2]
   [“c”,3]
   [“d”,4]

In Java implementing Iterator design pattern would again involve having an interface for accessing and traversing the elements which is further implemented by the concrete class. Thus a class who needs to access the list will need to call the interface class. [7] Thus Ruby helps in implementing this design pattern easily by providing functions like “each” which does the handling of the concrete class implementation.

Decorator Pattern

It is used to increase the functionality of the existing object dynamically. It adds the behavior at runtime. The below code is an example of decorators where we just create a place holder and we can use this function to perform different functions depending on the parameters supplied to it. The need for this kind of pattern is so as to increase the functionality of a particlar class. In this example we talk about how a coffee class can have many addtions to it like coffee with cream, sprinkles, milk etc and creating a class for each of them will not be the correct solution.In this example the cost can be calculated according to what is sent to it.Cost of the coffee is 2 and the cost of White coffee (coffee + milk) = 2.4. So what happens when we execute the Sprinkles.new(Whip.new(Milk.new(Coffee.new))).cost we get the result as 2.9 as the cost of the coffee is 2, the decorator object (now only coffee) is sent to the Milk and it becomes 2.4, which now is then sent to the Whip which is going to be 2.4 + 02 = 2.6 and finally to Sprinkles which becomes 2.6 + 0.3 = 2.9.

Decorator Example in Ruby

   module Decorator
     def initialize(decorated)
       @decorated = decorated
     end

   def method_missing(method, *args)
       args.empty? ? @decorated.send(method) : @decorated.send(method, args)
   end
  end

   class Whip
     include Decorator
     def cost 
       @decorated.cost + 0.2
     end
   end

   class Sprinkles
     include Decorator

     def cost
       @decorated.cost + 0.3
     end
   end

   Whip.new(Coffee.new).cost
   #=> 2.2
   Sprinkles.new(Whip.new(Milk.new(Coffee.new))).cost 
   #=> 2.9