Ruby Decorators

Let us consider a problem in which we have a Car class. The top speed of any car is a maximum of 100kmph but by adding extra power packs to the car we can increase the speed of the car. These packs are Nitro and Boost. Nitro increases the speed of the car by 30kmph and Boost increases it by 50kmph. Also a car can add any number of these utilities to reach super high speeds.

Solving using Inheritance

   class Car
     def top_speed
       100
     end
   end
   class CarWithNitro < Car
     def top_speed
       super + 30
     end
   end
   class CarWithBoost < Car
     def top_speed
       super + 50
     end
   end

The drawback of this pattern is

• The subclasses are tightly coupled to the super class. Suppose if the we change the name of the method in the super class we need to make these changes to our subclasses as well.
• No way of adding any packs to a car dynamically. If in a certain scenario we need a car with two nitro packs and three boost packs it’s not easy with this architecture.

Enter the decorator pattern

The decorator pattern allows you to attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

• A decorator is a wrapper object on the component class.
• Has the same interface as the component it is decorating so that it’s presence is transparent to the clients
• Delegates requests to components
• Performs additional actions before or after delegating

Applying decorator pattern to our problem step by step

• The component class that we would want to add new features to or “Decorate” is “Car”
• The decorators would be in this case “Nitro” and “Boost”

Now that we have identified what decorators we need let’s write it

   class Nitro
     # Wrapping the component class, Car object in this case
     def initialize(component)
       @component = component
     end
     # maintain the same public interface as component
     def top_speed
       # after delegation modify the value
       @component.top_speed + 30
     end
   end
   class Boost
     def initialize(component)
       @component = component
     end
     def top_speed
       @component.top_speed + 50
     end
   end

Now we can start using our decorators to add new packs to a car object.

   Nitro.new(Car.new).top_speed #130
   Nitro.new(Nitro.new(Car.new)).top_speed #160
   Boost.new(Nitro.new(Car.new)).top_speed #180

That’s it, our shiny new power packed cars are ready to be driven ;)