A bit about decorators and presenters

Posted by Ilija Eftimov on May 20, 2016

Object-oriented programming and design is (or, was?) a revolutionary way of thinking and designing programs. It introduced classes, objects, inheritance, polymorphism and many other ways to think about programming. As an addition, some very smart folks identified some pitfalls and patterns that occur in object-oriented programming and put them in books. That’s how we got a list of general code smells, design patterns and refactoring patterns that we can use in our everyday work.

But, is it that simple? Unfortunately, some of the patterns look very similar on the surface, and have subtle differences. Also, the craft lies in applying these patterns, over learning them by heart. In the past, I’ve been always confused between the decorator pattern and the presenter pattern. And if you are (or used to be) confused by these, believe me - it’s normal. Even the naming is so similar, I would be surprised if someone new on the block is not confused. Let’s go over some examples and see what makes the similar, but different.

Decorators

If you think about these patterns in a hierarchical way, the decorator pattern would live on the top of the hierarchy. This means that, a presenter is kind of subpattern of the decorator pattern. The decorator is a general purpose pattern, whose role is to attach additional responsibilities to an object dynamically. While “dynamically attach additional responsibilities” might sounds fancy, feel free to think about it as “adding functionalities on the fly, when needed”. Also, the decorator pattern as a side-effect, provides a flexible alternative to subclassing.

Decorating over subclassing

Very often, when we have some hierarchy of classes, we use subclassing (inheritance) to add some specific functionality to our subclass, while sharing the inherited functionality from the superclass. In cases like this, the decorator pattern gives us the flexibility to apply a decorator (or more decorators) to a class, instead of creating a class hierarchy, to get additional (or more specific) functionality on the class.

Also, the downside of subclassing is that it’s static, which means it’s applied to a whole class. On the other hand, a decorator can be applied on runtime, meaning it will decorate the object when needed.

Example

Let’s see a small example of what a decorator might look like. Ruby makes defining decorators so flexible and easy, which makes it hard to choose which way to do it.

Our base class will be a CoffeeMachine:

class CoffeeMachine
  attr_reader :price

  def initialize(price)
    @price = price
  end
end

Now, some coffee machines have a milk steamer attached. To make a SteamedMilkCoffeeMachineDecorator decorator, you can go couple of ways. The first one would be to make it a PORO:

class SteamedMilkCoffeeMachineDecorator
  def initialize(obj)
    @obj = obj
  end

  def price
    @obj.price + 300
  end

  def can_steam_milk?
    true
  end
end

Basically, when the object is decorated, it will override the price method on the CoffeeMachine object, and it will also decorate a method can_steam_milk?. The downside of using this approach is that we will need to add some method_missing magic, because we want all of the methods that are not present on the decorator to be delegated to the decorated object.

Or, we can use Ruby’s SimpleDelegator :

class SteamedMilkCoffeeMachineDecorator < SimpleDelegator
  def price
    super + 300
  end

  def can_steam_milk?
    true
  end
end

This decorator will have the same functionality as the PORO decorator above, with the addition of delegating the methods, that are unknown to the decorator, to the wrapped object.

Using the decorator is plain simple:

coffee_machine = CoffeeMachine.new(500)
with_steamer = SteamedMilkCoffeeMachineDecorator.new(coffee_machine)

coffee_machine.price
#=> 500

with_steamer.can_steam_milk?
#=> true

with_steamer.price
#=> 800

If you would like to see a nice summary of decorator implementations, I would recommend you read Evaluating Alternative Decorator Implementations In Ruby by Dan Croak.

The Presenter pattern

Now that we have a good grasp of the decorator pattern, let’s see what it is, and where the presenter pattern shines. As we mentioned in the beginning of this article, the presenter is a “subpattern” of the decorator. The main difference between them is how close they live to the view. Presenters live very close to the view layer, while decorators being very broad, can live near the business logic of your application.

Within a Rails application, presenters can be seen in various shapes. Most often, presenters are used to keep logic out of the views:

class Apartment
  attr_reader :area

  def initialize(sq_meters: sq_meters)
    @area = sq_meters
  end
end

class ApartmentPresenter < Decorator
  decorates :apartment

  def size
    if area < 30
      "Small"
    elsif area < 50
      "Cozy"
    elsif area < 80
      "Big"
    else
      "Huge"
    end
  end
end

As you can tell, this example uses the very popular draper gem that enables easy creation of presenters. We can use our newly created presenter as:

apartment = ApartmentPresenter.new(Apartment.new(sq_meters: 75))
apartment.area #=> 75
apartment.size #=> "Big"

With presenters like this, you can always simplify your views. Whatever the type of logic that you usually would put into helper methods, you can use presenters for it. Presenters are more object-oriented way to achieve the same goal.

Wrapping up

If you think just a bit more broadly about decorators, you can see that basically decorators are the open/closed principle (the O, from the SOLID principles) put into practice. The open/closed principle states that a class should be open for extension, but closed for modification, which if applied in our context, easily paints the picture of how decorators extend classes.

As it usually goes, when something is easy to create, abusing it even easier. Gems like Draper allow us to create decorators without any hassle, so it’s pretty easy to create fat decorators or to use them in weird ways. Also, another thing to keep in mind is that although decorators help with applying SOLID principles to your code, the decorators themselves need to oblige to these principle at the same time.

Nevertheless, learning about the decorator pattern is a very useful addition to your design patterns toolbelt. Although at time using it in the proper manner can require some additional thinking (and maybe even prototyping), it can improve the flexibility of your design.

Additional reading

These are some links that I would recommend you if you would like to learn more about the decorator pattern:


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