A factory's job is to manufacture the correct player of a role. Factories don't know what to do, they know how to choose who does.

Code that knows:

why one condition applies over another
the behavior needed in that case

is evil! 😼

For example, look at this contrived code:

  def get_button_type(number)
    case number
    when 1
      number > 1 ? 1 : 0
    when 2
      2
    when number > 8
      number % 2 == 0 ? number : number + 1
    else
      number * 3
    end
  end

This example knows the answers to a few subtle questions:

How many different kinds of numbers are there?
Which numbers are most alike? In what way?
Which numbers are most different? In what way?
What is the rule to determine the behavior given a number?

This code is hard to change because it has intimate knowledge of so many details.

Instead we can use factories with polymorphism. The factory will figure out which class to use based on a single condition. It knows nothing of the behavior. This separation of concerns makes our code resiliant to change, easy to reason about, and very extensible.

Factories can be open or closed.

#  Open factory
#  ------------
#  + No conditionals
#  + Very extensible if we follow the pattern
#  - Harder to understand because it uses metaprogramming
#  - No explicit class references in code
#  - Ignores classes whose name does not follow convention
module ActionButton
  class Factory
    def self.for(type, options={})
      begin
        const_get("ActionButton::{type.to_s.camelize}")
      rescue NameError
        Default
      end.new(options)
    end
  end
end

#  Closed factory
#  --------------
#  + Understands the reasons for switching
#  + Knows the name of the class that supplies the behavior
#  + Not coupled to a naming convention
#  - Uses conditionals
#  - Not open
#  - More complex to extend
module ActionButton
  class Factory
    def self.for(type, options={})
      case type
      when :success
        ActionButton::Success
      when :recommend
        ActionButton::Recommend
      when :recommended
        ActionButton::Recommended
      when :like_with_count
        ActionButton::LikeWithCount
      when :unlike_with_count
        ActionButton::UnlikeWithCount
      else
        ActionButton::Default
      end.new(options)
    end
  end
end

Files and directories to look at

app/factories
app/controllers/action_buttons_controller.rb
routes.rb
app/views/action_buttons

byelipk/polymorphic-button-factories

Files and directories to look at