programming-examples/ruby/Basics/single_responsibility.rb

# The Single Responsibility Principle is the most abstract of the bunch. It
# helps keep classes and methods small and maintainable. In addition to keeping
# classes small and focused it also makes them easier to understand.

# While we all agree that focusing on a single responsibility is important, it’s
# difficult to determine what a class’s responsibility is. Generally, it is said
# that anything that gives a class a reason to change can be viewed as a
# responsibility. By change I am talking about structural changes to the class
# itself (as in modifying the code in the class’s file, not the object’s
# in-memory state).
# In the below class we have a single command interface that processes
# commission payments for deals. At first glance the class seems simple enough,
# but let’s look at reasons we might want to change this class. Any change in
# how we calculate commissions would require a change to this class. We could
# introduce new commission rules or strategies that would cause our
# calculate_commission method to change. For instance, we might want to vary
# the percentage based on deal amount. Any change in the steps required to mark
# a deal as processed in the mark_deal_processed method would result in a change
# in the file as well. An example of this might be adding support for sending an
# email summary of a specific person’s commissions after marking a deal
# processed. The fact that we can identify multiple reasons to change signals a
# violation of the Single Responsibility Principle.

class DealProcessor
  attr_reader :deals

  def initialize(deals)
    @deals = deals
  end

  def process
    deals.each do |deal|
      # Here we calculate commission and create instance of Commission
      Commission.create(deal: deal, amount: calculate_commission(deal))
      mark_deal_processed
    end
  end

  private

  def mark_deal_processed
    # Implementation
  end

  def calculate_commission(deal)
    deal.amount * 0.05
  end
end

class Commission
  # Implementation
end

# Solution

class DealProcessor
  attr_reader :deals

  def initialize(deals)
    @deals = deals
  end

  def process
    deals.each do |deal|
      # Now we call calculator in one operation, all logic now in it
      CommissionCalculator.create_commission(deal) if mark_deal_processed
    end
  end

  private

  def mark_deal_processed
    # Implementation
  end
end

class CommissionCalculator
  def self.create_commission(deal)
    Commission.new(deal: deal, amount: calculate(deal))
  end

  private

  def self.calculate(deal)
    deal.amount * 0.05
  end
end

class Commission
  # Implementation
end