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# The Single Responsibility Principle is the most abstract of the bunch. It
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# helps keep classes and methods small and maintainable. In addition to keeping
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# classes small and focused it also makes them easier to understand.
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# While we all agree that focusing on a single responsibility is important, it’s
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# difficult to determine what a class’s responsibility is. Generally, it is said
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# that anything that gives a class a reason to change can be viewed as a
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# responsibility. By change I am talking about structural changes to the class
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# itself (as in modifying the code in the class’s file, not the object’s
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# in-memory state).
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# In the below class we have a single command interface that processes
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# commission payments for deals. At first glance the class seems simple enough,
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# but let’s look at reasons we might want to change this class. Any change in
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# how we calculate commissions would require a change to this class. We could
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# introduce new commission rules or strategies that would cause our
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# calculate_commission method to change. For instance, we might want to vary
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# the percentage based on deal amount. Any change in the steps required to mark
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# a deal as processed in the mark_deal_processed method would result in a change
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# in the file as well. An example of this might be adding support for sending an
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# email summary of a specific person’s commissions after marking a deal
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# processed. The fact that we can identify multiple reasons to change signals a
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# violation of the Single Responsibility Principle.
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class DealProcessor
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attr_reader :deals
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def initialize(deals)
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@deals = deals
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end
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def process
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deals.each do |deal|
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# Here we calculate commission and create instance of Commission
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Commission.create(deal: deal, amount: calculate_commission(deal))
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mark_deal_processed
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end
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end
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private
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def mark_deal_processed
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# Implementation
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end
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def calculate_commission(deal)
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deal.amount * 0.05
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end
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end
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class Commission
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# Implementation
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end
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# Solution
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class DealProcessor
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attr_reader :deals
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def initialize(deals)
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@deals = deals
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end
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def process
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deals.each do |deal|
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# Now we call calculator in one operation, all logic now in it
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CommissionCalculator.create_commission(deal) if mark_deal_processed
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end
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end
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private
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def mark_deal_processed
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# Implementation
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end
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end
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class CommissionCalculator
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def self.create_commission(deal)
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Commission.new(deal: deal, amount: calculate(deal))
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end
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private
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def self.calculate(deal)
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deal.amount * 0.05
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end
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end
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class Commission
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# Implementation
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end
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