Working through the DCI example from Singapore Ruby (2013-1-8)

dci_spec.rb

require 'minitest/autorun'

class Account < Struct.new :balance
end

class MoneyTransfer < Struct.new :source, :destination
  def initialize source, destination
    super
    source.extend Transferrer
  end

  def execute amount
    source.transfer_to destination, amount
  end

  private

  module Transferrer
    def transfer_to destination, amount
      self.balance -= amount
      destination.balance += amount
    end
  end
end

describe MoneyTransfer do
  let(:rich_account) { Account.new 100 }
  let(:poor_account) { Account.new 25 }

  before do
    transfer_context = MoneyTransfer.new rich_account, poor_account
    transfer_context.execute 25
  end

  it('should redistrict wealth from the rich man') { rich_account.balance.must_equal 75 }
  it('to the poor man') { poor_account.balance.must_equal 50 }
end