What DCI gets us: from Procedure to Context

01.rb

# Ruby-esque pseudocode

def transfer_money_between_accounts(acct1, acct2, amount)
  puts "Source balance is: #{acct1.balance}"
  puts "Destination balance is: #{acct2.balance}"

  acct1.balance -= amount
  acct2.balance += amount

  puts "Source balance is now: #{acct1.balance}"
  puts "Destination balance is now: #{acct2.balance}"
end

transfer_money_between_accounts(Account.new(1000), Account.new(0), 245)

02.rb

# Better signature and variable names

def transfer_money(Account source, Account destination, amount)
  puts "Source balance is: #{source.balance}"
  puts "Destination balance is: #{destination.balance}"

  source.balance -= amount
  destination.balance += amount

  puts "Source balance is now: #{source.balance}"
  puts "Destination balance is now: #{destination.balance}"
end

transfer_money(Account.new(1000), Account.new(0), 245)

03.rb

# Use domain language do describe the actions

def transfer_money(Account source, Account destination, amount)
  puts "Source balance is: #{source.balance}"
  puts "Destination balance is: #{destination.balance}"

  # Withdraw from source account
  source.balance -= amount

  # Deposit into destination account
  destination.balance += amount

  puts "Source balance is now: #{source.balance}"
  puts "Destination balance is now: #{destination.balance}"
end

transfer_money(Account.new(1000), Account.new(0), 245)

04.rb

# Move actions onto respective objects

class TransferMoney
  module Source
    def withdraw(amount)
      self.balance -= amount
    end
  end

  module Destination
    def deposit(amount)
      self.balance += amount
    end
  end

  def execute(Account source, Account destination, amount)
    source.extend Source
    destination.extend Destination

    puts "Source balance is: #{source.balance}"
    puts "Destination balance is: #{destination.balance}"

    source.withdraw(amount)
    destination.deposit(amount)

    puts "Source balance is now: #{source.balance}"
    puts "Destination balance is now: #{destination.balance}"
  end
end

TransferMoney.execute(Account.new(1000), Account.new(0), 245)

05.rb

# Prepare for other entry points

class TransferMoney
  module Source
    def withdraw(amount)
      self.balance -= amount
    end
  end

  module Destination
    def deposit(amount)
      self.balance += amount
    end
  end

  def initialize(Account source, Account destination, amount)
    @source = source
    @source.extend Source

    @destination = destination
    @destination.extend Destination

    @amount = amount
  end

  def execute
    puts "Source balance is: #{@source.balance}"
    puts "Destination balance is: #{@destination.balance}"

    @source.withdraw(@amount)
    @destination.deposit(@amount)

    puts "Source balance is now: #{@source.balance}"
    puts "Destination balance is now: #{@destination.balance}"
  end
end

TransferMoney.new(Account.new(1000), Account.new(0), 245).execute

06.rb

# Other entry points

class TransferMoney
  module Source
    def withdraw(amount)
      self.balance -= amount
    end
  end

  module Destination
    def deposit(amount)
      self.balance += amount
    end
  end

  def initialize(Account source, Account destination, amount)
    @source = source
    @source.extend Source

    @destination = destination
    @destination.extend Destination

    @amount = amount
  end

  def transfer
    @source.withdraw(@amount)
    @destination.deposit(@amount)
  end

  def transfer_verbosely
    puts "Source balance is: #{@source.balance}"
    puts "Destination balance is: #{@destination.balance}"

    transfer

    puts "Source balance is now: #{@source.balance}"
    puts "Destination balance is now: #{@destination.balance}"
  end
end

TransferMoney.new(Account.new(1000), Account.new(0), 245).transfer_verbosely