demeter_example.rb

# This example is from Dan Manges's blog, dcmanges.com

class Wallet ; attr_accessor :cash ; end
class Customer
  # assume Customer's initialize() method sets the wallet attribute
  #  to be a wallet instance full of money
  attr_accessor :wallet
end
class Paperboy
  def collect_money(customer, amount)
    # VIOLATION OF DEMETER: Paperboy may know about Customer, but
    #  we're exposing things about implementation of Wallet too.  Also,
    #  we're handling the problem of "not enough cash" in Paperboy,
    #  not in Wallet (where it arguably belongs)
    if customer.wallet.cash < amount
      raise InsufficientFundsError
    else
      customer.wallet.cash -= due_amount
      @collected_amount += due_amount
    end
  end
end

# Imagine testing the above code:
describe "collecting money" do
  it "should raise error if customer doesn't have enough money" do
    # "Mock trainwreck" is a warning of a Demeter violation
    wallet = mock('wallet', :cash => 5.00)
    customer = mock('customer', :wallet => wallet)
    lambda { @paperboy.collect(customer, 10.00) }.should raise_error(...)
  end
end

# This is better: we *delegate* the cash attribute via Customer.
# Now Paperboy only "talks to" Customer.

class Customer
  def cash
    self.wallet.cash
  end
end

class Paperboy
  def collect_money(amount)
    if customer.cash >= amount
      customer.cash -= due_amount
      @collected_amount += due_amount
    else
      raise InsufficientFundsError
    end
  end
end

# This is the best, because the *behavior* is delegated.  The implementation
# of the behavior can now be changed without affecting Paperboy.

class Wallet
  attr_reader :cash # no longer attr_accessor!
  def withdraw(amount)
     raise InsufficientFundsError if amount > cash
     cash -= amount
     amount
  end
end
class Customer
  # behavior delegation
  def pay(amount)
    wallet.withdraw(amount)
  end
end
class Paperboy
  def collect_money(customer, due_amount)
    @collected_amount += customer.pay(due_amount)
  end
end