Protection Proxy Pattern example that respects Interface Segregation

protection_proxy.rb

class BankAccount
  attr_reader :balance
  def initialize(starting_balance)
    @balance = starting_balance
  end

  # if this were #irl we'd need to worry about race conditions here
  def deposit(amount)
    @balance += amount
  end

  def withdraw(amount)
    @balance -= amount
  end
end

# The Etc module provides access to information typically stored in files
# in the /etc directory on Unix systems.
# require 'etc'

# one approach taken from the excellent book Design Patterns in Ruby by Russ Olsen# class AccountProtectionProxy
# this is great since we dont need to write proxy methods by hand, and this class will never need to change!
#
# class AccountProtectionProxy
#   def initialize(real_account, owner_name)
#     @real_account = real_account
#     @owner_name = owner_name
#   end

#   def method_missing(name, *args)
#     access_control!
#     @real_account.send(name, *args)
#   end

#   private def access_control!
#     raise("Access denied! #{Etc.getlogin} is unknown") unless Etc.getlogin == @owner_name
#   end
# end

# the downside, in my opinion, is that this AccountProtectionProxy class will blindly
# proxy any object you give it
#
# the essence of the Interface Segregation Principle is that a client should be provided with an
# interface limited only to the methods needed in the context of the client's interaction
#
# so, let's be more explicit about what we want to expose but still obviate the need to
# write out each proxied method manually. we use the Forwardable module to do so:

require 'etc'
require 'forwardable'

class AccountProtectionProxy
  extend Forwardable
  def_delegators :@real_account, :balance, :deposit, :withdraw

  def initialize(real_account, owner_name)
    @real_account = real_account
    @owner_name = owner_name
  end

  private def access_control!
    raise("Access denied! #{Etc.getlogin} is unknown") unless Etc.getlogin == @owner_name
  end
end

ba = BankAccount.new(100)
proxy = AccountProtectionProxy.new(ba, 'bob')
proxy.deposit 20
puts proxy.balance
proxy.withdraw 120
puts proxy.balance