PROXY

(object structural pattern)

GoF definition (p. 207):

"Provide a surrogate or placeholder for another object to control access to it."

A Proxy must

• maintain a reference that lets the proxy access the real subject
• provide an interface identical to the subject's
• control access to the real subject

Most common types of proxies

1. Remote proxy: provides a local representative for an object in an address space.
2. Virtual proxy: serves as a placeholder for more expensive objects.
3. Protection proxy: controls access to the original object.

Pros:

• A remote proxy can hide the fact that an object is not stored locally.
• A virtual proxy can create more complicated objects on demand.

Examples

Subject

class BankAccount
  def initialize(balance)
    @balance = balance
  end

  def deposit(amount)
    @balance += amount
  end

  def withdraw(amount)
    @balance -= amount
  end

  def balance
    @balance
  end
end

Proxy

(in this example, a protection proxy)

class AccountProtectionProxy
  def initialize(real_account, password)
    @subject = real_account
    @password = password
  end

  def deposit(amount)
    check_password
    @subject.deposit(amount)
  end

  def withdraw(amount)
    check_password
    @subject.withdraw(amount)
  end

  def balance
    check_password
    @subject.balance
  end

  private

  def check_password
    puts "Password:"
    password = gets.chomp
    raise "Invalid password" unless password == @password
  end
end

Implementation

my_account = BankAccount.new(1000)
proxy = AccountProtectionProxy.new(my_account, "supersecret")
proxy.withdraw(100)
puts proxy.balance