Open-closed Principle: Objects or entities should be open for extension, but closed for modification.

ocp.rb

# Violates OCP
class InvoiceNotifier
  def initialize(invoice)
    @invoice = invoice
  end

  def notify(channel)
    case channel
    when 'email'
      send_email
    when 'sms'
      send_sms
    when 'letter'
      send_letter
    else
      raise NotImplementedError
    end
  end

  def send_email
    # Logic to send email notification
  end

  def send_sms
    # Logic to send sms notification
  end

  def send_letter
    # Logic to send letter notification
  end
end

# Enforces OCP
class InvoiceNotifier
  def initialize(invoice, notifier: InvoiceMailer.new)
    @invoice = invoice
    @notifier = notifier
  end

  def notify
    @notifier.send(@invoice)
  end
end

class InvoiceMailer
  def send(invoice)
    # Send notification using email
  end
end

class InvoiceSMS
  def send(invoice)
    # Send notification using sms
  end
end

class InvoiceLetter
  def send(invoice)
    # Send notification using letter
  end
end

# Some examples

# Send invoice email
InvoiceNotifier.new(@invoice).notify

# Send invoice SMS message
InvoiceNotifier.new(@invoice, notifier: InvoiceSMS.new).notify

# Send invoice regular email letter
InvoiceNotifier.new(@invoice, notifier: InvoiceLetter.new).notify