Dependency Injection Principle: Entities must depend on abstractions not on concretions. It states that the high level module must not depend on the low level module, but they should depend on abstractions.

dip.rb

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

  def mail_notification
    InvoiceMailer.new.send(@invoice)
  end

  def sms_notification
    InvoiceSMS.new.send(@invoice)
  end

  def letter_notification
    InvoiceLetter.new.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

# Enforces DIP
class InvoiceNotifier
  def initialize(invoice)
    @invoice = invoice
  end

  def notify(notifier: InvoiceMailer.new)
    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