Flexibility with and without Dependency Injection

dependency_injection_example.rb

require 'minitest/autorun'

class TaxCodeBase
  class AbstractMethod < StandardError
  end

  attr_accessor :user_id

  def initialize(user_id=nil)
    self.user_id = user_id
  end

  def generate
    raise AbstractMethod, "Implement in subclass"
  end
end

class UsTaxCode < TaxCodeBase
  def generate
    "US-#{user_id}"
  end
end

class BrazilTaxCode < TaxCodeBase
  def generate
    "#{user_id}-BR"
  end
end

class Customer
  attr_accessor :id

  def initialize(id)
    self.id = id
  end

  def tax_code(tax_code_generator = UsTaxCode.new)
    tax_code_generator.user_id = self.id
    tax_code_generator.generate
  end
end

class WhimsicalTaxCode < TaxCodeBase
  def generate
    "****#{user_id.to_s.reverse}****"
  end
end

describe Customer do
  it "has an american tax code by default" do
    Customer.new(123456).tax_code.must_equal "US-123456"
  end

  it "can compute any type of tax code" do
    customer = Customer.new(123456)
    customer.tax_code(BrazilTaxCode.new).must_equal "123456-BR"
    customer.tax_code(UsTaxCode.new).must_equal "US-123456"
    customer.tax_code(WhimsicalTaxCode.new).must_equal "****654321****"
  end
end