Using RSpec Mocks' and_yield

rspec_and_yield_example.rb

class CSVOddUpcaser
  # Pretty contrived, but whatever.
  #
  # Reads rows from a CSV object, upcases the data in the odd numbered
  # rows and writes rows to a new CSV object.


  def initialize(csv)
    @csv = csv
  end

  def write(new_csv)
    @csv.each_with_index do |row, i|
      if i % 2 == 0
        new_csv << row.map(&:upcase)
      else
        new_csv << row
      end
    end
  end
end


describe CSVOddUpcaser do
  # We stub a CSV instance because:
  #
  # 1. We'd like to define our test data as ruby data structures
  # instead of csv strings
  #
  # 2. We don't want to couple our test to CSV. Any class that has
  # #each_with_index and #<< can be used with CSVOddUpcaser.

  let(:source_csv) { double }
  let(:dest_csv) { double }

  before do
    allow(source_csv).to receive(:each_with_index).
      and_yield(['first', 'row', 'data'], 0).
      and_yield(['second', 'row', 'data'], 1).
      and_yield(['third', 'row', 'data'], 2).
      and_yield(['fourth', 'row', 'data'], 3)
  end

  describe '#write' do
    it 'should upcase only the odd rows' do
      expect(dest_csv).to receive(:<<).with(['FIRST', 'ROW', 'DATA']).ordered
      expect(dest_csv).to receive(:<<).with(['second', 'row', 'data']).ordered
      expect(dest_csv).to receive(:<<).with(['THIRD', 'ROW', 'DATA']).ordered
      expect(dest_csv).to receive(:<<).with(['fourth', 'row', 'data']).ordered

      upcaser = CSVOddUpcaser.new(source_csv)
      upcaser.write(dest_csv)
    end
  end
end