gistfile1.rb

# http://blog.firsthand.ca/2011/12/example-using-rspec-double-mock-and.html

describe Transfer do
  context "transfer with amount of 10" do
    let(:source_account)      { double :source_account, :decrease => nil }
    let(:destination_account) { double :destination_account, :increase => nil }

    def transfer_amount(amount)
      Transfer.new(source_account, destination_account, amount).call
    end

    it "should decrease source account by 10" do
      source_account.should_receive(:decrease).with 10

      transfer_amount 10
    end

    it "should increase destination account by 10" do
      destination_account.should_receive(:increase).with 10

      transfer_amount 10
    end
  end
end