Example of using RSpec doubles with processor expecting proc

Gemfile

source 'https://rubygems.org'

gem 'rspec'

processor.rb

class Processor
  # Receives:
  # params: a set of arguments which we use
  # block: a callable proc, which we use for some other need.

  def initialize(params, block)
    @params = params
    @block = block
  end

  # execute some complicated logic, returning some result, probably
  # including some influence from the passed-in block
  def run
    @block.call
  end
end

processor_spec.rb

require './processor'

describe Processor do
  let(:params) { {} }

  context do
    it "can be called with a block" do                              # See how the code behaves when we control the result from the block
      my_code = Proc.new { "some calculation result" }              # This block pretends to be what is used in production
      confirmer = described_class.new(params, my_code)
      expect(confirmer.run).to eq("some calculation result")        # Ensure that the result of confirmer.run is influenced by the result from block.call
    end

    it "can ensure it calls the block" do                           # Ensure that the block is called, without caring about the result
      my_code = double                                              # RSpec double
      expect(my_code).to receive(:call).and_return("some result")   # Set an expectation, which says:
                                                                    #  1) I expect 'call' to be invoked. (raise an exception at the end of this spec if this isn't true)
                                                                    #  2) I will return 'some result' when that is called.
      confirmer = described_class.new(params, my_code)
      confirmer.run
    end
  end
end