RSpec easy loop testing

decider.rb

# frozen_string_literal: true
class Decider
  def self.undecided
    loop do
      break if decision?
    end
  end

  def self.decision?
    # code yielding true or false
  end
end

decider_spec.rb

# frozen_string_literal: true
require 'rails_helper'

RSpec.describe Decider, type: :model do
  describe '.undecided' do
    subject { Decider.undecided }

    after { subject }

    context 'loop testing method 1' do
      # Break the loop by passing a breaking result/value in the spec setup.

      context 'a decision has been made' do
        before do
          allow(Decider).to receive(:decision?) { true }
        end

        it 'breaks the loop' do
          expect(Decider).to receive(:decision?).once
        end
      end

      context 'no decision has been made' do
        # `.and_return()` takes multiple arguments for iterative results
        # The last arg of true breaks the loop in the application code

        before do
          allow(Decider).to receive(:decision?).and_return(false, false, true)
        end

        it 'continues to loop' do
          expect(Decider).to receive(:decision?).exactly(3).times
        end
      end
    end

    context 'loop testing method 2' do
      # Take control of the loop with `.and_yield`, which can be
      # chained together for additional passes through the loop

      before do
        allow(Decider).to receive(:loop).and_yield.and_yield
      end

      context 'a decision has been made' do
        before do
          allow(Decider).to receive(:decision?) { true }
        end

        it 'breaks the loop' do
          expect(Decider).to receive(:decision?).once
        end
      end

      context 'no decision has been made' do
        before do
          allow(Decider).to receive(:decision?).and_return(false, false)
        end

        it 'continues to loop' do
          expect(Decider).to receive(:decision?).exactly(:twice)
        end
      end
    end
  end
end