dummy_spec.rb

class Foo
	attr_reader :bar

	def change_bar
		bar ? @bar = false : @bar = true
	end
end

describe 'Foo' do
	let(:foo) { Foo.new }
	
	describe '#change_bar' do
		context 'when bar is true' do
			before { allow(foo).to receive(:bar).and_return(true) }

			it 'sets bar to false' do
				foo.change_bar

				allow(foo).to receive(:bar).and_call_original #this line is ugly

				expect(foo.bar).to eq(false)
			end
		end
	end
end