minitest_mock.rb

require 'minitest'
require 'minitest/mock'

class Test
  def initialize
    @vals = [1, 2, 3, 4, 5]
  end

  def a
    b
  end

  def b
    @vals.shift
  end
end

mock = MiniTest::Mock.new
mock.expect(:call, 'a')
mock.expect(:call, 'b')

test = Test.new

test.stub(:b, mock) do
  puts test.a
  puts test.a
end

mock.verify