the beginning of what testdouble.rb might look like

testdouble.rb

class TestDouble
  attr_accessor :instance, :when_stack

  def initialize(instance)
    @instance = instance
    @when_stack = []
  end

  def when(&block)
    fake_method_call = FakeMethodCall.new(block)
    if instance.respond_to?(fake_method_call.method_name) # also check arity
      add_to_when_stack(block)
    else
      raise StandardError.new("you tried calling #{fake_method_call.method_name} but your instance doesn't respond to that. Use when! if that is intentional")
    end
  end

  def when!(&block)
    add_to_when_stack(block)
  end

  private

  def add_to_when_stack(&block)
    fake_method_call = FakeMethodCall.new(block)
    @when_stack << fake_method_call
    fake_method_call
  end

  def method_missing(m, *args, &blk)
    if stubbed_method = @when_stack.find {|when_stack_item| when_stack_item.method_name == m && when_stack_item.method_args == args}
      stubbed_method.return_value
    else
      super
    end
  end
end

class FakeMethodCall
  attr_accessor :method_name, :method_args, :return_value

  def initialize(block)
    instance_eval(&block)
  end

  def then_return(return_value)
    @return_value = return_value
  end

  def method_missing(m, *args, &blk)
    @method_name, @method_args = m, args
  end
end

class MyClass
  def do_something
end

# THIS IS THE API
my_class_test_double = TestDouble.new(MyClass.new)
my_class_test_double.when{do_something(23)}.then_return('Josh')
puts my_class_test_double.do_something(23)