Techie Me

Suppose you have the following scenario:

class Foo
  def bar
    [:foo,:bar,:zoo].each { |var| instance_variable_set("@#{var.to_s}", rand(5)) }
  end
end

And you want to specify that Foo should receive instance_variable_set with :foo, :bar and :zoo. You can try this way:

describe Foo
  describe "#bar"
    subject { Foo.new }
    [...]
    it "sets the instance variable @zoo with a random number" do
      subject.should_receive(:instance_variable_set).with(:zoo)
      subject.bar
    end
  end
end

But this way when #bar calls instance_variable_set with :foo or :bar, you get a Spec::Mocks::MockExpectationError: “#<Foo:0x7f770e216d08> received :instance_variable_set with unexpected arguments”. Great, ain’t it? How are we supposed to fix that?

Here’s something that should work in this case:

describe Foo
  describe "#bar"
    subject { Foo.new }
    [...]
    it "sets the instance variable @zoo with a random number" do
      subject.stub(:instance_variable_set).as_null_object
      subject.should_receive(:instance_variable_set).with(:zoo)
      subject.bar
    end
  end
end

Since you don’t care, in this example, what instance_variable_set does, you can stub it as a null_object, which won’t complain when it gets unexpected messages. So instead of the #bar method actually calling instance_variable_set, it calls the stub!

Do you still want to assert that instance_variable_set is doing its job on setting variables? Then it’s a behavior that should be specified on another example! An example would be the following:

describe Foo
  describe "#bar"
    subject { Foo.new }
    [...]
    it "sets the instance variable @zoo with a random number" do
      subject.stub(:rand => (fake_random = double('fake-random')))
      subject.bar
      subject.instance_variable_get("@zoo").should == fake_random
    end
  end
end 

This approach is fairly simple and works as it should, letting you specify your code behavior without bothering with the need to assert that “ruby is doing its job right”.

What about you? Do you have any magic you would like to share about setting message expectations? Feel invited to comment!