RSpec: how to test if a method was called?

it "should call 'bar' with appropriate arguments" do
  expect(subject).to receive(:bar).with("an argument I want")
  subject.foo
end

In the new rspec expect syntax this would be:

expect(subject).to receive(:bar).with("an argument I want")

The below should work

describe "#foo"
  it "should call 'bar' with appropriate arguments" do
     subject.stub(:bar)
     subject.foo
     expect(subject).to have_received(:bar).with("Invalid number of arguments")
  end
end

Documentation: https://github.com/rspec/rspec-mocks#expecting-arguments

To fully comply with RSpec ~> 3.1 syntax and rubocop-rspec's default option for rule RSpec/MessageSpies, here's what you can do with spy:

Message expectations put an example's expectation at the start, before you've invoked the code-under-test. Many developers prefer using an arrange-act-assert (or given-when-then) pattern for structuring tests. Spies are an alternate type of test double that support this pattern by allowing you to expect that a message has been received after the fact, using have_received.

# arrange.
invitation = spy('invitation')

# act.
invitation.deliver("[email protected]")

# assert.
expect(invitation).to have_received(:deliver).with("[email protected]")

If you don't use rubocop-rspec or using non-default option. You may, of course, use RSpec 3 default with expect.

dbl = double("Some Collaborator")
expect(dbl).to receive(:foo).with("[email protected]")

• Official Documentation: https://relishapp.com/rspec/rspec-mocks/docs/basics/spies
• rubocop-rspec: https://docs.rubocop.org/projects/rspec/en/latest/cops_rspec/#rspecmessagespies