How to do argument capture with spock framework?

I got it:

    SaveCommentEvent firedEvent

    given:
     ...

    when:
     ....

    then:
    1 * eventBus.fireEvent(_) >> {arguments -> firedEvent=arguments[0]}
    firedEvent instanceof SaveModelEvent
    firedEvent.newModel == newModel
    firedEvent.oldModel == oldModel

Same idea with @Alex Luya but put the assertions in the closure and use assert on each of them. cf. Spock Framework Reference Documentation.

    then:
    1 * eventBus.fireEvent(_) >> {
        def firedEvent = it[0]
        assert firedEvent instanceof SaveModelEvent
        assert firedEvent.newModel == newModel
        assert firedEvent.oldModel == oldModel
    }

then:
     1*eventBus.fireEvent(
                            {
                               it.source.getClass()==SaveCommentEvent;
                               it.oldComment==oldComment;
                               it.newComment==newComment
                             }
                          )            

In your code it is a Groovy Closure Implicit Variable reference to a mock eventBus Interface which has no fields. How could you verify them?

Also, I think the order of events that has to happen to use Spock Mocks is not necessarily intuitive. I would write it up here except it would not be as good as Kenneth Kousen's explanation.