Using a mock of an S4 class

Hello,

I have a function that applies to an S4 object which contains a slot called @analysis:

function calculation(myObject) {
  tmp <- myObjects at analysis
  result <- ...operations on analysis...
  return result
}

I am writing a unit test for this function.  So I was hoping to create a mock object but I can't figure out how to do it:

test_that("test calculation function", {
  mockMyObject<- mock(?????)  #I am not sure what to put here
  r<-calculation(mockMyObject)
  expect_true(r,0.83625)
})

How can I create a mock S4 object??

Thanks in advance,
Ramiro