[spyrefused]

I found quite useful an example I read in the book "The Art of Unit Testing" by Roy Osherove, it was something like this (I don't have it at hand, I repeat it from memory):

Suppose you have to test the irrigation system of a tree in your garden, you want to test how many times and how much water is used, you have two ways to do it:

1) State-based: run the system for a few hours, during which it should water the tree several times. At the end of that time, you check the state of the watered tree: Is the soil moist enough, is the tree well, are the leaves green, etc. (Obviously this is just an example as this test would be complex in real life, but it is to get the idea across).

2) Interaction: We place a flow meter, a device that registers how much water is flowing through the device and at what times. At the end of the day, check that the device has been called the correct number of times, with the correct amount of water each time, and we don't worry about checking the status of the tree.