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One of the crucial lessons of working with FtiNesse is that you should not place all the logic in the fixture. In actual fact it’s bad practice to place your business logic in the fixture. The fixture should be used as a gateway to your business classes. What we actually want to do is to create a separation between your business logic and the fitnesse harness as shown here
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This model makes it possible to view the contents of the business objects through any channel, a GUI, Web page or even a fixture. If we didn't provide the Business Session Object above, each of the channels shown would have to provide the same logic to coordinate the behaviour of the Business Objects. So what you would actually see is that the channels are poluted polluted with Business Object informationand information and the Business Objects are polluted with channel information. Which This would make changing either side difficult and after a while the whole code base would become unmaintable.maintainable.
View the above model through the prism of a sequence diagram
A better way to understand this model is to think of it in terms of UML use cases
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