2 Why We Need Model-Based Testing

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B(2) C()
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Systems with Finite Models 127
Figure 7.12. M1 × M2 projected onto M2.
FSM(0, AcceptingStates(), Transitions(t(0,A(),1), t(1,B(2),2)))
Figure 7.13. M1 as an FSM text file.
executes an action of M2, that transition is added to the projection onto M2. In this
example the projection of M1 × M2 onto M2 looks like the original M2 (Figure 7.7),
except the action B() from M2 is replaced by the action B(2) from the product.
Sometimes the projection has fewer transitions than the original program. This is
significant when composition is used for analysis (Section 7.5).
7.3.2 FSMs for scenario control
We often write a scenario model program to compose with a contract model program
for scenario control. It is often convenient to express a scenario model program as
an FSM. The library and tools support several styles for coding model programs.
All of the examples so far have used just one of these styles, the style introduced
in Chapter 5 and described in Appendix A. In this section we introduce two more
styles that make it easier to code FSMs for scenario control.
First we show how to code an FSM in the familiar style. Figure 7.13 shows
program M1 from Section 7.3.1, coded in C#, in the same style as the other model
programs we have seen. There is just a single state variable, which corresponds
to the state numbers shown in the state bubbles in Figure 7.6. We call a program
coded in this style a library model program. There is a factory method that invokes
the constructor for the LibraryModelProgram class, which is derived from the
ModelProgram base class. To display the FSM, compile the program and invoke mpv:
mpv /r:M1.dll M1.Factory.Create
To compose and display two model programs, for example, M1 and M2 as in
Figure 7.8,invokempv with both:
mpv /r:M1.dll /r:M2.dll M1.Factory.Create M2.Factory.Create
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