2 Why We Need Model-Based Testing

118 Structuring Model Programs with Features and Composition
7.2.1 Defining features
To define a feature, declare its state variables, action methods, and enabling methods
in a class. The feature name is the class name. Label the class with the [Feature]
attribute.
Each feature can be selectively included or excluded from the model program.
Classes that are not labeled with the [Feature] attribute are always included. State
variables that are declared within a feature should only be used by that feature. State
variables that are used by more than one feature should be declared in a class that is
not a feature, so they will always be included.
Figure 7.3 shows excerpts from a version of the client/server model program
where the client and server are separate features (compare to the original version in
Chapter 5, Figures 5.7–5.10). All of the state variables are used by both the client
and the server, so they remain in the ClientServer class, which is not a feature. The
state variables are declared internal to make them accessible in the feature classes.
The action methods and enabling conditions of the client and server are declared in
the new Client and Server classes, which are features.
7.2.2 Including or excluding features
Features are included in a model program by naming them in the LibraryModel-
Program constructor (Chapter 6, Section 6.2.1). This constructor has an optional
third argument, a set of strings that name the features to include. If this argument
is omitted, all features are included. You can write several factory methods, where
each one names a particular combination of features. When you invoke a tool, use
the factory method that names the features you want to include.
In Figure 7.3, the factory method CreateServerOnly names the server feature but
not the client. This command runs mpv on the client/server model program with just
the server feature included.
mpv /r:ClientServer.dll ClientServer.Factory.CreateServerOnly
Figure 7.4 shows the diagram that mpv displays. The client feature was excluded
from the model program, so only a few server actions are enabled (the ones that do
not involve communication with the client).
7.2.3 Combining features
Features can be combined to model interacting features in the implementation, or
to achieve scenario control. To combine features, give some of their action methods
the same names. Action methods in classes that are not features (which are always
included in the model program) can also have the same names. All of the action
methods that have the same name are parts of the same action; they are executed
together. The combined action is enabled only when all the enabling conditions for
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