2 Why We Need Model-Based Testing

Systems with Finite Models 69
conditions, preconditions, orguards. Enabling conditions are the control structures
of model programs. Writing enabling conditions is the only way to control the
sequence of actions that can occur when a model program executes. When coding
enabling conditions, it is helpful to refer to the sample traces you wrote during
preliminary analysis.
An enabling condition is a Boolean method that returns a bool value (a Boolean
method is sometimes called a predicate). The enabling conditions of an action
method have the same name as the action method, with the added suffix Enabled.
An action method that has no enabling condition is always enabled (as if it had an
enabling condition that always returned true).
An enabling condition describes a set of states: all the states where it returns true.
In this example, the enabling condition for the ShowTitles action method (described
before) is ShowTitlesEnabled. It expresses that ShowTitles is enabled in all states
where the page is a Topics page and the style is WithText. In this model program,
there are two such states, where the third state variable sort takes on both of its two
possible values.
static bool ShowTitlesEnabled()
{
return (page == Page.Topics && style == Style.WithText);
}
Enabling conditions may have parameters, which correspond to the parameters
of its action method. The parameters in the two methods are matched up by order,
so the first parameter in each must have the same type (etc.), and they are alwayes
called with the same arguments (the tools ensure this). An enabling condition may
have fewer parameters than its action method. An enabling condition may use its
parameters, as well as the state variables, to compute its return value.
An action method may have more than one enabling condition by overloading
(they have the same name but different parameters). All of the enabling conditions
must be true to enable the action. A static enabling condition method may be used
for an instance-based action method (Chapter 15).
An enabling condition must be a pure method: it can only return true or false;it
must not have any side effects that update state variables or parameters. An enabling
condition can include calls to helper methods, provided that those methods are also
pure.
You can code the enabling conditions of several action methods such that more
than one action is enabled in some states. For example, in our initial state both
SelectMessages and ShowTitles are enabled. This is typical. Our model program
exhibits nondeterminism: it is not possible to predict (from the model program itself)
which of the enabled actions will be invoked. Any enabled action can be chosen at
run time by the tool that executes the model program. A model program that exhibits
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