2 Why We Need Model-Based Testing

40 Why We Need Model-Based Analysis
The dispatcher is a collection of if statements. In the first if statement, Reset() is
the call to the handler and ResetEnabled() is the guard, a Boolean expression that
must be true when the handler is invoked. A handler whose guard is true is said to
be enabled. The design logic of the controller described in Section 3.1 is achieved
by coding the guards and the bodies of the handlers. For example, here is the guard
for the Reset handler:
bool ResetEnabled()
{
return (cevent == ControlEvent.Timeout
&& waitfor == WaitFor.Timout && sensor == Sensor.Error);
}
This guard causes the program to invoke the Reset handler when the time-out event
occurs, provided that the control program is waiting for a time-out and the sensor is
considered to be erroneous. In contrast, here is the guard for the Poll handler:
bool PollEnabled()
{
return (cevent == ControlEvent.Timeout
&& waitfor == WaitFor.Timeout && sensor == Sensor.OK);
}
This guard is similar to the previous guard, but it invokes Poll (instead of Reset)
after the time-out event when the sensor is considered to be working correctly.
Notice that different handlers might be invoked after the same event, depending on
the values of other variables. That is why we must provide a dispatcher, instead of
simply associating each handler with an event.
Each handler assigns some of the variables that are tested by the guards. Here
Reset assigns waitfor to indicate that the program is now waiting for a message
(instead of a time-out).
void Reset()
{
Console.WriteLine(" Reset");
ResetSensor(); // send reset command to sensor
StartTimer(MessageTimeout); // wait for response from sensor
waitfor = WaitFor.Message;
}
Also, the call to ResetSensor commands the sensor to reset, which should cause
another message event soon. But if no message arrives – if the sensor does not
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