UML Weekend Crash Course™ - To Parent Directory

Session 22—Modeling the Extended Features of the Statechart 233
control is shown by a single transition divided into multiple arrows pointing to multiple
states or substates. The divide is accomplished with the fork bar you used in the Activity
diagram and is illustrated in Figure 22-8.
Merge of control can be modeled as multiple transition arrows pointing to a synchronization
bar, as you saw earlier in learning about the Activity diagram. Synchronization is not
shown in Figure 22-8.
when temperature received(temperature)
[temperature>76F]/
^AirConditioner.turnOn()
Cooling
after tempdelay(seconds)
^Thermometer.getTemperature()
Monitor Cooling
Monitor Device
Exit:^AirConditioner.turnOff()
Figure 22-8
Split of control
Concurrency
By allowing multiple Statecharts within a state, the UML supports concurrency within a
state. To model concurrency, simply split the superstate internal transition compartment
into as many separate compartments as needed, one for each sub-Statechart. In the thermostat
example, the Thermostat is doing two jobs at the same time: monitoring the cooling
device and watching for problems with the device. So the Cooling state internal transition
compartment is split in two by a line.
Note that in this particular example each substate provides a different transition out of
the superstate. The Monitor Cooling substate is watching for the event “when temperature
received(temperature) [temperature < 70].” If it receives this event, it takes the Thermostat
out of the Cooling state and back to monitoring the temperature. So what happens to the
other substate Well, this hits on the definition of substate. A substate is a state within a
state — in this case, Monitor Device within Cooling. If the Thermostat leaves the Cooling
state, by definition it also leaves the Monitor Device state. This is illustrated in Figure 22-9.
The same is true if the Monitor Device state receives the event it is waiting for.