A State-Based Programming Model for Wireless Sensor Networks

94 Chapter 5. The Object-State Model
the entry actions of B and then be evaluated in the guard, selecting the actual
initial state. This is possible since the entry actions of a superstate are always
executed before the initial-state condition is evaluated.
A
g
C
e
f
e
f
inB1()
inB2()
inB3()
inB4()
B
var int a
[a=1]
[else]
Figure 5.10: Conditional entrance to the substates D and E of B, depending on
guards over the state variable a. The state variable a can be set in
the entry actions of B, for example, depending on which transition
was taken.
5.5.3 Substate Preemption
When a transition fires, it ends the lifetime of its source state. We say the transition
terminates the source state normally. If the source state of a transition is
a superstate, the transition also preempts its substates (i.e., the embedded state
machine). Then we say the transition preempts the source state’s substates. States
can only be left either through normal termination or through preemption. Substates
are preempted level-by-level, starting from the bottom-most state in the
hierarchy. A substate cannot prevent its superstate from being left (and thus
itself from being preempted).
If a state is preempted, its exit actions do not fire as during normal termination.
In order to allow the preempted state to react to the termination or preemption
of its superstate (and thus its own preemption), preemption actions are
introduced. Preemption actions are useful, for example, to release resources that
where initialized in the preempted states’ entry actions. Each state can have a
single preemption action. This preemption action is invoked as the state is preempted.
Preemption actions are invoked in the order in which their states are
preempted, that is, from the bottom of the hierarchy to its top. The substates’
preemption actions are invoked before the exit action of their superstate. Preemption
actions of parallel states are executed in any order.
The scope of the preemption action is the to-be-preempted substate. That is, a
preemption action can access all state variables its state before it is actually being
preempted plus all state variables of (direct and indirect) superstates. Note
however, that a transition terminating a state directly does not trigger that state’s
preemption action—any required operations can be performed in the regular
exit action. In a preemption action, there is no means to determine any information
of the actual transition causing the preemption, such as its trigger and its
source state.
D
E