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846Model-Based Requirements Documentation(e.g., “if automaton A is in state 4”). Figure 6-16shows a behavior modelfor a navigation device of a vehicle by means of a statechart. The naviga-tion device is initially in the state “navigation device inactive”.Figure 6-16 Simplified statechart of a vehicular navigation deviceTransition into super stateBy turning on the navigation device (event: “navigation device activated”),the system transitions into the super state “navigation device active” (moreprecisely, the system transitions into the initial state “no GPS signal” of thesuper state “navigation device active”). The super state “navigation deviceactive” is refined by a partial automaton that consists of two states. Forexample, if a GPS signal is received in the state “navigation device active:no GPS signal”8. the system transitions into the state “navigation deviceactive: GPS signal” and issues a notification. If the device is deactivatedwhile in the state “navigation device active” (event: “navigation device deac-tivated”), the system transitions into the state “navigation device inactive”. 6.7.2UML State DiagramsModeling reactive behaviorof a system using UMLIn order to model reactive system behavior, Unified Modeling Language(UML) [OMG 2007] offers so-called state machines that are essentiallybased on statecharts. Figure 6-17shows the most important modeling ele-ments of UML state diagrams. The notation of the modeling elements ofUML state diagrams has largely been adopted from statecharts. However,UML 2 extends the modeling elements of statecharts, e.g., by the ability todefine explicit entry and exit points of hierarchical states [OMG 2007]. 8.For unique identification, a state that is part of a super state is referenced by “super state:state”. The state “no GPS signal” in the super state “navigation device active” is thereforereferenced as “navigation device active: no GPS signal”.
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6.7Requirements Modeling in the Behavioral Perspective85Figure 6-17Modeling elements of state machines as defined by the UML 2States and transitionsJust as in statecharts, a state defines a period of time in which a systemshows a particular behavior and waits for a particular event to occur. Atransition is triggered by an event that occurs in a particular state anddescribes the change from one state to the next. A transition can bedependent on a condition. In addition, the system can perform actions thatare directed toward the system or its environment.Hierarchization and concurrencyDepending on the purpose of the model, state machines allow hierar-chically combining states into super states, thereby abstracting from thepotentially very complex behavior of these states. Aside from hierarchi-cally decomposing states by means of partial automata, a state can bedecomposed into several concurrent state machines. Just as in statecharts,synchronization of concurrent state machines can be achieved using con-ditions.
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