Smalltalk and Object Orientation: an Introduction - Free

18.2.3.1 A start point
A start point is the point at which the state diagram is initialized. In the figure, there are actually four
start points indicated (‘Start’, ‘lock’, ‘close’ and ‘unlock’). The ‘Start’ start point is the initial entry
point for the whole diagram, while the other three indicated start points are for sub State diagrams.
Any preconditions required by the State diagram c an be specified on the transition from the start
point (for example, in the figure the transmittedID must be the same as the memoryID memorized by
the system). It is the initial transition from which all other transitions emanate. This transition is
automatically taken when the State diagram is executed. Note that the initial ‘Start’ point is not
equivalent to a state.
18.2.3.2 Events
Events are one way asynchronous transmissions of information from one object to another. They can
possess parameters with names and types. The general format of an event is: eventName
(parameter:type, ...). Of course many events do not have any associated parameters.
18.2.3.3 A set of transitions
These are the statements which move the system from one state to another. In the state diagrams eac
transition is formed of four (optional) parts:
h
1. an event (e.g. lock).
2. a condition (e.g. transmittedID = memoryID]
3. the initiated event (e.g. ^EngineManagementUnit.locked)
4. an operation (e.g. /setDoorToLock)
The event is what triggers the transition, howev er the transition will only occur if the condition is
met. If the event occurs and the conditions are met, then the associated operation is performed. An
operation is a segment of code (equivalent to a statement or program or method within a programming
language) which causes the system state to be altered. Some transitions can also trigger an event which
should be sent to a specified object. For example, the above example sends an event locked to the
EngineManagementUnit. The process of sending a global event is a special case of sending an event to a
specified object.
The syntax of an event is therefore:
event(arguments) [condition] ^target.sendEvent(arguments) /operation(arguments)
18.2.3.4 A set of state variables
These are variables referred to in a State diagr
have the format name: type = value.
am (for example memoryID is a state variable). They
18.2.3.5 A set of states
A state represents a period of time during which an object is waiting for an event to occur. It is an
abstraction of the attribute values and links of a n object. A state is drawn as a rounded box containing
the (optional) name of the state. A state may often be composed of other states (the combination of
which represents the higher level state). A state has duration, that is it occupies an interval of time.
A state box can contain two additional sections, one section containing a list of state variables and
the other section containing a list of triggered operations (as illustrated by Figure 18.5).
car alarmed
sensors: List = {}
entry / set alarm active
do / check sensors
exit / set alarm inactive
Figure 18.5: State box with state variables and triggered operations
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