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cannot represent the receiver of the signal in the diagram, though this information
is present in the model. Here the receiver is the card reader that switches its
light-emitting diodes as a result of the signal (Section 2.4.5).
There is also an action to receive a signal??”an AcceptEventAction ??”modeled in
Figure 3.42 . Here we receive the signal timeout reservation system . It is a timing
signal, shown by a symbolized hourglass. The left-hand receive symbol from Figure
3.43 is used for all other signal variants.
The AcceptEventAction in Figure 3.42 is not within a fl ow, like SendSignal
Actions , which is also permitted. In this case, it plays the special role of receiving
a signal to terminate the interruptible activity region.
The action concept is not only known in UML. There are a number of textual
languages that map elementary actions and support a runtime environment. The
Action Specifi cation Language ( ASL ) was developed in the early 1990s. Later versions
of the language consider the action model of UML 1.


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