Action C starts only
once action B has terminated, since there are now tokens present at both incoming
edges. In total we have the execution sequence A , B , C , and object S is transported
by A past B directly to C .
Turn to page 99; you will see that we used the Detour pattern in Figure 2.62 .
After most actions in an activity there is some checking as to whether or not
they were successful. The decision after an action is based on the outgoing object
fl ow: The object is either present or not, or the action was successful or unsuccessful.
The fact the no object is present doesn ??™ t mean that there is no object fl ow.
What fl ows here is a so-called null token ??”an empty object. The UML specifi cation
says that the edge must go to and end in an object node. This means that the
action following that action needs an input pin for formal reasons although it may
do without input data. A zero token is not very informative.
FIGURE 2-94
Using a Detour pattern.
act Detour pattern
A
S
S
S
B
C
A
B
C
137 2.8 Marginal Notes
UML knows so-called control pins , which accept object tokens, so that they
can cause an action to execute, but without passing the object on to the action.
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