A stream property is denoted with the word {stream} on the parameter
( Figure 3.37 ).
Playing marbles: The fl ow of an activity is determined by the so-called token
fl ow . A token can be thought of as a marble that rolls along the edges through the
activity. If it encounters an action, then this action can be activated.
??? Playing marbles ??? begins as soon as the activity is invoked. One marble is placed
on every initial node and every input parameter of the activity. More specifi cally, a
control token each is put on the initial nodes and an object token each is put on the
input parameters. An object token is a marble, which additionally describes that the
specifi ed object is present at the respective position within the fl ow. It can then be
consumed, for example, by an action as an input object over a pin.
Two conditions have to be met in order for the marble to roll over an edge:
1. The edge has to be ready to transport the marble. For example, this can be
infl uenced by a condition denoted at the edge.
3.6 The Activity Diagram
FIGURE 3-36
Notation for activity parameters.
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