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We use this information to
derive a state model.
To this end, we select a use case and take the pertaining sequence diagram that
describes the standard interaction between the blocks ( Figure 2.75 ). Our goal is to
derive state machines of the blocks that occur in the interaction we are looking at.
We think along the lifelines to see what state that block is in at the respective
position. The state could theoretically have changed after each event that
occurred on that lifeline, e.g., when the block received a message and responded
to it. We can denote the current state as a state invariant directly on the lifeline in
the appropriate position ( Figure 2.84 ).
In the next step, we use the states we found to describe a state machine for
each block, which may be very trivial. However, we have looked at only one fl ow
of a use case so far. So we will take more interactions and the activities of that use
case and extend the state machine accordingly. Each fl ow variant has to refl ect in
Table 2.21 Summary: Creating a state model.


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