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8.2 Flow Transitions
Given a collection of action states, how
are those action states related to
one another? Flow transitions address this issue. As discussed in
Chapter 2, a flow transition
shows how action states are ordered or sequenced. There are various
types of flow transitions, including control-flow and object-flow
transitions, which are mentioned in Chapter 2 and
discussed here.
8.2.1 Control-Flow Transitions
A control-flow transition indicates the
order of action states. That is,
once a source action state completes its processing, a target action
state starts its processing. For example, the project management
system may have the following order of action states for the task of
generating a report:
The Project Manager Enters Report Criteria action
state occurs first, because the project manager must enter the report
criteria before the system can generate a report.
The Project Management System Generates Report
action state occurs next, because the project management system must
generate the report before the printer can print the report.
The Printer Prints Report action state occurs
last, once the report has been generated by the project management
system.
In the UML, a control-flow transition is shown as a solid line from a
source action state to a target action state. Figure 8-3 shows the order of action states associated
with the project management system.
Figure 8-3. Control-flow transitions
Figure 8-3 also shows a control-flow transition
originating from the initial state to indicate the first action
state, and a transition to the final state to indicate the last
action state.Control-flow transitions are also known as default
transitions or automatic transitions,
because they are unlabeled and are immediately triggered after the
source action state completes processing.
8.2.2 Object-Flow Transitions
An object-flow transition indicates that
an action state inputs or outputs an
object. For example, the action states shown in Figure 8-3 may have inputs and outputs as follows:
Project Manager Enters Report Criteria
Outputs a Report Criteria
object and may be renamed as the Project Manager
Enters Data action state
Project Management System Generates Report
Inputs the Report Criteria
object and outputs a Report object and may be
renamed as the Project Management System Generates
Information action state
Printer Prints Report
Inputs the Report object and may be renamed as the
Printer Prints
Information action state
In the UML, an object-flow transition is shown as a dashed arrow
between an action state and an object. An action state that uses an
object as input is shown with the object-flow transition arrow
pointing from the object to the action state. An action state that
updates or produces an object as output is shown with the object-flow
transition arrow pointing from the action state to the object. Figure 8-4 shows the objects used by the action states
associated with the project management system.Notice the new names for the action states in Figure 8-4 as compared to Figure 8-3.
In Figure 8-3, the action state names conveyed some
idea of the inputs and outputs from each state. Figure 8-4 however, shows these inputs and outputs
explicitly, so there's no longer any need to imply
them redundantly in the action state names. The advantage is the
action state names can now focus purely on the actions, while the
object-flow transitions indicate clearly the inputs and outputs to
and from the actions.
Figure 8-4. Control-flow and object-flow transitions
A control-flow transition may be omitted when an object-flow
transition indicates the ordering of action states; that is, when an
action state produces an output that is input to a subsequent action
state, the object-flow transition implies a control-flow transition
and an explicit control-flow transition is not necessary. Figure 8-5 updates Figure 8-4 by
removing the unnecessary control-flow transitions.
Figure 8-5. Control-flow and object-flow transitions without unnecessary control-flow transitions
