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7.3 Advanced State Diagrams
To be flexible and powerful, the UML allows you to
put one or more state diagrams inside a single state to indicate that
when an element is in that state, other elements inside of it have
their own states. For example, within the Active
state of the project management system, each object and subsystem has
its own state, and the state diagram for each element may be shown
inside the Active state of the project management
system or by using the element's own state diagram.
This recursive nature of state diagrams is very powerful, because it
allows you to learn the basic notation of state diagrams and reapply
it to nested state diagrams without having to learn more notation.In the UML, a state may be shown as a rectangle with rounded corners
and three compartments separated by horizontal solid lines. Figure 7-5 shows the Active state of
the project management system with three compartments. The top
compartment of a state contains the name of the state or a
description of the situation of the element.The middle compartment of a state may contain transitions for the
following special events:
entry
Indicates the action to be performed when the element enters the state
exit
Indicates the action to be performed when the element exits the state
do
Indicates the action to be performed continuously while the element
is in the state
Figure 7-5 shows that when the project management
system enters the Active state, a message is
logged using the LogMessage action. When the
project management system exits the Active state,
the user's date and time is retrieved using the
getDateAndTime action of the OS
object. Finally, when the project management system is in the
Active state, it continuously monitors its usage
using the MonitorUsage action.The bottom compartment of a state may contain one or more nested
state diagrams separated by dashed lines, each with a title that
describes the element to which the state diagram pertains. When a
state becomes the current state of an element, all its nested state
diagrams become active concurrently.Figure 7-5 shows that when the project management
system enters the Active state, the following
nested state diagrams each has a current state:
Main Graphical User Interface Window or Foreground Processing
Describes the lifecycle of the user interface. There are two states,
including the Shown state, which indicates that
the user interface is visible to the user, and the
Hidden state, which indicates that the user
interface is not visible to the user. The Hide
event makes the user interface not visible to the user, and the
Show event makes the user interface visible to the
user.
Background Processing
Describes the lifecycle of how the system works as it is handling
requests from the user. There are two states, including the
Idle state, which indicates that the project
management system is not handling any user request, and the
Busy state, which indicates that the project
management system is handling a user request. The
DoProcessing event makes the project management
system busy handling a user request, and the
ProcessingComplete event makes the project
management system idle waiting to handle a user request.
Figure 7-5. Advanced state diagrams
All the transitions going from a state pertain to each nested state
diagram; if one of these nested state diagrams activates an outgoing
transition, all nested state diagrams are forcibly exited. Figure 7-5 indicates that when the project management
system is in the Active state and a
Severe Error event occurs, both nested state
diagrams are terminated independent of which nested state diagram
caused the event to occur.
