Common Mechanisms of UML

Advanced States and Transitions

• Entry and Exit Actions 
• Internal Transitions
• Do-Activities
• Deferred Event

Entry and Exit Actions: 

• Entry actions are actions done upon entry of a state and are shown by the keyword event 'entry' with an appropriate action 
• Exit actions are actions done upon exit from a state marked by the keyword event 'exit', together with an appropriate action

Internal Transitions: 

• Internal transitions are events handled internally without leaving the state 
• Internal transitions may have events with parameters and guard conditions

Do-Activities: 

• Activities that make use of object's idle time when inside a state
• 'do' transition specifies the work done inside a state after the entry action is dispatched

Deferred Event: 

• A deferred event is a list of events whose occurrence in the state is postponed until a state in which the listed events are not deferred 
• It becomes active at the time they occur and may trigger transitions as if they had just occurred 
• A deferred event is specified by listing the event with the special action 'defer'

SUB STATES

• Sequential Substates
• History States
• Concurrent Substates

Sequential Substates

• Sequential substates are those substates in which an event common to the composite states can easily be exercised by each state inside it at any time 
• Sequential substates partition the state space of the composite state into disjoint states 
• A nested sequential state machine may have at most one initial state and one final state

History States 

• It allows composite state that contains sequential substates to remember the last substate that was active in it prior to the transition from the composite state 
• A shallow history state is represented as a small circle containing the symbol 'H' 
• The first time entry to a composite state doesn't have any history 
• The symbol 'H' designates a shallow history, which remembers only the history of the immediate nested state machine 
• The symbol 'H*' designates deep history, which remembers down to the innermost nested state at any depth 
• If there is only one level of nesting, shallow and deep history states are semantically equivalent

Concurrent Substates 

• Concurrent substates specify two or more state machines that execute in parallel, in the context of the enclosing object 
• Execution of these concurrent substates continues in parallel 
• These substates wait for each other to finish to join back into one flow 
• A nested concurrent state machine does not have an initial, final, or history state