Features of StateCharts Nested states and hierarchy Improves scalability and understandability Helps describing preemption Concurrency two or more states can be viewed as simultaneously active Nondeterminism there are properties which are irrelevant Example FSM Hierarchy When FSM is in any sub states of S an event k would preempt the FSM to transition to z Concurrency Soda Machines Coke Machine Pepsi Machine Coke machine and Pepsi machine are simultaneously active The use of AND state avoids exponential state explosion Definitions Current states of FSMs are also called active states States which are not composed of other states are called basic states States containing other states are called super states For each basic state s the super states containing s are called ancestor states Super states S are called OR super states if exactly one of the substates of S is active whenever S is active superstate ancestor state of E substates Edge Labels Transition may be taken if event occurred in the last step and condition is true Action is carried out when the transition is taken Action performs assignments to variables and generation of events Assignments to variables do not take effect until the following step Default State Default state is entered whenever super state is entered Effectively hides the internal details of a state from external states In example A is the default state of S History In state Z on input m FSM enters the state S was in before S was left can be A B C D or E If S is entered for the very first time or history has been cleared the default state is entered STATEMATE Semantics Changes in a given step should take effect in the next step Pros deterministic and simple Cons induce synchronization overhead for concurrent software simulation Race Condition X has value 0 prior to action Action X X 1 X X 2 What are the possible values of X after the action is executed 0 1 2 3 Basic Configuration A basic configuration is a set of basic states that a system can be put in simultaneously Basic configurations are B1 B2 C1 D1 C1 D2 C2 D1 and C2 D2
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