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Transition Action Types

Transitions can have different action types, which include event or message triggers, conditions, condition actions, and transition actions. The action types follow the label notation with this general format:

event_or_message trigger[condition]{condition_action}/{transition_action}

The following example shows typical transition label syntax:

TransitionEvent TriggerConditionCondition ActionTransition Action
State A to state Cevent1temp > 50func1()None
State A to state Bevent2NoneNonedata1 = 5

Event or Message Triggers

In transition label syntax, event or message triggers appear first as the name of an event or message. They have no distinguishing special character to separate them from other actions in a transition label. In the example in Transition Action Types, both transitions from state A have event triggers. The transition from state A to state B has the event trigger event2 and the transition from state A to state C has the event trigger event1.

Event triggers specify an event that causes the transition to be taken, provided the condition, if specified, is true. Specifying an event is optional. Message triggers specify the transition to be taken if the message is present in the message queue. The absence of an event or message indicates that the transition is taken upon the occurrence of any event. Multiple events or messages are specified using the OR logical operator (|).

Conditions

In transition label syntax, conditions are Boolean expressions enclosed in square brackets ([]). In the example in Transition Action Types, the transition from state A to state C has the condition temp > 50.

A condition is a Boolean expression to specify that a transition occurs given that the specified expression is true. Follow these guidelines for defining and using conditions:

  • The condition expression must be a Boolean expression that evaluates to true (1) or false (0).

  • The condition expression can consist of any of the following:

  • The condition expression can call a graphical function, truth table function, or MATLAB® function that returns a numeric value.

    For example, [test_function(x, y) < 0] is a valid condition expression.

    Note

    If the condition expression calls a function with multiple return values, only the first value applies. The other return values are not used.

  • The condition expression should not call a function that causes the chart to change state or modify any variables.

  • Boolean expressions can be grouped using & for expressions with AND relationships and | for expressions with OR relationships.

  • Assignment statements are not valid condition expressions.

  • Unary increment and decrement actions are not valid condition expressions.

Condition Actions

In transition label syntax, condition actions follow the transition condition and are enclosed in curly braces ({}). In the example in Transition Action Types, the transition from state A to state C has the condition action func1(), a function call.

Condition actions are executed as soon as the condition is evaluated as true, but before the transition destination has been determined to be valid. If no condition is specified, an implied condition evaluates to true and the condition action is executed.

Note

If a condition is guarded by an event, it is checked only if the event trigger is active. If a condition is guarded by a message, it is checked only if the message is present.

Transition Actions

In transition label syntax, transition actions are preceded with a forward slash (/) and are enclosed in curly braces ({}). In the example in Transition Action Types, the transition from state A to state B has the transition action data1 = 5. In C charts, transition actions are not required to be enclosed in curly braces. In charts that use MATLAB as the action language, the syntax is auto corrected if the curly braces are missing from the transition action. See Auto Correction When Using MATLAB as the Action Language.

Transition actions execute only after the complete transition path is taken. They execute after the transition destination has been determined to be valid, and the condition, if specified, is true. If the transition consists of multiple segments, the transition action executes only after the entire transition path to the final destination is determined to be valid.

Transition actions are supported only in Stateflow® charts in Simulink® models.