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Transitions

A transition is a line with an arrowhead that links one graphical object to another. In most cases, a transition represents the passage of the system from one mode (state) to another. A transition typically connects a source and a destination object. The source object is where the transition begins and the destination object is where the transition ends. The following chart shows a transition from a source state, B, to a destination state, A.

Junctions divide a transition into transition segments. In this case, a full transition consists of the segments taken from the origin to the destination state. Each segment is evaluated in the process of determining the validity of a full transition.

The following example has two segmented transitions: one from state On to state Off, and the other from state On to itself:

A default transition is a special type of transition that has no source object. See Default Transitions for details.

Transition Hierarchy

Transitions cannot contain other objects the way that states can. However, transitions are contained by states. The hierarchy for a transition is described in terms of its parent, source, and destination states. The parent is the lowest level that contains the source and destination of the transition. Consider the parents for the transitions in the following example:

The following table resolves the parentage of each transition in the preceding example. The / character represents the chart. Each level in the hierarchy of states is separated by the period (.) character.

Transition Label

Transition Parent

Transition Source

Transition Destination

switch_off

/

/Power_on.Low.Heat

/Power_off

switch_high

/Power_on

/Power_on.Low.Heat

/Power_on.High

switch_cold

/Power_on.Low

/Power_on.Low.Heat

/Power_on.Low.Cold

Transition Labels

A transition label can consist of an event or message, a condition, a condition action, and a transition action. Each part of the label is optional. The ? character is the default transition label. Transition labels have this overall format:

event_or_message[condition]{condition_action}/transition_action

This example illustrates the parts of a transition label.

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 Transitions, 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 Transitions, 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.

  • 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 Transitions, 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.

Transition Actions

In transition label syntax, transition actions are preceded with a forward slash (/) and are enclosed in curly braces ({}). In the example in Transitions, 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.

Valid Transitions

Usually, a transition is valid when the source state of the transition is active and the transition label is valid. Default transitions are different because there is no source state. Validity of a default transition to a substate is evaluated when there is a transition to its superstate, assuming the superstate is active. This labeling criterion applies to both default transitions and general case transitions. The following table lists possible combinations of valid transition labels.

Transition Label

Is Valid If...

Event only

That event occurs

Event and condition

That event occurs and the condition is true

Message only

That message occurs

Message and condition

That message occurs and the condition is true

Condition only

Any event occurs and the condition is true

Action only

Any event occurs

Not specified

Any event occurs

Transition Connections

Transitions to and from Exclusive (OR) States

This example shows simple transitions to and from exclusive (OR) states.

The following transition...Is valid when...
B to AState B is active and the event E1 occurs.
A1 to A2State A1 is active and event E2 occurs.

See Transition Between Exclusive States for more information on the semantics of this notation.

Transitions to and from Junctions

The following chart shows transitions to and from connective junctions.

The chart uses temporal logic to determine when the input u equals 1.

If the input equals 1...A transition occurs from...
Before t = 2Start to Fast
Between t = 2 and t = 5Start to Good
After t = 5Start to Slow

For more information about temporal logic, see Control Chart Execution by Using Temporal Logic. For more information on the semantics of this notation, see Transition from a Common Source to Multiple Destinations.

Transitions to and from Exclusive (OR) Superstates

This example shows transitions to and from an exclusive (OR) superstate and the use of a default transition.

The chart has two states at the highest level in the hierarchy, Power_off and Power_on. By default, Power_off is active. The event Switch toggles the system between the Power_off and Power_on states. Power_on has three substates: First, Second, and Third. By default, when Power_on becomes active, First also becomes active. When Shift equals 1, the system transitions from First to Second, Second to Third, Third to First, for each occurrence of the event Switch, and then the pattern repeats.

For more information on the semantics of this notation, see Control Chart Execution by Using Default Transitions.

Transitions to and from Substates

The following example shows transitions to and from exclusive (OR) substates.

For details on how this chart works, see Debounce Signals with Fault Detection. For information on the semantics of this notation, see Transition from a Substate to a Substate with Events.

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