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 Label Notation

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.

Event or Message Trigger

Specifies an event or message that causes the transition to occur when the condition is true. Specify multiple events using the OR logical operator (|). Specifying an event or message is optional. The absence of an event or message indicates that the transition takes place on the occurrence of any event. For more information, see Synchronize Model Components by Broadcasting Events and Communicate with Stateflow Charts by Sending Messages.

In the previous example, the broadcast of event E triggers the transition from On to Off if the condition [off_count==0] is true.

Condition

Specifies a Boolean expression that, when true, validates a transition for the specified event or message trigger. Enclose the condition in square brackets ([]). If no condition is specified, an implied condition evaluates to true. For more information, see Conditions.

In the previous example, when the event E occurs, the condition [off_count==0] must evaluate as true for the transition from On to Off to be valid.

Condition Action

Executes after the condition for the transition is evaluated as true, but before the transition to the destination is determined to be valid. Enclose the condition action in curly braces ({}) following the condition. For more information, see Condition Action Behavior.

In the previous example, if the event E occurs and the condition [off_count==0] is true, then the condition action {off_count = off_count + 1} is immediately executed.

Transition Action

Executes after the transition to the destination is determined to be valid. If the transition consists of multiple segments, then the transition action is executed when the entire transition path to the final destination is determined to be valid. Transition actions occur after the exit actions of the source state and before the entry actions of the destination state. Precede the transition action with a /. For more information, see Condition and Transition Action Behavior.

In the preceding example, if the event E occurs and the condition [off_count==0] is true, then the transition action {Light_off()} is executed when the transition from On to Off is determined to be valid. The transition action occurs after On becomes inactive, but before Off becomes active.

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 `A`	State `B` is active and the event `E1` occurs.
`A1` to `A2`	State `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 = 2	`Start` to `Fast`
Between t = 2 and t = 5	`Start` to `Good`
After t = 5	`Start` 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.