About State diagrams

A state diagram shows the sequences of states that an object or an interaction goes through during its life in response to received stimuli, together with its responses and actions.  A state machine is a graph of states and transitions that describes the response of an object of a given class to the receipt of outside stimuli. A state machine is attached to a class or a method.
 

Main State diagram elements

State

A state is a condition during the life of an object or an interaction during which it satisfies some condition, performs some action, or waits for some event. An object remains in a state for a finite (non-instantaneous) time.  Actions are atomic and uninterruptible. A state may correspond to ongoing activity. Such activity is expressed as a nested state machine. Alternately, ongoing activity may be represented by a pair of actions, one that starts the activity on entry to the state and one that terminates the activity on exit from the state.

In a State diagram, states are represented by State node icons.  The State icon's context menu is documented in the User Reference: State Context Menu.
 

Internal Transitions

Internal transitions, also called Actions, are those that occur within the context of a single state, as opposed to transitions (see topic below) that occur between two states. The State node icon has two sections, Name and Internal Transitions.

Transition links

A simple transition is a relationship between two states indicating that an object in the first state will enter the second state and perform certain specified actions when a specified event occurs if specified conditions are satisfied. On such a change of state, the transition is said to fire. The trigger for a transition is the occurrence of the event labeling the transition. The event may have parameters which are available within actions specified on the transition, or within actions initiated in the sub-sequent state. Events are processed one at a time. If an event does not trigger any transition, it is simply ignored. If it triggers more than one transition within the same sequential region (i.e., not in different concurrent regions), only one will fire; the choice may be nondeterministic if a firing priority is not specified.

A complex transition may have multiple source states and target states. It represents a synchronization and/or a splitting of control into concurrent threads without concurrent substates.  A complex transition is enabled when all of the source states are occupied. After a complex transition fires, all of its destination states are occupied.

A complex transition is shown as a short heavy bar (a synchronization bar, which can represent synchronization, forking, or both). The bar may have one or more solid arrows from states to the bar (these are the source states); the bar may have one or more solid arrows from the bar to states (these are the destination states). A transition string may be shown near the bar. Individual arrows do not have their own transition strings.
 

Working with State diagrams

Creating Internal Transitions

To create an Internal Transition in a State:
1. Right click on the State
2. Choose New Internal Transition from the context menu.
You can view and edit Internal Transition properties in the Properties tab of the Inspector when the Internal Transition  is selected on the State node icon.

Creating Transitions

To create a  Transition between States:
1. Click on the Transition button on the State diagram toolbar.
2. In the Diagram pane, drag from one State node icon to a second State node icon and drop.
You can view and edit Transition properties in the Properties tab of the Inspector when the Action is selected on the diagram.

Creating a Transition to self

Just drag the transition link from a State and back again.  On releasing the mouse button, a transition link to from the State back to itself is created.

Creating complex Substate diagrams

A state can be decomposed into concurrent substates AND relationships, or by using OR relationships into mutually exclusive disjointed substates. A given state may only be refined in one of these two ways. Its substates may be refined in either of the same ways.  A newly-created object starts in its initial state. The event that creates the object may be used to trigger a transition from the initial state symbol.

By enlarging the State element on the diagram, you can use the State to model Substates, including Start State, End State, Syncbar and History as well as other States as Substates.  You can add elements to this State by clicking on the toolbar icon and then on the State or by dragging an existing element into the State nodes enlarged area.

Chapter contents

About State diagrams
Main State diagram elements
State
Internal Transitions
Transition Link
Working with State diagrams
Creating Internal Transitions
Creating Transitions
Creating a Transition to self
Creating complex Substate diagrams

 

GO TO TOP

CHAPTER 6

CONTENTS

GETTING STARTED

HELP CONTENTS

© 1997-98