PureMVC ActionScript 3 Utility: State Machine
This utility provides a simple yet effective Finite State Machine implementation, which allows the definition of discrete states, and the valid transitions to other states available from any given state, and the actions which trigger the transitions. A mechanism is provided for defining the entire state machine in XML and having a fully populated StateMachine injected into the PureMVC app.
Standard and MultiCore versions are included.
Production - Version 1.2
Demos Using This Utility
Platforms / Technologies
Changes in 1.2
- Added entry-guard cancel checking to StateMachine transition handling. This means both the entry and exit guards (the responders to 'entering' and 'exiting' announcements) have an opportunity to cancel the transition.
Changes in 1.1
- State-specific 'changed' notification. In addition to the 'entering' and 'exiting' notifications for a State, you may also define a 'changed' notification that will be sent when the state is finally entered. This notification is where you should begin any code that will possibly send another StateMachine.ACTION note.
- Passing data between States. When sending a StateMachine.ACTION notification, you may include any Object as the body of the notification and it will be included in the state-specific 'exiting', 'entering', and 'changed' notifications. This allows, for instance, the data collected from a UI form in the 'DATA_ENTRY' state to be passed to the 'DATA_SAVING' state when the form's 'Save' button is pressed, for instance.
- States can be re-entrant. For instance you might have a PRODUCT_MENU state, that could alternately be showing menswear, sporting goods, or telephones. While displaying menswear, you might click a sporting goods link, which leads to a StateMachine.ACTION note with the PRODUCT_MENU state specified in the type parameter, and a constant value for sporting goods passed as the body of the note. The state-specific 'changed' notification that is defined for your PRODUCT_MENU state might be a command that looks at the body of the note to determine what products to display.
- These changes are backward compatible. The StopWatch demo still works with this code, it just doesn't implement any of the new features. A more 'real-world' demo will be forthcoming.
Features in 1.0
- A StateMachine class, which extends Mediator and maintains the FSM representation as a set of discrete, named States. This Mediator is interested in StateMachine.ACTION and StateMachine.CANCEL notifications. It exposes methods for regsistering and removing States.
- The States held and navigated by the StateMachine are instances of a State class, which carries several critical pieces of information about that State. Each State has optional associated Notifications to be sent on entry into the State and exit from the State.
- The exiting notification carries a reference in the body to the State that we are transitioning to. This helps actors respond properly by anticipating the destination state.
- The entering notification for a State carries a reference in the body to the state we are entering as well, in case you've sub-classed State to pass data.
- It is up to the programmer to define and register commands or mediators with interest in these entering and exiting notifications, the state machine simply sends them at the appropriate times.
- The State class also exposes methods for defining and removing transitions. A transition simply maps an action name to a target State name.
- The transition from one state to the next is triggered by any actor sending a StateMachine.ACTION Notification. Include the name of the action in the Notification's type parameter.
- Actions are what trigger the StateMachine to initiate the transition from the one State to the next. There is no formal Action class at this time, it is merely a name that will trigger a State transition.
- It is up to the application to ensure any special conditions for making the transition are met before sending the StateMachine.ACTION Notification, which will initiate the transition immediately if one is defined for the input action given the current State.
- Any actor responding to the Notification sent when exiting a state may send a StateMachine.CANCEL notification, which will cause the StateMachine not to enter the next State. The programmer must insure that no other responses to the current State's exit notification need to be rolled back. This is best done by checking any items that could lead to a StateMachine.CANCEL being sent before initiating any exit activity such as visual transitions or form clearing/population, thus avoiding the need to rollback to restore the application to the state being exited.
- Finally, when a transition is complete, the StateMachine sends a StateMachine.CHANGED Notification, with a reference to the new State. This is sent once any exiting notification for the previous state, as well as any entering notification for the new state have both been executed, and the current state of the StateMachine has been updated to the new state.
- Also included in this release is useful class that allows you to define your FSM in an XML format, and pass it to the FSMInjector where it will be parsed, and a fully populated StateMachine instance will be created and registered via your Facade.
- The FSMInjector extends Notifier to give it a reference to the Facade for injecting the completed StateMachine.
- The XML format for the FSM Injector is simple. For instance here is the FSM for a door:
<fsm initial="CLOSED"> <state name="OPENED" entering="openingNote" exiting="aboutToCloseNote"> <transition action="CLOSE" target="CLOSED"/> </state> <state name="CLOSED" entering="closingNote"> <transition action="OPEN" target="OPENED"/> <transition action="LOCK" target="UNLOCKED"/> </state> <state name="LOCKED" entering="lockingNote"> <transition action="UNLOCK" target="CLOSED"/> </state>
- The above FSM defines three discrete states OPENED, CLOSED and LOCKED.
- The actions OPEN, CLOSE and LOCK are used to trigger state transitions.
- It is only possible to LOCK the door when it is CLOSED, because only the CLOSED state defines a transition targeting the LOCKED state.
- It is not possible to OPEN the door from the LOCKED state because no transition is defined targeting the OPEN state.
- And when you UNLOCK the door, it returns to the CLOSED state, where it is once again possible to OPEN or LOCK.
- An exiting notification is defined only for exiting the OPEN state to illustrate that entering and exiting notifications are optional.
PureMVC AS3 Utility - State Machine - Copyright © 2007-2008 Neil Manuell, Cliff Hall
PureMVC - Copyright © 2007-2012 Futurescale, Inc.
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