Finite State Machine(FSM)???

Example:

First off,let's overview this simple as3-based Finite State Machine System:
1.IState;
2.IFiniteStateMachine;
3.Agent;

Next,how to construct Finite State Machine(FSM) using AS3?

Code snippet:

IState;

/**
	 * Each state implements the IState interface.
	 * Each state implements enter and exit metheod
	 * for one-off action that the agent takes
	 * when entering and leaving the state,
	 * in addition to the update method
	 * which is run repeatedly while in the state.
	 * The time parameter in the update method
	 * is the duration of the frame we're excuting.
	 *
	 * @author Knight.zhou
	 *
	 */
	public interface IState
	{
		/**
		 *
		 * @param value is the description of monster's state.
		 *
		 */
		function set description(value:String):void;
		function get description():String;

		function enter():void;//called on entering the state
		function exit():void;//called on leaving the state
		function update(time:Number=0):void;//called every frame while in the state
	}

IFiniteStateMachine

/**
     * Our FiniteStateMachine must implment this interface.
     *
     * @author Knight.zhou
     *
     */
    public interface IFiniteStateMachine
    {
        function set globalState(s:IState):void;
        function get globalState():IState;

        function set currentState(s:IState):void;
        function get currentState():IState;

        function set previousState(s:IState):void;
        function get previousState():IState;

        function set nextState(s:IState):void;
        function get nextState():IState;
    }

Agent;

/**
	 * The agent uses an instance of the FiniteStateMachine class the handle its AI.
	 * The agent class is a lot more manageable without the FSM implementation
	 * and all the states inside it. and must have a single FSM class
	 * that can be used by all agents(who)can even share states across agents too.
	 *
	 * @author Knight.zhou
	 *
	 */
	public class Agent
	{
		private var fsm:FiniteStateMachine;
		public var name:String = "unnamed";
		public var carrier:SWFLoader;
		public var traceTarget:TextArea;
		/**
		 *
		 * @param the name of agent;
		 * @param the carrier of agent;
		 * @param the agen's traceTarget;
		 *
		 */
		public function Agent(name:String,carrier:SWFLoader,traceTarget:TextArea=null)
		{
			fsm = new FiniteStateMachine(this);
			this.name = name;
			this.carrier = carrier;
			this.traceTarget = traceTarget;
		}

		public function update():void
		{
			fsm.update();
		}

		public function getFSM():FiniteStateMachine
		{
			return fsm;
		}
	}

Supplmentary interpretation

Custom state class implement sample:

FiniteStateMachine

/**
	 * The FiniteStateMachine is responsible for
	 * managing the current state.
	 *
	 * Add a number of features to the FSM class to
	 * all chaining of states and returning to previous states.
	 *
	 * @author Knight.zhou
	 *
	 */
	public class FiniteStateMachine implements IFiniteStateMachine
	{
		private static const LOG:ILogger = LogUtil.getLogger(FiniteStateMachine);
	    private var _globalState:IState;//For globally track monster's state.
		private var _currentState:IState;
		private var _previousState:IState;
		private var _nextState:IState;
		protected var owner:Agent;

		public function FiniteStateMachine( owner:Agent,
		                                    currentState:IState=null,
		                                    previousState:IState=null,
		                                    nextState:IState=null
		                                  )
		{
		    this.owner = owner;
			this.currentState = currentState;
			this.previousState = previousState;
			this.nextState = nextState;
		}
		/**
         * Prepare a global state for globally tracking current state.
         *
         * @param s is anew state;
         *
         */
        public function set globalState( s:IState ):void
        {
            _globalState = s ;
        }
        public function get globalState():IState
        {
            return _globalState;
        }
        /**
         * Prepare a state for using the current state.
         *
         * @param s is anew state;
         *
         */
        public function set currentState( s:IState ):void
        {
            _currentState = s ;
        }
        public function get currentState():IState
        {
            return _currentState;
        }
		/**
		 * Prepare a state for use after the current state.
		 *
		 * @param s is anew state;
		 *
		 */
		public function set nextState( s:IState ):void
		{
			_nextState = s ;
		}
		public function get nextState():IState
        {
            return _nextState;
        }
		/**
         * Prepare a state for use before the current state.
         *
         * @param s is anew state;
         *
         */
        public function set previousState( s:IState ):void
        {
            _previousState = s ;
        }
        public function get previousState():IState
        {
            return _previousState;
        }
		/**
		 * Update the FSM.
		 */
		public function update():void
		{
			if(currentState)
			{
				currentState.update();
			}
		}
		/**
		 * Change to another state.
		 *
		 * @param s is anew state.
		 *
		 */
		public function changeState( s:IState ):void
		{
			LOG.info("exited:{0}",currentState);
			currentState.exit();
			previousState = currentState;
			globalState = s;
			currentState = s;
			currentState.enter();
			LOG.info("entered:{0}",currentState);
		}
		/**
		 * Go to the previous state.
		 */
		public function goToPreviousState():void
		{
			changeState( previousState );
		}
		/**
		 * Go to the next state.
		 */
		public function goToNextState():void
		{
			changeState( nextState );
		}

SadLow

/**
	 * Monster's all kinds of talk states
	 *
	 * @author Knight.zhou
	 *
	 */
	public class SadLow implements IState
	{
		private var agent:Agent;
		//----------------------------------
		//  description(native)
		//----------------------------------
		private var _description:String = "STATE_SAD_LOW";
		public function get description():String
		{
			return _description;
		}
		public function set description(value:String):void
		{
			_description = value;
		}
		public function SadLow(monster:Agent)
		{
			this.monster = monster;
		}

		public function enter():void
		{
			//TODO: implement function
		}

		public function exit():void
		{
			//TODO: implement function
		}

		public function update(time:Number=0):void
		{
                       //TODO: implement function
		}
	}

Supple Interpretation

XMLFiniteStateMachine

<?xml version="1.0" ?>
<fsm name="XML FiniteStateMachine">
	<states>
		<state name="stateName">
			<transition input="someInputValue" trigger="triggerName" source="head" next="nextTransition"/>
		</state>
       </states>
</fsm>