标题: 状态驱动的游戏智能体设计（中） [打印本页] 作者: 51testing 时间: 2007-11-30 14:49 标题: 状态驱动的游戏智能体设计（中） 作为一个如何利用有限状态机的创造智能体的实例，我们创建名为WestWorld的旧西部风格的淘金镇的游戏，并研究其中的智能体实现。一开始只存在一个名为Miner Bob的淘金者，随后他的妻子也出现。你可以想像风滚草、叽叽作响的淘金用具和沙漠的风把沙吹进你的眼睛，因为WestWorld只是一个简单的基于文本的控制台程序。所有的状态改变和状态动作产生的输出都作为文本传送到控制台窗口。我使用纯文本的原因是为了清晰地示范有限状态机的机制，不想增加代码以免搞得太过于复杂。
WestWorld有四个场景：一个金矿、一个储藏库（Bob把找到的金块存放在这里）、一个酒吧（喝水吃饭）和一个家（睡觉）。确切来讲就是他去哪里、做什么和什么去，都由Bob当前的状态决定。他根据饥渴度、疲惫度和从金旷获得的金块数量来改变状态。
在我们研究代码之前，我们先来看看WestWorld1可执行文件产生的输出：
Miner Bob: Pickin' up a nugget
Miner Bob: Pickin' up a nugget
Miner Bob: Ah'm leavin' the gold mine with mah pockets full o' sweet gold
Miner Bob: Goin' to the bank. Yes siree
Miner Bob: Depositin’ gold. Total savings now: 3
Miner Bob: Leavin' the bank
Miner Bob: Walkin' to the gold mine
Miner Bob: Pickin' up a nugget
Miner Bob: Ah'm leavin' the gold mine with mah pockets full o' sweet gold
Miner Bob: Boy, ah sure is thusty! Walkin' to the saloon
Miner Bob: That's mighty fine sippin liquor
Miner Bob: Leavin' the saloon, feelin' good
Miner Bob: Walkin' to the gold mine
Miner Bob: Pickin' up a nugget
Miner Bob: Pickin' up a nugget
Miner Bob: Ah'm leavin' the gold mine with mah pockets full o' sweet gold
Miner Bob: Goin' to the bank. Yes siree
Miner Bob: Depositin' gold. Total savings now: 4
Miner Bob: Leavin' the bank
Miner Bob: Walkin' to the gold mine
Miner Bob: Pickin' up a nugget
Miner Bob: Pickin' up a nugget
Miner Bob: Ah'm leavin' the gold mine with mah pockets full o' sweet gold
Miner Bob: Boy, ah sure is thusty! Walkin' to the saloon
Miner Bob: That's mighty fine sippin' liquor
Miner Bob: Leavin' the saloon, feelin' good
Miner Bob: Walkin' to the gold mine
Miner Bob: Pickin' up a nugget
Miner Bob: Ah'm leavin' the gold mine with mah pockets full o' sweet gold
Miner Bob: Goin' to the bank. Yes siree
Miner Bob: Depositin' gold. Total savings now: 5
Miner Bob: Woohoo! Rich enough for now. Back home to mah li'l lady
Miner Bob: Leavin' the bank
Miner Bob: Walkin' home
Miner Bob: ZZZZ...
Miner Bob: ZZZZ...
Miner Bob: ZZZZ...
Miner Bob: ZZZZ...
Miner Bob: What a God-darn fantastic nap! Time to find more gold
BaseGameEntity的声明如下：
class BaseGameEntity
{
private:
//every entity has a unique identifying number
int m_ID;
//this is the next valid ID. Each time a BaseGameEntity is instantiated
//this value is updated
static int m_iNextValidID;
//this is called within the constructor to make sure the ID is set
//correctly. It verifies that the value passed to the method is greater
//or equal to the next valid ID, before setting the ID and incrementing
//the next valid ID
void SetID(int val);
public:
BaseGameEntity(int id)
{
SetID(id);
}
virtual ~BaseGameEntity(){}
//all entities must implement an update function
virtual void Update()=0;
Miner类
Miner类从BaseGameEntity类派生，它包括健康、疲惫程度和位置等数据成员。像前文描述过的Troll例子，Miner也有一个指向State类实例的指针，当然也少不了用以改变State指针所指向的实例的方法。
Class Miner : public BaseGameEntity
{
private:
//a pointer to an instance of a State
State* m_pCurrentState;
// the place where the miner is currently situated
location_type m_Location;
//how many nuggets the miner has in his pockets
int m_iGoldCarried;
//how much money the miner has deposited in the bank
int m_iMoneyInBank;
//the higher the value, the thirstier the miner
int m_iThirst;
//the higher the value, the more tired the miner
int m_iFatigue;
public:
Miner(int ID);
//this must be implemented
void Update();
//this method changes the current state to the new state
void ChangeState(State* pNewState);
重温状态设计模式
之前已经对这个模式作了简单介绍，但不够深入。每一个游戏智能体的状态机都作为唯一的类来实现，智能体拥有一个指向当前状态实例的指针。智能体需要实现ChangeState成员函数以实现状态切换。决定状态转换的逻辑包含在每一个State派生类的内部。所有的状态类都从一个抽象类派生，以获得统一接口。现在，你已经知道足够多关于状态设计模式的知识了。
之前也提及过通常每一个状态都有相应的Enter和Exit动作，这将使得程序员能够编写仅在进入或者离开状态只执行一次的逻辑以增强FSM的可伸缩性。为了实现这一点，让我们来看看改进后的State基类。
class State
{
public:
virtual ~State(){}
//this will execute when the state is entered
virtual void Enter(Miner*)=0;
//this is called by the miner’s update function each update-step
virtual void Execute(Miner*)=0;
//this will execute when the state is exited
virtual void Exit(Miner*)=0;
}
这两个方法仅在Miner改变状态的时候调用，当发生一个状态转换，Miner::ChangeState方法首先调用当前状态的Exit方法，然后它为当前状态指派一个新的状态，最后调用新状态的Enter方法。我认为代码比言语更清晰，所有这里列出ChangeState方法的代码：
void Miner::ChangeState(State* pNewState)
{
//make sure both states are valid before attempting to
//call their methods
assert (m_pCurrentState && pNewState);
//call the exit method of the existing state
m_pCurrentState->Exit(this);
//change state to the new state
m_pCurrentState = pNewState;
//call the entry method of the new state
m_pCurrentState->Enter(this);
}
因为具类只是简单地实现虚基类State定义的接口，它们的声明非常简明：
class EnterMineAndDigForNugget : public State
{
private:
EnterMineAndDigForNugget(){}
/* copy ctor and assignment op omitted */
public:
//this is a singleton
static EnterMineAndDigForNugget* Instance();
virtual void Enter(Miner* pMiner);
virtual void Execute(Miner* pMiner);
virtual void Exit(Miner* pMiner);
};
如你所见，这只是一个模式，让我们来看看其它方法。
EnterMineAndDigForNugget::Enter
下面是EnterMineAndDigForNugget的Enter方法：
void EnterMineAndDigForNugget::Enter(Miner* pMiner)
{
//if the miner is not already located at the goldmine, he must
//change location to the gold mine
if (pMiner->Location() != goldmine)
{
cout << "\n" << GetNameOfEntity(pMiner->ID()) << ": "
<< "Walkin' to the goldmine";
EnterMineAndDigForNugget::Execute
Execute有点复杂，它包含了改变淘金者状态的逻辑。（不要忘记Miner::Update在每一个更新帧都会调用Execute方法。）
void EnterMineAndDigForNugget::Execute(Miner* pMiner)
{
//the miner digs for gold until he is carrying in excess of MaxNuggets.
//If he gets thirsty during his digging he stops work and
//changes state to go to the saloon for a beer.
pMiner->AddToGoldCarried(1);
//digging is hard work
pMiner->IncreaseFatigue();
cout << "\n" << GetNameOfEntity(pMiner->ID()) << ": "
<< "Pickin' up a nugget";
//if enough gold mined, go and put it in the bank
if (pMiner->PocketsFull())
{
pMiner->ChangeState(VisitBankAndDepositGold::Instance());
}
//if thirsty go and get a beer
if (pMiner->Thirsty())
{
pMiner->ChangeState(QuenchThirst::Instance());
}
}
