1.意图

State pattern属于行为型模式。

允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。

简单来说，就是将 “一个大 class + 一堆 if else” 替换为 “一堆小 class”。一堆小 class 就是一堆状态，用一堆状态代替 if else 会更好拓展与维护。

在实际工程代码中，一般是用FSM(有限状态机)来实现。FSM在许多不同的情况下用于建模复杂的实体状态。

2.UML类图

3.GOF角色说明

Context：定义客户感兴趣的接口；维护一个ConcreteState子类的实现。

State：状态接口，即定义一个接口以封装与Context的一个特定状态相关的行为。

ConcreteState subclasses：具体状态子类。每一个子类实现一个与Context的一个状态相关的行为。

4.代码实现

下面例子模拟银行卡的存/取钱流程，客户的帐号状态，随着余额改变而不断变化。

GreenState(余额充足)----状态迁移---->YellowState(收支平衡)----状态迁移---->RedState(透支)---->状态迁移---->GreenState/YellowState，不断循环。

头文件fsm.h实现：

#pragma once#include 
   
    //前向声明class Depositor;//UML中的"State"抽象类class State{public:    //当前状态    std::string GetCurrState() const { return _currState; }    //储户id    Depositor* GetDepositor() const { return _depositor; }    void SetDepositor(Depositor* dp) { _depositor = dp; }    //余额    double GetBalance() const { return _balance; }    void SetBalance(const double balance) { _balance = balance; }    virtual void Deposit(const double money) = 0;  //存钱    virtual void Withdraw(const double money) = 0; //取钱    virtual void PayInterest() = 0;                //利息    protected:    std::string _currState;    Depositor* _depositor;    double _balance;    double _interest;    double _lowerLimit;    double _upperLimit;};//UML中的"ConcreteState"类//绿色状态, 余额充足class GreenState : State{public:    GreenState(State* state);    void Deposit(const double money);    void Withdraw(const double money);    void PayInterest();    void TransferNextState();  //扭转到下一个状态private:    GreenState();    void Initialize();};//UML中的"ConcreteState"类//黄色状态, 收支基本平衡. 无透支, 但也没有太多余额class YellowState : State{public:    YellowState(State* state);    YellowState(const double balance, Depositor* dp);    void Deposit(const double money);    void Withdraw(const double money);    void PayInterest();    void TransferNextState();  //扭转到下一个状态private:    YellowState();    void Initialize();};//UML中的"ConcreteState"类//红色状态,入不敷出,已透支class RedState : State{public:    RedState(State* state);    void Deposit(const double money);    void Withdraw(const double money);    void PayInterest();    void TransferNextState();  //扭转到下一个状态private:    RedState();    void Initialize();    double _serviceFee;};//UML中的"Context"类//储户帐号和状态class Depositor{public:    Depositor(const std::string id);    ~Depositor();    double GetBalance() const;    void Deposit(const double money);    void Withdraw(const double money);    void PayInterest();    void SetState(State* state) { _state = state; }    State* GetState() { return _state; }private:    State* _state;    const std::string _id;    Depositor();};

main.cpp 主逻辑实现

#include 
   
    #include "fsm.h"Depositor::Depositor(const std::string id) : _id(id) {    //账户默认初始化为余额为0的黄色状态    _state = reinterpret_cast
    
     (new YellowState(0.0, this));}Depositor::~Depositor() {    if (_state != nullptr) {        delete _state;        _state = nullptr;    }}double Depositor::GetBalance() const {    return _state->GetBalance();}void Depositor::Deposit(const double money) {    _state->Deposit(money);    std::cout << "Deposited $" << money << std::endl;    std::cout << "Balance   $" << GetBalance() << std::endl;    std::cout << "CurrStatus " << _state->GetCurrState() << std::endl;    std::cout << std::endl;}void Depositor::Withdraw(const double money) {    _state->Withdraw(money);    std::cout << "Withdraw  $" << money << std::endl;    std::cout << "Balance   $" << GetBalance() << std::endl;    std::cout << "CurrStatus " << _state->GetCurrState() << std::endl;    std::cout << std::endl;}void Depositor::PayInterest() {    _state->PayInterest();    std::cout << "------Interest Pay------" << std::endl;    std::cout << "Balance   $" << GetBalance() << std::endl;    std::cout << "CurrStatus " << _state->GetCurrState() << std::endl;    std::cout << std::endl;}void GreenState::Initialize() {    _currState = "Green";    _interest = 5.0;    _lowerLimit = 1000.0;    _upperLimit = 100000000.0;}GreenState::GreenState(State* state) {    _depositor = state->GetDepositor();    _balance = state->GetBalance();    Initialize();}void GreenState::Deposit(const double money) {    _balance += money;    TransferNextState();}void GreenState::Withdraw(const double money) {    _balance -= money;    TransferNextState();}void GreenState::PayInterest() {    _balance = _balance * _interest;    TransferNextState();}void GreenState::TransferNextState() {    if (_balance < 0.0) {        _depositor->SetState(reinterpret_cast
     
      (new RedState(this)));        delete this;        return;    }    else if (_balance < _lowerLimit) {        _depositor->SetState(reinterpret_cast
      
       (new YellowState(this)));        delete this;        return;    }    else if (_balance > _upperLimit) {        std::cout << "Current depositor is too big" << std::endl;    }}void YellowState::Initialize() {    _currState = "Yellow";    _interest = 1.0;    _lowerLimit = 0.0;    _upperLimit = 1000.0;}YellowState::YellowState(State* state) {    _balance = state->GetBalance();    _depositor = state->GetDepositor();    Initialize();}YellowState::YellowState(const double balance, Depositor* dp) {    _balance = balance;    _depositor = dp;    Initialize();}void YellowState::Deposit(const double money) {    _balance += money;    TransferNextState();}void YellowState::Withdraw(const double money) {    _balance -= money;    TransferNextState();}void YellowState::PayInterest() {    _balance = _balance * _interest;    TransferNextState();}void YellowState::TransferNextState() {    if (_balance < _lowerLimit) {        _depositor->SetState(reinterpret_cast
       
        (new RedState(this)));        delete this;        return;    }    else if (_balance > _upperLimit) {        _depositor->SetState(reinterpret_cast
        
         (new GreenState(this))); delete this; return; }}void RedState::Initialize() { _currState = "Red"; _interest = 0.0; //最多允许透支1000 _lowerLimit = -1000.0; _upperLimit = 0.0; //每次透支的服务费是20 _serviceFee = 20.0;}RedState::RedState(State* state) { _balance = state->GetBalance(); _depositor = state->GetDepositor(); Initialize();}void RedState::Deposit(const double money) { _balance += money; TransferNextState();}void RedState::Withdraw(const double money) { double newFee = money + _serviceFee; if (_balance - newFee < _lowerLimit) { //已到透支上限,无法执行 std::cout << "Can not withdraw due to overdraw too much" << std::endl; } else { _balance -= newFee; }}void RedState::PayInterest() { //not available}void RedState::TransferNextState() { if (_balance > _upperLimit) { _depositor->SetState(reinterpret_cast
         
          (new YellowState(this))); delete this; return; }}int main() { Depositor* dp = new Depositor("Sam"); dp->Withdraw(10.0); dp->Withdraw(1000.0); dp->Deposit(1500.0); dp->Deposit(2000.0); dp->Withdraw(5000.0); dp->Withdraw(3000.0); delete dp; return 0;}

运行结果：

Withdraw $10

Balance $-10

CurrStatus Red

Can not withdraw due to overdraw too much

Withdraw $1000

Balance $-10

CurrStatus Red

Deposited $1500

Balance $1490

CurrStatus Yellow

Deposited $2000

Balance $3490

CurrStatus Green

Withdraw $5000

Balance $-1510

CurrStatus Red

Can not withdraw due to overdraw too much

Withdraw $3000

Balance $-1510

CurrStatus Red

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