【30】c++设计模式——＞状态模式

状态模式概述

状态模式是一种行为型设计模式，它可以让一个对象在其内部状态发生变化时更改其行为。通过将每个状态封装成一个独立的类，我们可以使状态之间互相独立，并且使得状态的切换变得更加灵活、可扩展。（多个状态之间可以相互转换）
在状态模式中，我们通常会定义一个抽象状态类（Abstract State），以及多个具体状态类（Concrete States）。每个具体状态都会实现抽象状态类中定义的各种操作，并且在需要时执行状态转换。
此外，还有一个环境类（Context），它包含了当前状态，并且在状态发生变化时调用各个状态类的方法来实现状态转换。因为这些状态类都实现了同一个接口，所以环境类不需要知道具体状态的细节，只需要调用相应的方法即可。

如何理解状态类和环境类

电视遥控器	人
开电视	学习
关电视	睡觉
静音	打游戏

电视遥控器的例子

一个电视遥控器。电视遥控器有三种状态：开机状态、关机状态和静音状态。我们可以通过简单地按遥控器上的按钮来更改状态。为了实现这个功能，首先我们需要定义一个抽象状态类，该类将定义所有可能的操作：

class TVState {
public:virtual void onButtonPressed(TVRemote* remote) = 0;virtual void offButtonPressed(TVRemote* remote) = 0;virtual void muteButtonPressed(TVRemote* remote) = 0;
};

TVState中包含了三种操作：开机、关机和静音。这些操作在不同的状态下可能会有不同的实现方式，因此我们需要在具体状态类中进行实现。

// 具体状态类：开机状态
class OnState : public TVState {
public:void onButtonPressed(TVRemote* remote) override {std::cout << "The TV is already on." << std::endl;}void offButtonPressed(TVRemote* remote) override {std::cout << "Turning off the TV." << std::endl;remote->setState(new OffState());}void muteButtonPressed(TVRemote* remote) override {std::cout << "Muting the TV." << std::endl;remote->setState(new MuteState());}
};// 具体状态类：关机状态
class OffState : public TVState {
public:void onButtonPressed(TVRemote* remote) override {std::cout << "Turning on the TV." << std::endl;remote->setState(new OnState());}void offButtonPressed(TVRemote* remote) override {std::cout << "The TV is already off." << std::endl;}void muteButtonPressed(TVRemote* remote) override {std::cout << "Cannot mute the TV when it's turned off." << std::endl;}
};// 具体状态类：静音状态
class MuteState : public TVState {
public:void onButtonPressed(TVRemote* remote) override {std::cout << "Unmuting the TV." << std::endl;remote->setState(new OnState());}void offButtonPressed(TVRemote* remote) override {std::cout << "Turning off the TV." << std::endl;remote->setState(new OffState());}void muteButtonPressed(TVRemote* remote) override {std::cout << "The TV is already muted." << std::endl;}
};

在这些具体状态类中，我们重写了TVState中定义的所有操作，并且在需要时执行状态转换。例如，在开机状态下按下静音键会将遥控器的状态更改为“静音状态”。
接下来，让我们定义环境类（Context）：

class TVRemote {
private:TVState* currentState;public:TVRemote() {currentState = new OffState();}void setState(TVState* state) {currentState = state;}void pressOnButton() {currentState->onButtonPressed(this);}void pressOffButton() {currentState->offButtonPressed(this);}void pressMuteButton() {currentState->muteButtonPressed(this);}
};

在环境类中，我们维护了当前状态，并且在状态发生变化时调用相应的具体状态类方法。我们还定义了三个按键操作：开机、关机和静音。
现在，我们可以使用电视遥控器来测试状态模式的实现了：

int main() {TVRemote remote;remote.pressOnButton();    // Turning on the TV.remote.pressOnButton();    // The TV is already on.remote.pressMuteButton();  // Muting the TV.remote.pressMuteButton();  // The TV is already muted.remote.pressOffButton();   // Turning off the TV.remote.pressOffButton();   // The TV is already off.remote.pressMuteButton();  // Cannot mute the TV when it's turned off.remote.pressOnButton();    // Turning on the TV.remote.pressMuteButton();  // Unmuting the TV.return 0;
}

通过上面的代码，我们可以看到当我们按下不同的键时，电视遥控器的状态会发生相应的变化。

完整代码

remote.cpp

#include "remote.h"
#include "state.h"TVRemote::TVRemote(TVState* State )
{currentState = State;
}void TVRemote::setState(TVState* state)
{currentState = state;
}void TVRemote::pressOnButton()
{currentState->onButtonPressed(this);
}void TVRemote::pressOffButton()
{currentState->offButtonPressed(this);
}void TVRemote::pressMuteButton()
{currentState->muteButtonPressed(this);
}

remote.h

#pragma once
class TVState; //这里没声明，报了一堆错class TVRemote
{
private:TVState* currentState;public:TVRemote(TVState* State);void setState(TVState* state);void pressOnButton();void pressOffButton();void pressMuteButton();
};

state.h

#pragma once
#include"remote.h"class TVState {
public:virtual void onButtonPressed(TVRemote* remote) = 0; // 开机virtual void offButtonPressed(TVRemote* remote) = 0; // 关机virtual void muteButtonPressed(TVRemote* remote) = 0; // 静音
};// 具体状态类：关机状态
class OffState : public TVState
{
public:void onButtonPressed(TVRemote* remote) override;void offButtonPressed(TVRemote* remote) override;void muteButtonPressed(TVRemote* remote) override;
};// 具体状态类：开机状态
class OnState : public TVState
{
public:void onButtonPressed(TVRemote* remote) override;void offButtonPressed(TVRemote* remote) override;void muteButtonPressed(TVRemote* remote) override;
};// 具体状态类：静音状态
class MuteState : public TVState {
public:void onButtonPressed(TVRemote* remote) override;void offButtonPressed(TVRemote* remote) override;void muteButtonPressed(TVRemote* remote) override;
};

state.cpp

#include<iostream>
#include "state.h"
#include "remote.h"// 具体状态类：关机状态
void OffState::onButtonPressed(TVRemote* remote) 
{std::cout << "Turning on the TV." << std::endl;remote->setState(new OnState());
}void OffState::offButtonPressed(TVRemote* remote) 
{std::cout << "The TV is already off." << std::endl;
}void OffState::muteButtonPressed(TVRemote* remote)
{std::cout << "Cannot mute the TV when it's turned off." << std::endl;
}// 具体状态类：开机状态
void OnState::onButtonPressed(TVRemote* remote) 
{std::cout << "The TV is already on." << std::endl;
}void OnState::offButtonPressed(TVRemote* remote) 
{std::cout << "Turning off the TV." << std::endl;remote->setState(new OffState());
}void OnState::muteButtonPressed(TVRemote* remote)
{std::cout << "Muting the TV." << std::endl;remote->setState(new MuteState());
}// 具体状态类：静音状态
void MuteState::onButtonPressed(TVRemote* remote)
{std::cout << "Unmuting the TV." << std::endl;remote->setState(new OnState());
}void MuteState::offButtonPressed(TVRemote* remote) 
{std::cout << "Turning off the TV." << std::endl;remote->setState(new OffState());
}void MuteState::muteButtonPressed(TVRemote* remote)
{std::cout << "The TV is already muted." << std::endl;
}

main.cpp

#include <iostream>
using namespace std;
#include "state.h"
#include "remote.h"int main() {TVState* off = new MuteState;TVRemote remote(off);remote.pressOnButton();    // Turning on the TV.remote.pressOnButton();    // The TV is already on.remote.pressMuteButton();  // Muting the TV.remote.pressMuteButton();  // The TV is already muted.remote.pressOffButton();   // Turning off the TV.remote.pressOffButton();   // The TV is already off.remote.pressMuteButton();  // Cannot mute the TV when it's turned off.remote.pressOnButton();    // Turning on the TV.remote.pressMuteButton();  // Unmuting the TV.return 0;
}