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状态模式（State Pattern）是一种行为设计模式，它允许一个对象在内部状态改变时改变其行为，使对象看起来像是改变了其类。

核心概念

设计原则

状态模式遵循以下设计原则：

1. 单一职责原则：将状态相关行为分离到不同类中

2. 开闭原则：可以新增状态而不修改现有代码

3. 封装性：状态转换逻辑封装在状态类中

主要优点

1. 清晰状态转换：将状态转换逻辑组织化

2. 减少条件语句：消除庞大的条件状态判断

3. 可扩展性：容易添加新状态和转换

4. 行为局部化：状态特定行为集中在对应状态类中

模式结构

主要组件

1. Context（上下文）

   • 维护当前状态的引用

   • 将状态相关请求委托给当前状态对象

2. State（状态接口）

   • 定义状态接口

   • 封装与Context特定状态相关的行为

3. ConcreteState（具体状态）

   • 实现状态接口

   • 每个子类实现与Context状态相关的行为

完整代码示例

#include <iostream>
#include <memory>
#include <string>// 前置声明
class State;// ==================== 上下文类 ====================
class Player {std::unique_ptr<State> state_;std::string name_;public:explicit Player(const std::string& name);void setState(std::unique_ptr<State> state);std::string getName() const;// 玩家行为void play();void pause();void stop();void lock();void unlock();
};// ==================== 状态接口 ====================
class State {
public:virtual ~State() = default;virtual void play(Player* player) = 0;virtual void pause(Player* player) = 0;virtual void stop(Player* player) = 0;virtual void lock(Player* player) = 0;virtual void unlock(Player* player) = 0;protected:void changeState(Player* player, std::unique_ptr<State> state) {player->setState(std::move(state));}
};// ==================== 具体状态 ====================
class ReadyState : public State {
public:void play(Player* player) override;void pause(Player* player) override { /* 无效操作 */ }void stop(Player* player) override { /* 无效操作 */ }void lock(Player* player) override;void unlock(Player* player) override { /* 已在解锁状态 */ }
};class PlayingState : public State {
public:void play(Player* player) override { /* 已在播放状态 */ }void pause(Player* player) override;void stop(Player* player) override;void lock(Player* player) override;void unlock(Player* player) override { /* 无效操作 */ }
};class PausedState : public State {
public:void play(Player* player) override;void pause(Player* player) override { /* 已在暂停状态 */ }void stop(Player* player) override;void lock(Player* player) override;void unlock(Player* player) override { /* 无效操作 */ }
};class LockedState : public State {
public:void play(Player* player) override { /* 锁定状态下不能播放 */ }void pause(Player* player) override { /* 锁定状态下不能暂停 */ }void stop(Player* player) override { /* 锁定状态下不能停止 */ }void lock(Player* player) override { /* 已在锁定状态 */ }void unlock(Player* player) override;
};// ==================== 上下文方法实现 ====================
Player::Player(const std::string& name) : name_(name) {state_ = std::make_unique<ReadyState>();
}void Player::setState(std::unique_ptr<State> state) {state_ = std::move(state);
}std::string Player::getName() const {return name_;
}void Player::play() {state_->play(this);
}void Player::pause() {state_->pause(this);
}void Player::stop() {state_->stop(this);
}void Player::lock() {state_->lock(this);
}void Player::unlock() {state_->unlock(this);
}// ==================== 具体状态方法实现 ====================
void ReadyState::play(Player* player) {std::cout << player->getName() << ": 开始播放" << std::endl;changeState(player, std::make_unique<PlayingState>());
}void ReadyState::lock(Player* player) {std::cout << player->getName() << ": 锁定播放器" << std::endl;changeState(player, std::make_unique<LockedState>());
}void PlayingState::pause(Player* player) {std::cout << player->getName() << ": 暂停播放" << std::endl;changeState(player, std::make_unique<PausedState>());
}void PlayingState::stop(Player* player) {std::cout << player->getName() << ": 停止播放" << std::endl;changeState(player, std::make_unique<ReadyState>());
}void PlayingState::lock(Player* player) {std::cout << player->getName() << ": 锁定播放器" << std::endl;changeState(player, std::make_unique<LockedState>());
}void PausedState::play(Player* player) {std::cout << player->getName() << ": 恢复播放" << std::endl;changeState(player, std::make_unique<PlayingState>());
}void PausedState::stop(Player* player) {std::cout << player->getName() << ": 停止播放" << std::endl;changeState(player, std::make_unique<ReadyState>());
}void PausedState::lock(Player* player) {std::cout << player->getName() << ": 锁定播放器" << std::endl;changeState(player, std::make_unique<LockedState>());
}void LockedState::unlock(Player* player) {std::cout << player->getName() << ": 解锁播放器" << std::endl;changeState(player, std::make_unique<ReadyState>());
}// ==================== 客户端代码 ====================
int main() {std::cout << "=== 状态模式演示: 音乐播放器 ===" << std::endl;Player player("我的播放器");// 正常流程player.play();player.pause();player.play();player.stop();// 锁定测试std::cout << "\n测试锁定功能:" << std::endl;player.play();player.lock();player.play();  // 应该无效player.unlock();player.play();// 无效操作测试std::cout << "\n测试无效操作:" << std::endl;player.stop();player.stop();  // 应该无效player.pause(); // 应该无效return 0;
}

模式变体

1. 状态表驱动

class StateMachine {std::unordered_map<std::string, std::unordered_map<std::string, std::function<void()>>> transitions_;std::string currentState_;public:void addTransition(const std::string& from, const std::string& event, const std::string& to, std::function<void()> action) {transitions_[from][event] = [this, to, action] {action();currentState_ = to;};}void handleEvent(const std::string& event) {auto& stateTransitions = transitions_[currentState_];if (stateTransitions.find(event) != stateTransitions.end()) {stateTransitions[event]();}}
};

2. 共享状态对象

class SharedState : public State {// 可以共享的状态数据
};class ConcreteStateA : public SharedState {// 实现特定行为
};// 使用时可以共享同一个状态实例
auto sharedState = std::make_shared<ConcreteStateA>();
context1.setState(sharedState);
context2.setState(sharedState);

实际应用场景

1. 工作流引擎：如订单状态流转

2. 游戏开发：角色行为状态管理

3. UI系统：控件不同状态下的行为

4. 网络协议：协议状态机实现

5. 硬件控制：设备状态管理