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状态模式

概念

状态模式（State Pattern）是一种行为设计模式，它允许一个对象在其内部状态改变时改变其行为，使其看起来像改变了自身的类。通过将状态相关的行为抽取到独立的状态类中，状态模式能有效实现状态与行为的分离。

核心组件与职责

使用场景

优点与缺点

与其他模式对比

实现代码

C++ 实现

#include <iostream>
#include <memory>

// State Interface
class State {
public:
    virtual ~State() {}
    virtual void handle() = 0;
};

// ConcreteStateA
class ConcreteStateA : public State {
public:
    void handle() override {
        std::cout << "Handling in State A\n";
    }
};

// ConcreteStateB
class ConcreteStateB : public State {
public:
    void handle() override {
        std::cout << "Handling in State B\n";
    }
};

// Context
class Context {
private:
    std::unique_ptr<State> state;

public:
    void setState(std::unique_ptr<State> newState) {
        state = std::move(newState);
    }

    void request() {
        if (state) {
            state->handle();
        } else {
            std::cout << "No state set\n";
        }
    }
};

// Example
int main() {
    Context context;

    context.setState(std::make_unique<ConcreteStateA>());
    context.request();

    context.setState(std::make_unique<ConcreteStateB>());
    context.request();

    return 0;
}

C# 实现

using System;

// State Interface
public interface IState {
    void Handle();
}

// ConcreteStateA
public class ConcreteStateA : IState {
    public void Handle() {
        Console.WriteLine("Handling in State A");
    }
}

// ConcreteStateB
public class ConcreteStateB : IState {
    public void Handle() {
        Console.WriteLine("Handling in State B");
    }
}

// Context
public class Context {
    private IState state;

    public void SetState(IState newState) {
        state = newState;
    }

    public void Request() {
        if (state != null) {
            state.Handle();
        } else {
            Console.WriteLine("No state set");
        }
    }
}

// Example
public class Program {
    public static void Main(string[] args) {
        Context context = new Context();

        context.SetState(new ConcreteStateA());
        context.Request();

        context.SetState(new ConcreteStateB());
        context.Request();
    }
}

类图

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