【设计模式】【行为型模式（Behavioral Patterns）】之状态模式（State Pattern）

1. 设计模式原理说明

状态模式（State Pattern） 是一种行为设计模式，它允许对象在其内部状态发生变化时改变其行为。这个模式的核心思想是使用不同的类来表示不同的状态，每个状态类都封装了与该状态相关的特定行为。当对象的状态发生改变时，对象会切换到另一个状态对象，从而改变了它的行为。

主要角色

Context（上下文）：定义客户感兴趣的接口，维护一个对当前状态对象的引用。
State（抽象状态）：定义一个接口，用以封装与Context的一个特定状态相关的行为。
ConcreteState（具体状态）：实现State接口，每种状态都对应一个具体的状态类，封装了与该状态相关的特定行为。

2. UML 类图及解释

UML 类图

+-----------------+
|     Context     |
|-----------------|
| - state: State  |
| + setState(state: State) |
| + request()     |
+-----------------+
           ^
           |
           |
           v
+-----------------+
|     State       |
|-----------------|
| + handle(context: Context) |
+-----------------+
           ^
           |
           |
           v
+-----------------+
| ConcreteStateA  |
|-----------------|
| + handle(context: Context) |
+-----------------+
           ^
           |
           |
           v
+-----------------+
| ConcreteStateB  |
|-----------------|
| + handle(context: Context) |
+-----------------+

类图解释

Context：定义客户感兴趣的接口，维护一个对当前状态对象的引用。当状态变化时，调用setState方法更新当前状态。
State：定义一个接口，用以封装与Context的一个特定状态相关的行为。
ConcreteStateA 和 ConcreteStateB：实现State接口，每种状态都对应一个具体的状态类，封装了与该状态相关的特定行为。

3. 代码案例及逻辑详解

Java 代码案例

// 抽象状态
interface State {
    void handle(Context context);
}

// 具体状态 A
class ConcreteStateA implements State {
    @Override
    public void handle(Context context) {
        System.out.println("Current state is ConcreteStateA");
        context.setState(new ConcreteStateB());
    }
}

// 具体状态 B
class ConcreteStateB implements State {
    @Override
    public void handle(Context context) {
        System.out.println("Current state is ConcreteStateB");
        context.setState(new ConcreteStateA());
    }
}

// 上下文
class Context {
    private State state;

    public Context(State state) {
        this.state = state;
    }

    public void setState(State state) {
        this.state = state;
    }

    public void request() {
        state.handle(this);
    }
}

// 客户端
public class Client {
    public static void main(String[] args) {
        Context context = new Context(new ConcreteStateA());
        context.request();  // 输出: Current state is ConcreteStateA
        context.request();  // 输出: Current state is ConcreteStateB
    }
}

C++ 代码案例

#include <iostream>

// 抽象状态
class State {
public:
    virtual void handle(Context* context) = 0;
};

// 具体状态 A
class ConcreteStateA : public State {
public:
    void handle(Context* context) override {
        std::cout << "Current state is ConcreteStateA" << std::endl;
        context->setState(new ConcreteStateB());
    }
};

// 具体状态 B
class ConcreteStateB : public State {
public:
    void handle(Context* context) override {
        std::cout << "Current state is ConcreteStateB" << std::endl;
        context->setState(new ConcreteStateA());
    }
};

// 上下文
class Context {
private:
    State* state;
public:
    Context(State* state) : state(state) {}

    void setState(State* state) {
        delete this->state;
        this->state = state;
    }

    void request() {
        state->handle(this);
    }
};

// 客户端
int main() {
    Context* context = new Context(new ConcreteStateA());
    context->request();  // 输出: Current state is ConcreteStateA
    context->request();  // 输出: Current state is ConcreteStateB

    delete context;
    return 0;
}

Python 代码案例

# 抽象状态
class State:
    def handle(self, context):
        pass

# 具体状态 A
class ConcreteStateA(State):
    def handle(self, context):
        print("Current state is ConcreteStateA")
        context.set_state(ConcreteStateB())

# 具体状态 B
class ConcreteStateB(State):
    def handle(self, context):
        print("Current state is ConcreteStateB")
        context.set_state(ConcreteStateA())

# 上下文
class Context:
    def __init__(self, state):
        self.state = state

    def set_state(self, state):
        self.state = state

    def request(self):
        self.state.handle(self)

# 客户端
if __name__ == "__main__":
    context = Context(ConcreteStateA())
    context.request()  # 输出: Current state is ConcreteStateA
    context.request()  # 输出: Current state is ConcreteStateB

Go 代码案例

package main

import (
	"fmt"
)

// 抽象状态
type State interface {
	handle(context *Context)
}

// 具体状态 A
type ConcreteStateA struct{}

func (s *ConcreteStateA) handle(context *Context) {
	fmt.Println("Current state is ConcreteStateA")
	context.setState(&ConcreteStateB{})
}

// 具体状态 B
type ConcreteStateB struct{}

func (s *ConcreteStateB) handle(context *Context) {
	fmt.Println("Current state is ConcreteStateB")
	context.setState(&ConcreteStateA{})
}

// 上下文
type Context struct {
	state State
}

func (c *Context) setState(state State) {
	c.state = state
}

func (c *Context) request() {
	c.state.handle(c)
}

// 客户端
func main() {
	context := &Context{state: &ConcreteStateA{}}
	context.request()  // 输出: Current state is ConcreteStateA
	context.request()  // 输出: Current state is ConcreteStateB
}