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

news 2025/6/30 11:12:54

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 {@Overridepublic void handle(Context context) {System.out.println("Current state is ConcreteStateA");context.setState(new ConcreteStateB());}
}// 具体状态 B
class ConcreteStateB implements State {@Overridepublic 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 ConcreteStateAcontext.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 ConcreteStateAcontext->request();  // 输出: Current state is ConcreteStateBdelete 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 = statedef set_state(self, state):self.state = statedef request(self):self.state.handle(self)# 客户端
if __name__ == "__main__":context = Context(ConcreteStateA())context.request()  # 输出: Current state is ConcreteStateAcontext.request()  # 输出: Current state is ConcreteStateB

Go 代码案例

package mainimport ("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 ConcreteStateAcontext.request()  // 输出: Current state is ConcreteStateB
}