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【设计模式】【行为型模式（Behavioral Patterns）】之策略模式（Strategy Pattern）

article 2024/11/27 19:47:02

1. 设计模式原理说明

策略模式（Strategy Pattern） 是一种行为设计模式，它允许你定义一系列算法，并将每个算法封装起来，使它们可以互换。策略模式让算法的变化独立于使用算法的客户。通过这种方式，客户端可以根据不同的情况选择不同的算法，而不需要修改其内部结构。

主要角色

Context（上下文）：使用某个策略类的对象。它维护着对Strategy对象的引用，用于在运行时切换策略。
Strategy（策略接口）：定义所有支持的算法的公共接口。Context使用这个接口来调用具体的策略。
ConcreteStrategy（具体策略）：实现了Strategy接口的具体算法。

2. UML 类图及解释

UML 类图

+------------------+                +-----------------------+
|      Context     |                |   Strategy Interface  |
|------------------|                |-----------------------|
| - strategy: Strategy |            | - execute(context): void |
|------------------|                +-----------------------+
| - setStrategy(strategy: Strategy) |                ^
| - performOperation()              |                |
+------------------+                +-----------------------+
                                                   |
                                                   |
                                                   v
                                    +---------------------------+
                                    | ConcreteStrategyA         |
                                    |--------------------------|
                                    | - execute(context): void  |
                                    +---------------------------+
                                                   ^
                                                   |
                                    +---------------------------+
                                    | ConcreteStrategyB         |
                                    |--------------------------|
                                    | - execute(context): void  |
                                    +---------------------------+

类图解释

Context：维护对Strategy对象的引用，并提供一个方法performOperation()来执行策略。客户端通过这个方法来调用具体的策略。
Strategy：定义了所有支持的算法的公共接口。Context使用这个接口来调用具体的策略。
ConcreteStrategyA 和 ConcreteStrategyB：实现了Strategy接口的具体算法。客户端可以选择不同的具体策略来执行不同的算法。

3. 代码案例及逻辑详解

Java 代码案例

// 策略接口
interface Strategy {
    void execute();
}

// 具体策略A
class ConcreteStrategyA implements Strategy {
    @Override
    public void execute() {
        System.out.println("Executing strategy A");
    }
}

// 具体策略B
class ConcreteStrategyB implements Strategy {
    @Override
    public void execute() {
        System.out.println("Executing strategy B");
    }
}

// 上下文
class Context {
    private Strategy strategy;

    public Context(Strategy strategy) {
        this.strategy = strategy;
    }

    public void setStrategy(Strategy strategy) {
        this.strategy = strategy;
    }

    public void performOperation() {
        strategy.execute();
    }
}

// 客户端
public class Client {
    public static void main(String[] args) {
        Context context = new Context(new ConcreteStrategyA());
        context.performOperation(); // 输出: Executing strategy A

        context.setStrategy(new ConcreteStrategyB());
        context.performOperation(); // 输出: Executing strategy B
    }
}

C++ 代码案例

#include <iostream>

// 策略接口
class Strategy {
public:
    virtual void execute() = 0;
    virtual ~Strategy() {}
};

// 具体策略A
class ConcreteStrategyA : public Strategy {
public:
    void execute() override {
        std::cout << "Executing strategy A" << std::endl;
    }
};

// 具体策略B
class ConcreteStrategyB : public Strategy {
public:
    void execute() override {
        std::cout << "Executing strategy B" << std::endl;
    }
};

// 上下文
class Context {
private:
    Strategy* strategy;
public:
    Context(Strategy* strategy) : strategy(strategy) {}

    void setStrategy(Strategy* strategy) {
        this->strategy = strategy;
    }

    void performOperation() {
        strategy->execute();
    }

    ~Context() {
        delete strategy;
    }
};

// 客户端
int main() {
    Context context(new ConcreteStrategyA());
    context.performOperation(); // 输出: Executing strategy A

    context.setStrategy(new ConcreteStrategyB());
    context.performOperation(); // 输出: Executing strategy B

    return 0;
}

Python 代码案例

# 策略接口
class Strategy:
    def execute(self):
        raise NotImplementedError

# 具体策略A
class ConcreteStrategyA(Strategy):
    def execute(self):
        print("Executing strategy A")

# 具体策略B
class ConcreteStrategyB(Strategy):
    def execute(self):
        print("Executing strategy B")

# 上下文
class Context:
    def __init__(self, strategy: Strategy):
        self._strategy = strategy

    def set_strategy(self, strategy: Strategy):
        self._strategy = strategy

    def perform_operation(self):
        self._strategy.execute()

# 客户端
if __name__ == "__main__":
    context = Context(ConcreteStrategyA())
    context.perform_operation()  # 输出: Executing strategy A

    context.set_strategy(ConcreteStrategyB())
    context.perform_operation()  # 输出: Executing strategy B

Go 代码案例

package main

import (
	"fmt"
)

// 策略接口
type Strategy interface {
	execute()
}

// 具体策略A
type ConcreteStrategyA struct{}

func (c *ConcreteStrategyA) execute() {
	fmt.Println("Executing strategy A")
}

// 具体策略B
type ConcreteStrategyB struct{}

func (c *ConcreteStrategyB) execute() {
	fmt.Println("Executing strategy B")
}

// 上下文
type Context struct {
	strategy Strategy
}

func NewContext(strategy Strategy) *Context {
	return &Context{strategy: strategy}
}

func (c *Context) SetStrategy(strategy Strategy) {
	c.strategy = strategy
}

func (c *Context) PerformOperation() {
	c.strategy.execute()
}

// 客户端
func main() {
	context := NewContext(&ConcreteStrategyA{})
	context.PerformOperation() // 输出: Executing strategy A

	context.SetStrategy(&ConcreteStrategyB{})
	context.PerformOperation() // 输出: Executing strategy B
}