1、介绍

        观察者模式（Observer Pattern）是一种行为设计模式，它定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。当主题对象状态发生变化时，它的所有依赖者（观察者）都会自动收到通知并更新。

包含组件如下：

        （1）主题（Subject）：主题是一个接口，可以把所有观察者对象登记到其观察者列表中，并且当它的内部状态发生改变时，会自动通知所有登记过的观察者对象。【在类内部建立一个vector记录所有的观察者，当主体发生变化时，遍历通知】。【一个抽象类】。

        （2）具体主题（ConcreteSubject）：具体主题实现了主题接口，在内部状态发生改变时，会主动通知所有登记过的观察者对象。【抽象类的子类】。

        （3）观察者（Observer）：观察者是一个接口，为所有的具体观察者定义一个更新接口，使得在得到主题的通知时更新自己。【一个抽象类】。

        （4）具体观察者（ConcreteObserver）：具体观察者实现了观察者接口，该接口保持一个指向具体主题的引用，以便在主题状态发生改变时得到通知。【抽象类的子类】。

2、示例

#include <iostream>  
#include <list>  
#include <string>  // 观察者接口  
class Observer {
public:virtual ~Observer() {}virtual void update(const std::string& message) = 0;
};// 具体观察者1 
class ConcreteObserver1 : public Observer {
private:std::string name;
public:ConcreteObserver1(const std::string& name) : name(name) {}void update(const std::string& message) override {std::cout << name << " 1111 received: " << message << std::endl;}
};// 具体观察者2 
class ConcreteObserver2 : public Observer {
private:std::string name;
public:ConcreteObserver2(const std::string& name) : name(name) {}void update(const std::string& message) override {std::cout << name << " 2222 received: " << message << std::endl;}
};// 主题接口  
class Subject {
public:virtual ~Subject() {}virtual void attach(Observer* observer) = 0;virtual void detach(Observer* observer) = 0;virtual void notifyObservers() = 0;
};// 具体主题1 
class ConcreteSubject1 : public Subject {
private:std::list<Observer*> observers;std::string state;
public:// 添加观察者void attach(Observer* observer) override {observers.push_back(observer);}// 删除观察者void detach(Observer* observer) override {observers.remove(observer);}// 通知观察者void notifyObservers() override {for (auto observer : observers) {observer->update(state);}}// 设置状态void setState(const std::string& newState) {state = newState;notifyObservers();}
};// 具体主题2
class ConcreteSubject2 : public Subject {
private:std::list<Observer*> observers;std::string state;
public:// 添加观察者void attach(Observer* observer) override {observers.push_back(observer);}// 删除观察者void detach(Observer* observer) override {observers.remove(observer);}// 通知观察者void notifyObservers() override {for (auto observer : observers) {observer->update(state);}}// 设置状态void setState(const std::string& newState) {state = newState;notifyObservers();}
};int main() {ConcreteSubject1 subject1;ConcreteSubject2 subject2;Observer* observer1 = new ConcreteObserver1("Observer 1111");Observer* observer2 = new ConcreteObserver1("Observer 1112");Observer* observer3 = new ConcreteObserver2("Observer 2221");Observer* observer4 = new ConcreteObserver2("Observer 2222");subject1.attach(observer1);subject1.attach(observer2);subject1.attach(observer3);subject1.attach(observer4);subject1.setState("Hello, observers!");// 不再需要观察者时，应该将其从主题中分离并删除  subject1.detach(observer1);subject1.detach(observer4);subject1.setState("Hello, observers 2 and 3!");delete observer1;delete observer2;delete observer3;delete observer4;std::cout << "===============================================" << std::endl;Observer* observer5 = new ConcreteObserver1("Observer 1111");Observer* observer6 = new ConcreteObserver1("Observer 1112");Observer* observer7 = new ConcreteObserver2("Observer 2221");Observer* observer8 = new ConcreteObserver2("Observer 2222");subject2.attach(observer5);subject2.attach(observer6);subject2.attach(observer7);subject2.attach(observer8);subject2.setState("Hello, observers!");// 不再需要观察者时，应该将其从主题中分离并删除  subject2.detach(observer6);subject2.detach(observer7);subject2.setState("Hello, observers 5 and 8!");delete observer5;delete observer6;delete observer7;delete observer8;return 0;
}

结果：

Observer 1111 1111 received: Hello, observers!
Observer 1112 1111 received: Hello, observers!
Observer 2221 2222 received: Hello, observers!
Observer 2222 2222 received: Hello, observers!
Observer 1112 1111 received: Hello, observers 2 and 3!
Observer 2221 2222 received: Hello, observers 2 and 3!
===============================================
Observer 1111 1111 received: Hello, observers!
Observer 1112 1111 received: Hello, observers!
Observer 2221 2222 received: Hello, observers!
Observer 2222 2222 received: Hello, observers!
Observer 1111 1111 received: Hello, observers 5 and 8!
Observer 2222 2222 received: Hello, observers 5 and 8!