C++设计模式——Observer观察者模式

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一，观察者模式的定义

观察者模式是一种行为型设计模式，又被称为"发布-订阅"模式，它定义了对象之间的一对多的依赖关系，当一个对象的状态发生变化时，所有依赖于它的对象都会收到通知并自动更新。

观察者模式的关注点是对象之间的通信以及被观察对象的状态。

观察者模式在现实生活中的抽象实例：

报纸订阅：报纸的内容发生变化时，订阅了该报纸的读者们都会收到通知并阅读最新的内容。

股票投资：股票的价格发生波动时，投资者们会根据最新价格修改相应的投资决策。

天气预报：当天气发生变化时，订阅了该服务的用户们会收到通知。

网络论坛：当论坛中有新的帖子或回复出现时，论坛的用户们会收到通知并可以参与讨论。

二，观察者模式的结构

观察者模式主要包含以下组件：

1.被观察者(Subject)：

被观察的对象，它的内部包含了观察者对象的集合，并提供了添加、通知和删除观察者对象的统一接口。

2.观察者(Observer)：

接收Subject通知的对象，它订阅了Subject的状态，并提供了更新操作的统一接口。

3.具体的被观察者(ConcreteSubject)：

包含Subject类接口的具体实现，维护了观察者的列表，自身状态发生变化时通知所有的观察者。

4.具体的观察者(ConcreteObserver)：

包含Observer类接口的具体实现，提供了更新操作的具体实现细节，一旦收到Subject的通知便进行更新操作。

组件之间的工作步骤如下：

1.被观察者维护一个观察者的列表，并提供了管理和通知观察者的方法。

2.观察者与被观察者绑定(attach)，并将自己添加到观察者列表中。

3.当被观察者的状态发生变化时，开始通知观察者，通知的方式一般是遍历观察者列表，遍历时会调用每个观察者的更新方法。

4.观察者完成具体的更新操作。

对应UML类图：

三，观察者模式代码样例

Demo1：subject只完成通知

#include <iostream>
#include <vector>class Observer {
public:virtual void update() = 0;
};class ConcreteObserver : public Observer {
public:ConcreteObserver(std::string name){observer_name = name;}void update() {std::cout << observer_name <<  " received notify." << std::endl;}
private:std::string observer_name = "";
};class Subject {
private://观察者集合std::vector<Observer*> observers;
public://添加观察者void attach(Observer* observer) {observers.push_back(observer);}//移除观察者void detach(Observer* observer) {for (auto it = observers.begin(); it != observers.end(); ++it) {if (*it == observer) {observers.erase(it);break;}}}//通知观察者void notify() {for (auto observer : observers) {observer->update();}}
};int main() {Subject subject;ConcreteObserver observer1("observer_1");ConcreteObserver observer2("observer_2");subject.attach(&observer1);subject.attach(&observer2);subject.notify();subject.detach(&observer2);subject.notify();return 0;
}

运行结果：

observer_1 received notify.
observer_2 received notify.
observer_1 received notify.

Demo2：subject完成通知并传参

#include <iostream>
#include <vector>class Observer {
public:virtual void update(int data) = 0;
};class ConcreteObserver : public Observer {
public:ConcreteObserver(std::string name){observer_name = name;}void update(int data) override {std::cout << observer_name << " received data: " << data << std::endl;}
private:std::string observer_name = "";
};class Subject {
public:virtual void attach(Observer* observer) = 0;virtual void detach(Observer* observer) = 0;virtual void notify(int data) = 0;
};class ConcreteSubject : public Subject {
private:std::vector<Observer*> observers;
public:void attach(Observer* observer) override {observers.push_back(observer);}void detach(Observer* observer) override {for (auto it = observers.begin(); it != observers.end(); ++it) {if (*it == observer) {observers.erase(it);break;}}}void notify(int data) override {for (auto observer : observers) {observer->update(data);}}
};int main() {ConcreteSubject subject;ConcreteObserver observer1("observer_1");ConcreteObserver observer2("observer_2");ConcreteObserver observer3("observer_3");subject.attach(&observer1);subject.attach(&observer2);subject.attach(&observer3);subject.notify(30);subject.detach(&observer1);subject.detach(&observer2);subject.notify(40);return 0;
}

运行结果：

observer_1 received data: 30
observer_2 received data: 30
observer_3 received data: 30
observer_3 received data: 40

四，观察者模式的应用场景

五，观察者模式的优缺点

六，代码实战

#include <iostream>
#include <vector>class Subject;class Observer
{
public:virtual ~Observer() = default;virtual void Update(Subject&) = 0;
};class Subject
{
public:virtual ~Subject() = default;void Attach(Observer& o) { observers.push_back(&o); }void Detach(Observer& o){observers.erase(std::remove(observers.begin(), observers.end(), &o));}void Notify(){for (auto* o : observers) {o->Update(*this);}}
private:std::vector<Observer*> observers;
};class ClockTimer : public Subject
{
public:void SetTime(int hour, int minute, int second){this->hour = hour;this->minute = minute;this->second = second;Notify();}int GetHour() const { return hour; }int GetMinute() const { return minute; }int GetSecond() const { return second; }
private:int hour;int minute;int second;
};class DigitalClock : public Observer
{
public:explicit DigitalClock(ClockTimer& s) : subject(s) { subject.Attach(*this); }~DigitalClock() { subject.Detach(*this); }void Update(Subject& theChangedSubject) override{if (&theChangedSubject == &subject) {Draw();}}void Draw(){int hour = subject.GetHour();int minute = subject.GetMinute();int second = subject.GetSecond();std::cout << "Digital time is " << hour << ":"<< minute << ":"<< second << std::endl;}
private:ClockTimer& subject;
};class AnalogClock : public Observer
{
public:explicit AnalogClock(ClockTimer& s) : subject(s) { subject.Attach(*this); }~AnalogClock() { subject.Detach(*this); }void Update(Subject& theChangedSubject) override{if (&theChangedSubject == &subject) {Draw();}}void Draw(){int hour = subject.GetHour();int minute = subject.GetMinute();int second = subject.GetSecond();std::cout << "Analog time is " << hour << ":"<< minute << ":"<< second << std::endl;}
private:ClockTimer& subject;
};int main()
{ClockTimer timer;DigitalClock digitalClock(timer);AnalogClock analogClock(timer);timer.SetTime(14, 41, 36);timer.SetTime(18, 00, 00);
}

运行结果：

Digital time is 14:41:36
Analog time is 14:41:36
Digital time is 18:0:0
Analog time is 18:0:0

Demo2：基于观察者模式实现的模拟天气预报

#include <iostream>
#include <vector>class Observer {
public:virtual void update(float temperature, float humidity, float pressure) = 0;
};class WeatherStation {
private:float temperature;float humidity;float pressure;std::vector<Observer*> observers;
public:void registerObserver(Observer* observer) {observers.push_back(observer);}void removeObserver(Observer* observer) {}void notifyObservers() {for (Observer* observer : observers) {observer->update(temperature, humidity, pressure);}}void setMeasurements(float temp, float hum, float press) {temperature = temp;humidity = hum;pressure = press;notifyObservers();}
};class Display : public Observer {
public:void update(float temperature, float humidity, float pressure) {std::cout<< " Display: Temperature = " << temperature<< " °C, Humidity = " << humidity<< " %, Pressure = " << pressure << " hPa"<< std::endl;}
};int main() {WeatherStation weatherStation;Display display1;Display display2;weatherStation.registerObserver(&display1);weatherStation.registerObserver(&display2);weatherStation.setMeasurements(25.5, 60, 1013.2);weatherStation.setMeasurements(24.8, 58, 1014.5);return 0;
}

运行结果：

Display: Temperature = 25.5 °C, Humidity = 60 %, Pressure = 1013.2 hPa
Display: Temperature = 25.5 °C, Humidity = 60 %, Pressure = 1013.2 hPa
Display: Temperature = 24.8 °C, Humidity = 58 %, Pressure = 1014.5 hPa
Display: Temperature = 24.8 °C, Humidity = 58 %, Pressure = 1014.5 hPa

七，参考阅读

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