Observer

动机

在软件构建过程中，我们需要为某些对象建立一种“通知依赖关系”——一个对象（目标对象）的状态发生改变，所有的依赖对象（观察者对象）都将得到通知。如果这样的依赖关系过于紧密，将使软件不能很好地抵御变化。
使用面向对象技术，可以将这种依赖关系弱化，并形成一种稳定的依赖关系。从而实现软件体系结构的松耦合

模式定义

定义对象间的一种一对多（变化）的依赖关系，以便当一个对象(Subject)的状态发生改变时，所有依赖于它的对象都得到通知并自动更新。

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#include <iostream>
#include <vector>//或者Listclass Subject; // 稳定的class Observer // 稳定的
{
public:virtual ~Observer() {}virtual int getState() = 0;virtual void update(Subject *subject) = 0;// ...
};class ConcreteObserver : public Observer
{
public:ConcreteObserver(const int state) : observer_state(state) {}~ConcreteObserver() {}int getState(){return observer_state;}void update(Subject *subject);// ...private:int observer_state;// ...
};class Subject // 稳定的
{
public:virtual ~Subject() {}void attach(Observer *observer){observers.push_back(observer);}void detach(const int index){observers.erase(observers.begin() + index);}void notify(){for (unsigned int i = 0; i < observers.size(); i++){//自动更新observers.at(i)->update(this);}}virtual int getState() = 0;virtual void setState(const int s) = 0;// ...private:std::vector<Observer *> observers;// ...
};class ConcreteSubject : public Subject
{
public:~ConcreteSubject() {}int getState(){return subject_state;}void setState(const int s){subject_state = s;}// ...private:int subject_state;// ...
};void ConcreteObserver::update(Subject *subject)
{observer_state = subject->getState();std::cout << "Observer state updated." << std::endl;
}int main()
{ConcreteObserver observer1(1);ConcreteObserver observer2(2);std::cout << "Observer 1 state: " << observer1.getState() << std::endl;std::cout << "Observer 2 state: " << observer2.getState() << std::endl;Subject *subject = new ConcreteSubject();subject->attach(&observer1); // push_backsubject->attach(&observer2);subject->setState(10);subject->notify();std::cout << "Observer 1 state: " << observer1.getState() << std::endl;std::cout << "Observer 2 state: " << observer2.getState() << std::endl;delete subject;return 0;}
/*
Observer 1 state: 1
Observer 2 state: 2
Observer state updated.
Observer state updated.
Observer 1 state: 10
Observer 2 state: 10
*/