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
案例：
案例：文件分割器，在界面上显示分割进度
C++中尽量不要多继承，因为会有很多问题。但是可以继承一个主基类，然后再继承多个接口，这样一般是没问题的
抽象：将“通知依赖”关系抽象为1个接口，比如案例中splitter就是被观察者，界面窗口（进度条）、控制台就是观察者
如何支持多个观察者：vector
要点总结：
使用面向对象的抽象，Observer模式使得我们可以独立地改变目标与观察者，从而使二者之间的依赖关系达致松耦合。
目标发送通知时，无需指定观察者，通知（可以携带通知信息作为参数）会自动传播。
观察者自己决定是否需要订阅通知，目标对象对此一无所知。
Observer模式是基于事件的UI框架中非常常用的设计模式，也是 MVC模式的一个重要组成部分。
https://github.com/zhtsmailbox/design-patterns-cpp/blob/master/docs/Observer.md
/*
* C++ Design Patterns: Bridge
* Author: Jakub Vojvoda [github.com/JakubVojvoda]
* 2016
*
* Source code is licensed under MIT License
* (for more details see LICENSE)
*
*/
#include <iostream>
#include <vector>
class Subject;
/*
* Observer
* defines an updating interface for objects that should be notified
* of changes in a subject
*/
class Observer
{
public:
virtual ~Observer() {}
virtual int getState() = 0;
virtual void update( Subject *subject ) = 0;
// ...
};
/*
* Concrete Observer
* stores state of interest to ConcreteObserver objects and
* sends a notification to its observers when its state changes
*/
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;
// ...
};
/*
* Subject
* knows its observers and provides an interface for attaching
* and detaching observers
*/
class Subject
{
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++ )
{
observers.at( i )->update( this );
}
}
virtual int getState() = 0;
virtual void setState( const int s ) = 0;
// ...
private:
std::vector<Observer*> observers;
// ...
};
/*
* Concrete Subject
* stores state that should stay consistent with the subject's
*/
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 );
subject->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;
}