观察者模式

【1】什么是观察者模式？

观察者模式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。

这个主题对象在状态上发生变化时，会通知所有观察者对象，让他们能够自动更新自己 

【2】观察者模式代码示例：

代码示例如下：
#include <iostream>
#include <string>
#include <list>
using namespace std;

class Observer;

class Subject
{
protected:
    list<Observer*> observers;
public:
    string action;
public:
    virtual void attach(Observer*) = 0;
    virtual void detach(Observer*) = 0;
    virtual void notify() = 0;
};

class Observer
{
protected:
    string name;
    Subject *sub;
public:
    Observer(string name, Subject *sub)
    {
        this->name = name;
        this->sub = sub;
    }
    string getName()
    {
        return name;
    }
    virtual void update() = 0;
};

class StockObserver : public Observer
{
public:
    StockObserver(string name, Subject *sub) : Observer(name, sub)
    {
    }
    void update();
};

void StockObserver::update()
{
    cout << name << " 收到消息：" << sub->action << endl;
    if (sub->action == "梁所长来了!")
    {
        cout << "我马上关闭股票，装做很认真工作的样子！" << endl;
    }
}

class NBAObserver : public Observer
{
public:
    NBAObserver(string name, Subject *sub) : Observer(name, sub)
    {
    }
    void update();
};

void NBAObserver::update()
{
    cout << name << " 收到消息：" << sub->action << endl;
    if (sub->action == "梁所长来了!")
    {
        cout << "我马上关闭NBA，装做很认真工作的样子！" << endl;
    }
}

class Secretary : public Subject
{
    void attach(Observer *observer)
    {
        observers.push_back(observer);
    }
    void detach(Observer *observer)
    {
        list<Observer *>::iterator iter = observers.begin();
        while (iter != observers.end())
        {
            if ((*iter) == observer)
            {
                cout << "erase:" << observer->getName() << endl;
                observers.erase(iter++);
            }
            else
            {
                ++iter;
            }
        }
    }
    void notify()
    {
        list<Observer *>::iterator iter = observers.begin();
        while (iter != observers.end())
        {
            (*iter)->update();
            ++iter;
        }
    }
};


int main()
{
    Subject *dwq = new Secretary();

    Observer *xs = new NBAObserver("xiaoshuai", dwq);
    Observer *zy = new NBAObserver("zouyue", dwq);
    Observer *lm = new StockObserver("limin", dwq);

    dwq->attach(xs);
    dwq->attach(zy);
    dwq->attach(lm);

    dwq->action = "去吃饭了！";
    dwq->notify();
    cout << endl;

    dwq->action = "梁所长来了!";
    dwq->notify();

    dwq->detach(lm);
    dwq->detach(zy);
    dwq->detach(xs);

    return 0;
}