观察者模式

 1 #include<iostream>
 2 #include<list>
 3 #include<string> 
 4 using namespace std;
 5 class Observer;
 6 class Subject        //主题
 7 {
 8 public:
 9     virtual void notify()=0;
10     virtual void attach(Observer *o)=0;
11     virtual void detach(Observer *o)=0;
12     string getState()
13     {
14         return state;
15     }
16     void setState(string s)
17     {
18         state = s;
19     }
20 protected:    
21     string state;
22     list<Observer*> l;
23     /*
24     我的理解：
25         关键是主题类中有观察者实例的指针；
26         通过指针调用观察者的函数（函数传参，发消息）。
27     注：子类指针赋值给基类，没有问题（不会丢失子类独有的方法等、没有语法问题）。
28     */
29 };
30 class Observer
31 {
32 public:
33     virtual void update(Subject *s)
34     {
35         cout << "Observer::update" << endl;
36     }
37 };
38 class ConcreteSubject:public Subject
39 {
40 public:
41     virtual void notify()
42     {
43         list<Observer*>::iterator it = l.begin();
44         while(it != l.end())
45         {
46             (*it)->update(this);
47             it++;
48         }
49     }
50     virtual void attach(Observer *o)
51     {
52         l.push_back(o);
53     }
54     virtual void detach(Observer *o)
55     {
56         l.remove(o);
57     }
58 };
59 class Observer1:public Observer
60 {
61 public:
62     virtual void update(Subject *s)
63     {
64         cout << "Observer1::update Subject‘s state is " << s->getState() << endl;
65     }
66 };
67 class Observer2:public Observer
68 {
69 public:
70     virtual void update(Subject *s)
71     {
72         cout << "Observer2::update Subject‘s state is " << s->getState() << endl;
73     }
74 };
75  
76 int main()
77 {
78     Observer1 o1;
79     Observer2 o2;
80     ConcreteSubject cs;
81     cs.attach(&o1);
82     cs.attach(&o2);
83     cs.setState("state1");
84     cs.notify();
85     cs.setState("state2");
86     cs.notify();
87     return 0;
88 }