轉(zhuǎn)載自：
http://patmusing.blog.163.com/blog/static/13583496020101501923571/
也稱為Dependents或Publish-Subscribe模式。
定義對(duì)象間的一種一對(duì)多的依賴關(guān)系，以便當(dāng)一個(gè)對(duì)象的狀態(tài)發(fā)生改變時(shí)，所有依賴于它的對(duì)象都得到通知并自動(dòng)更新。
“Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.” – GoF
將一個(gè)系統(tǒng)分解成一系列的相互協(xié)作的類會(huì)產(chǎn)生一個(gè)普遍存在的副作用：需要維護(hù)相關(guān)對(duì)象之間的一致性。要維護(hù)各對(duì)象之間的一致性，通常的做法就是使這些對(duì)象之間彼此緊耦合，但是這樣又會(huì)降低他們的可重用性。
比如，許多圖形界面工具包將用戶界面的展示和底層應(yīng)用數(shù)據(jù)分開。用來定義應(yīng)用數(shù)據(jù)和界面展示的類可以獨(dú)立重復(fù)使用，也可以一起使用。表格對(duì)象和柱狀圖對(duì)象可以同時(shí)展示同一個(gè)應(yīng)用數(shù)據(jù)對(duì)象中的數(shù)據(jù)，但表格對(duì)象和柱狀圖對(duì)象，彼此無需知道對(duì)方的存在。但實(shí)際情況中，它們卻似乎又知道對(duì)方的存在，比如當(dāng)一個(gè)用戶改動(dòng)表格中的信息，柱狀圖也會(huì)隨之改變，反之亦然。
這種行為暗示了表格對(duì)象和柱狀圖對(duì)象都依賴了數(shù)據(jù)對(duì)象。如果一個(gè)用戶改變了表格中的數(shù)據(jù)，這種改動(dòng)會(huì)立即被反映到數(shù)據(jù)對(duì)象中，數(shù)據(jù)對(duì)象中的數(shù)據(jù)發(fā)生了這種改變，會(huì)立即通知柱狀圖對(duì)象，柱狀圖對(duì)象得到通知后，會(huì)對(duì)柱狀圖的顯示做出相應(yīng)的改變。
Observer模式描述如何建立這些關(guān)系。本模式中的關(guān)鍵對(duì)象是Subject和Observer。一個(gè)Subject可以有任意多獨(dú)立的Observer。如果Subject的內(nèi)容發(fā)生改變，所有的Observer都會(huì)得到通知，每個(gè)Observer則查詢Subject，以保持與其狀態(tài)同步。
這種交互也稱為publish-subscribe(發(fā)布者-注冊(cè))。Subject就是publisher，任意數(shù)量的Observer可以注冊(cè)以接收通知。
UML類圖：
業(yè)務(wù)案例：
說明：
- 通過ATM、InternetBankingService和MobileBankingService均可改動(dòng)銀行賬戶余額；
- 銀行賬戶余額還可以有其他方式改變，比如柜臺(tái)(此例并未用及，用來模擬銀行賬戶自身的余額變化)；
- 只要一上其中之一改變了銀行賬戶中的余額，其他相關(guān)各方均可以收到余額變動(dòng)信息；
- Bank Account是Subject，ATM、InternetBankingService和MobileBankingService是Observer。
C++實(shí)現(xiàn)上述業(yè)務(wù)案例的代碼如下：
// Observer.h
#include <iostream>
#include <vector>
using namespace std;
class Observer;
//Account抽象類，用做接口
class Account
{
protected:
vector<Observer *> observers; // 保存若干個(gè)Observer
double balance; // 賬戶余額
public:
virtual void add_observer(Observer *o) = 0; // 注冊(cè)一個(gè)Observer
virtual void remove_observer(Observer *o) = 0; // 刪除一個(gè)Observer
virtual void notify_observer() = 0; // 通知所有注冊(cè)了的Observer
virtual void set_balance(double balance) = 0; // 改動(dòng)賬戶余額
virtual double get_balance() = 0; // 獲取賬戶月娥
public:
virtual ~Account()
{
cout << "in the destructor of Account..." << endl;
}
};
// Observer抽象類，用做接口
class Observer
{
protected:
Account *account; // 賬戶
public:
Observer(Account *account):account(account) // 構(gòu)造函數(shù)
{
}
virtual void handle_event() = 0; // 事件處理
virtual void set_account_balance(double balance) = 0; // 改動(dòng)賬戶余額
virtual ~Observer()
{
cout << "in the destructor of Observer..." << endl;
}
};
// BankAccout具體類，繼承Account抽象類
class BankAccount : public Account
{
public:
~BankAccount()
{
cout << "in the destructor of BankAccount..." << endl;
}
public:
void add_observer(Observer *o)
{
observers.push_back(o);
}
void remove_observer(Observer *o)
{
for(vector<Observer *>::iterator it = observers.begin(); it != observers.end(); it++)
{
if(o == *it) // 注意*it就是observers中的元素，其類型為Observer *
{
observers.erase(it); // 這里需要注意，o從observers中被刪除，但并未從內(nèi)存中刪除
return;
}
}
}
void notify_observer() // 向observers中的所有Observer發(fā)出通知，并進(jìn)行相關(guān)處理
{
for(vector<Observer *>::const_iterator it = observers.begin(); it != observers.end(); it++)
{
(*it)->handle_event();
}
}
void set_balance(double balance)
{
this->balance = balance;
notify_observer(); // 只要余額變動(dòng)，就向所有注冊(cè)了的Observer發(fā)出通知
}
double get_balance()
{
return balance;
}
};
class ATM : public Observer
{
public:
ATM(Account *account):Observer(account) // 將account傳遞給基類Observer的構(gòu)造函數(shù)
{
}
~ATM()
{
cout << "in the destructor of Observer..." << endl;
}
public:
void handle_event()
{
cout << "ATM: The balance of bank account is changed to: " << account->get_balance() << endl;
}
void set_account_balance(double balance) // 在ATM中改動(dòng)銀行賬戶余額，比如在ATM機(jī)上存取款
{
account->set_balance(balance);
}
};
class InternetBankingService : public Observer
{
public:
InternetBankingService(Account *account):Observer(account) // 將account傳遞給基類Observer的構(gòu)造函數(shù)
{
}
~InternetBankingService()
{
cout << "in the destructor InternetBankingService..." << endl;
}
public:
void handle_event()
{
cout << "IBS: The balance of bank account is changed to: " << account->get_balance() << endl;
}
void set_account_balance(double balance) // 網(wǎng)上銀行改動(dòng)銀行賬戶余額，比如通過網(wǎng)上銀行轉(zhuǎn)賬
{
account->set_balance(balance);
}
};
class MobileBankingService : public Observer
{
public:
MobileBankingService(Account *account):Observer(account) // 將account傳遞給基類Observer的構(gòu)造函數(shù)
{
}
~MobileBankingService()
{
cout << "in the destructor of MobileBankingService..." << endl;
}
public:
void handle_event()
{
cout << "MBS: The balance of bank account is changed to: " << account->get_balance() << endl;
}
void set_account_balance(double balance) // 手機(jī)銀行改動(dòng)銀行賬戶余額，比如通過網(wǎng)上銀行轉(zhuǎn)賬
{
account->set_balance(balance);
}
};
// Observer.cpp
#include "Observer.h"
int main(int argc, char **argv)
{
Account *account = new BankAccount();
Observer *atm_observer = new ATM(account);
Observer *ibs_observer = new InternetBankingService(account);
Observer *mbs_observer = new MobileBankingService(account);
// 注冊(cè)Observer
account->add_observer(atm_observer);
account->add_observer(ibs_observer);
account->add_observer(mbs_observer);
// 銀行賬戶余額變動(dòng)(比如在柜臺(tái)存取款)
account->set_balance(1000.12);
// ATM機(jī)觸發(fā)銀行賬戶余額變動(dòng)
cout << "\nEvent triggered by atm_observer: " << endl;
cout << "================================" << endl;
atm_observer->set_account_balance(1000.23);
// 網(wǎng)上銀行觸發(fā)銀行賬戶余額變動(dòng)
cout << "\nEvent triggered by ibs_observer: " << endl;
cout << "================================" << endl;
ibs_observer->set_account_balance(1000.36);
// 手機(jī)銀行觸發(fā)銀行賬戶余額變動(dòng)
cout << "\nEvent triggered by mbs_observer: " << endl;
cout << "================================" << endl;
mbs_observer->set_account_balance(1000.68);
// 刪除(unsubscribe)網(wǎng)上銀行Observer，并變動(dòng)銀行賬戶余額
cout << "\nRemove ibs_observer: " << endl;
cout << "====================" << endl;
account->remove_observer(ibs_observer);
account->set_balance(2000.23);
cout << "\nDestroy all dynamically created objects: " << endl;
cout << "========================================" << endl;
delete account;
delete atm_observer;
delete ibs_observer;
delete mbs_observer;
}
運(yùn)行結(jié)果：
ATM: The balance of bank account is changed to: 1000.12
IBS: The balance of bank account is changed to: 1000.12
MBS: The balance of bank account is changed to: 1000.12
Event triggered by atm_observer:
================================
ATM: The balance of bank account is changed to: 1000.23
IBS: The balance of bank account is changed to: 1000.23
MBS: The balance of bank account is changed to: 1000.23
Event triggered by ibs_observer:
================================
ATM: The balance of bank account is changed to: 1000.36
IBS: The balance of bank account is changed to: 1000.36
MBS: The balance of bank account is changed to: 1000.36
Event triggered by mbs_observer:
================================
ATM: The balance of bank account is changed to: 1000.68
IBS: The balance of bank account is changed to: 1000.68
MBS: The balance of bank account is changed to: 1000.68
Remove ibs_observer:
====================
ATM: The balance of bank account is changed to: 2000.23
MBS: The balance of bank account is changed to: 2000.23
Destroy all dynamically created objects:
========================================
in the destructor of BankAccount...
in the destructor of Account...
in the destructor of Observer...
in the destructor of Observer...
in the destructor InternetBankingService...
in the destructor of Observer...
in the destructor of MobileBankingService...
in the destructor of Observer...
Observer設(shè)計(jì)模式和Mediator設(shè)計(jì)模式的主要區(qū)別
Observer設(shè)計(jì)模式：某個(gè)特定的對(duì)象（subject或者observable）的狀態(tài)發(fā)生改變時(shí)，其他對(duì)象（observers）將做出相應(yīng)的反應(yīng)。
Mediator設(shè)計(jì)模式：任何對(duì)象的狀態(tài)發(fā)生變化，其他對(duì)象都將做出相應(yīng)的反應(yīng)，中間是通過Mediator來進(jìn)行協(xié)調(diào)的。
Observer設(shè)計(jì)模式有一個(gè)實(shí)際意義上的“中心”。Mediator設(shè)計(jì)模式?jīng)]有實(shí)際意義上的中心，相對(duì)而言更分布式一些，其邏輯上的中心是由于純?cè)O(shè)計(jì)上的需要而引入的“Mediator”，即Observer模式中，事件源在subject或observable對(duì)象；在Mediator模式中，事件源在除Mediator之外的其他所有對(duì)象。
關(guān)于在STL容器中保存對(duì)象指針的情況，詳見：
http://patmusing.blog.163.com/blog/static/13583496020101831514657/