C++實現Behavioral - Observer模式 （轉）

轉http://patmusing.blog.163.com/blog/static/13583496020101501923571/

 

也稱爲Dependents或Publish-Subscribe模式。

 

定義對象間的一種一對多的依賴關係，以便當一個對象的狀態發生改變時，全部依賴於它的對象都獲得通知並自動更新。

「Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.」 – GoF

 

將一個系統分解成一系列的相互協做的類會產生一個廣泛存在的反作用：須要維護相關對象之間的一致性。要維護各對象之間的一致性，一般的作法就是使這些對象之間彼此緊耦合，可是這樣又會下降他們的可重用性。

 

好比，許多圖形界面工具包將用戶界面的展現和底層應用數據分開。用來定義應用數據和界面展現的類能夠獨立重複使用，也能夠一塊兒使用。表格對象和柱狀圖對象能夠同時展現同一個應用數據對象中的數據，但表格對象和柱狀圖對象，彼此無需知道對方的存在。但實際狀況中，它們卻彷佛又知道對方的存在，好比當一個用戶改動表格中的信息，柱狀圖也會隨之改變，反之亦然。

 

這種行爲暗示了表格對象和柱狀圖對象都依賴了數據對象。若是一個用戶改變了表格中的數據，這種改動會當即被反映到數據對象中，數據對象中的數據發生了這種改變，會當即通知柱狀圖對象，柱狀圖對象獲得通知後，會對柱狀圖的顯示作出相應的改變。

 

Observer模式描述如何創建這些關係。本模式中的關鍵對象是Subject和Observer。一個Subject能夠有任意多獨立的Observer。若是Subject的內容發生改變，全部的Observer都會獲得通知，每一個Observer則查詢Subject，以保持與其狀態同步。

 

這種交互也稱爲publish-subscribe(發佈者-註冊)。Subject就是publisher，任意數量的Observer能夠註冊以接收通知。

 

UML類圖：

 

 

 

業務案例：

 

說明：

-          經過ATM、InternetBankingService和MobileBankingService都可改動銀行帳戶餘額；

-          銀行帳戶餘額還能夠有其餘方式改變，好比櫃檯(此例並未用及，用來模擬銀行帳戶自身的餘額變化)；

-          只要一上其中之一改變了銀行帳戶中的餘額，其餘相關各方都可以收到餘額變更信息；

-          Bank Account是Subject，ATM、InternetBankingService和MobileBankingService是Observer。

 

C++實現上述業務案例的代碼以下：

// Observer.h

#include <iostream>

#include <vector>

 

using namespace std;

 

class Observer;

 

//Account抽象類，用作接口

class Account

{

protected:

         vector<Observer *> observers;                                      // 保存若干個Observer

         double balance;                                                                // 帳戶餘額

 

public:

         virtual void add_observer(Observer *o) = 0;                // 註冊一個Observer

         virtual void remove_observer(Observer *o) = 0;         // 刪除一個Observer

         virtual void notify_observer() = 0;                                   // 通知全部註冊了的Observer

         virtual void set_balance(double balance) = 0;            // 改動帳戶餘額

         virtual double get_balance() = 0;                                   // 獲取帳戶月娥

 

public:

         virtual ~Account()

         {

                   cout << "in the destructor of Account..." << endl;

         }

};

 

// Observer抽象類，用作接口

class Observer

{

protected:

         Account *account;                                                                      // 帳戶

public:

         Observer(Account *account):account(account)                    // 構造函數

         {

         }

 

         virtual void handle_event() = 0;                                                // 事件處理

         virtual void set_account_balance(double balance) = 0;     // 改動帳戶餘額

         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，其類型爲Observer *

                            {

                                     observers.erase(it);  // 這裏須要注意，o從observers中被刪除，但並未從內存中刪除

                                     return;

                            }

                   }

         }

 

         void notify_observer()                                    // 向observers中的全部Observer發出通知，並進行相關處理

         {

                   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();                                  // 只要餘額變更，就向全部註冊了的Observer發出通知

         }

 

         double get_balance()

         {

                   return balance;

         }

};

 

class ATM : public Observer

{

public:

         ATM(Account *account):Observer(account)                          // 將account傳遞給基類Observer的構造函數

         {

         }

 

         ~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中改動銀行帳戶餘額，好比在ATM機上存取款

         {

                   account->set_balance(balance);

         }

};

 

class InternetBankingService : public Observer

{

public:

         InternetBankingService(Account *account):Observer(account)     // 將account傳遞給基類Observer的構造函數

         {

         }

 

         ~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)                                        // 網上銀行改動銀行帳戶餘額，好比經過網上銀行轉帳

         {

                   account->set_balance(balance);

         }

};

 

class MobileBankingService : public Observer

{

public:

         MobileBankingService(Account *account):Observer(account)          // 將account傳遞給基類Observer的構造函數

         {

         }

 

         ~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)                                          // 手機銀行改動銀行帳戶餘額，好比經過網上銀行轉帳

         {

                   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);

 

         // 註冊Observer

         account->add_observer(atm_observer);

         account->add_observer(ibs_observer);

         account->add_observer(mbs_observer);

         // 銀行帳戶餘額變更(好比在櫃檯存取款)

         account->set_balance(1000.12);

 

         // ATM機觸發銀行帳戶餘額變更

         cout << "\nEvent triggered by atm_observer: " << endl;

         cout << "================================" << endl;

         atm_observer->set_account_balance(1000.23);

 

         // 網上銀行觸發銀行帳戶餘額變更

         cout << "\nEvent triggered by ibs_observer: " << endl;

         cout << "================================" << endl;

         ibs_observer->set_account_balance(1000.36);

 

         // 手機銀行觸發銀行帳戶餘額變更

         cout << "\nEvent triggered by mbs_observer: " << endl;

         cout << "================================" << endl;

         mbs_observer->set_account_balance(1000.68);

 

         // 刪除(unsubscribe)網上銀行Observer，並變更銀行帳戶餘額

         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;

}

 

運行結果：

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設計模式和Mediator設計模式的主要區別

Observer設計模式：某個特定的對象（subject或者observable）的狀態發生改變時，其餘對象（observers）將作出相應的反應。

Mediator設計模式：任何對象的狀態發生變化，其餘對象都將作出相應的反應，中間是經過Mediator來進行協調的。

 

Observer設計模式有一個實際意義上的「中心」。Mediator設計模式沒有實際意義上的中心，相對而言更分佈式一些，其邏輯上的中心是因爲純設計上的須要而引入的「Mediator」，即Observer模式中，事件源在subject或observable對象；在Mediator模式中，事件源在除Mediator以外的其餘全部對象。

 

關於在STL容器中保存對象指針的狀況，詳見：

http://patmusing.blog.163.com/blog/static/13583496020101831514657/