设计模式
行为模式之观察者模式
假如你有两种类型的对象： 顾客和 商店 。 顾客对某个特定品牌的产品非常感兴趣 （例如最新型号的 iPhone 手机）， 而该产品很快将会在商店里出售。
顾客可以每天来商店看看产品是否到货。 但如果商品尚未到货时， 绝大多数来到商店的顾客都会空手而归。
另一方面， 每次新产品到货时， 商店可以向所有顾客发送邮件 （可能会被视为垃圾邮件）。 这样， 部分顾客就无需反复前往商店了， 但也可能会惹恼对新产品没有兴趣的其他顾客。
我们似乎遇到了一个矛盾： 要么让顾客浪费时间检查产品是否到货， 要么让商店浪费资源去通知没有需求的顾客。
main.cc: 概念示例
/** * Observer Design Pattern * * Intent: Lets you define a subscription mechanism to notify multiple objects * about any events that happen to the object they're observing. * * Note that there's a lot of different terms with similar meaning associated * with this pattern. Just remember that the Subject is also called the * Publisher and the Observer is often called the Subscriber and vice versa. * Also the verbs "observe", "listen" or "track" usually mean the same thing. */ #include <iostream> #include <list> #include <string> class IObserver { public: virtual ~IObserver(){}; virtual void Update(const std::string &message_from_subject) = 0; }; class ISubject { public: virtual ~ISubject(){}; virtual void Attach(IObserver *observer) = 0; virtual void Detach(IObserver *observer) = 0; virtual void Notify() = 0; }; /** * The Subject owns some important state and notifies observers when the state * changes. */ class Subject : public ISubject { public: virtual ~Subject() { std::cout << "Goodbye, I was the Subject.\n"; } /** * The subscription management methods. */ void Attach(IObserver *observer) override { list_observer_.push_back(observer); } void Detach(IObserver *observer) override { list_observer_.remove(observer); } void Notify() override { std::list<IObserver *>::iterator iterator = list_observer_.begin(); HowManyObserver(); while (iterator != list_observer_.end()) { (*iterator)->Update(message_); ++iterator; } } void CreateMessage(std::string message = "Empty") { this->message_ = message; Notify(); } void HowManyObserver() { std::cout << "There are " << list_observer_.size() << " observers in the list.\n"; } /** * Usually, the subscription logic is only a fraction of what a Subject can * really do. Subjects commonly hold some important business logic, that * triggers a notification method whenever something important is about to * happen (or after it). */ void SomeBusinessLogic() { this->message_ = "change message message"; Notify(); std::cout << "I'm about to do some thing important\n"; } private: std::list<IObserver *> list_observer_; std::string message_; }; class Observer : public IObserver { public: Observer(Subject &subject) : subject_(subject) { this->subject_.Attach(this); std::cout << "Hi, I'm the Observer \"" << ++Observer::static_number_ << "\".\n"; this->number_ = Observer::static_number_; } virtual ~Observer() { std::cout << "Goodbye, I was the Observer \"" << this->number_ << "\".\n"; } void Update(const std::string &message_from_subject) override { message_from_subject_ = message_from_subject; PrintInfo(); } void RemoveMeFromTheList() { subject_.Detach(this); std::cout << "Observer \"" << number_ << "\" removed from the list.\n"; } void PrintInfo() { std::cout << "Observer \"" << this->number_ << "\": a new message is available --> " << this->message_from_subject_ << "\n"; } private: std::string message_from_subject_; Subject &subject_; static int static_number_; int number_; }; int Observer::static_number_ = 0; void ClientCode() { Subject *subject = new Subject; Observer *observer1 = new Observer(*subject); Observer *observer2 = new Observer(*subject); Observer *observer3 = new Observer(*subject); Observer *observer4; Observer *observer5; subject->CreateMessage("Hello World! :D"); observer3->RemoveMeFromTheList(); subject->CreateMessage("The weather is hot today! :p"); observer4 = new Observer(*subject); observer2->RemoveMeFromTheList(); observer5 = new Observer(*subject); subject->CreateMessage("My new car is great! ;)"); observer5->RemoveMeFromTheList(); observer4->RemoveMeFromTheList(); observer1->RemoveMeFromTheList(); delete observer5; delete observer4; delete observer3; delete observer2; delete observer1; delete subject; } int main() { ClientCode(); return 0; }执行结果：
Hi, I'm the Observer "1". Hi, I'm the Observer "2". Hi, I'm the Observer "3". There are 3 observers in the list. Observer "1": a new message is available --> Hello World! :D Observer "2": a new message is available --> Hello World! :D Observer "3": a new message is available --> Hello World! :D Observer "3" removed from the list. There are 2 observers in the list. Observer "1": a new message is available --> The weather is hot today! :p Observer "2": a new message is available --> The weather is hot today! :p Hi, I'm the Observer "4". Observer "2" removed from the list. Hi, I'm the Observer "5". There are 3 observers in the list. Observer "1": a new message is available --> My new car is great! ;) Observer "4": a new message is available --> My new car is great! ;) Observer "5": a new message is available --> My new car is great! ;) Observer "5" removed from the list. Observer "4" removed from the list. Observer "1" removed from the list. Goodbye, I was the Observer "5". Goodbye, I was the Observer "4". Goodbye, I was the Observer "3". Goodbye, I was the Observer "2". Goodbye, I was the Observer "1". Goodbye, I was the Subject.