- 它是一种行为型模式,观察者模式(Observer Pattern),也是我们熟知的发布-订阅模式(publish-subscribe)。
- C++ 代码中观察者模式很常见, 特别是在 GUI组件编程中,不同UI在不与其他对象类耦合的前提下,针对逻辑层事件做出不同反应的方式。
- C++ 程序员必须掌握的核心设计模式之一
-
目的:
定义对象间的一(Subject)对多( Observer)的依赖关系,当一个对象的状态发生改变时,所有依赖于它的对象都获得变更通知,并被自动执行更新信息。 -
意义:
保证高度的协作,但低耦合,不同对象之间进行解耦并弱化关系。
#include
#include
#include
//====================================抽象接口======================================
class I_Observer {
public:
virtual ~I_Observer(){};
virtual void Update(const std::string& message_from_subject) = 0;
};
class I_Subject {
public:
virtual ~I_Subject(){};
virtual void Attach(std::shared_ptr<I_Observer> observer) = 0;
virtual void Detach(std::shared_ptr<I_Observer> observer) = 0;
virtual void Notify() = 0;
};
//====================================被观察者======================================
class Subject : public I_Subject {
public:
virtual ~Subject() { std::cout << "Subject delete"<<std::endl; }
void Attach(std::shared_ptr<I_Observer> observer) override { list_observer_.emplace_back(observer); }
void Detach(std::shared_ptr<I_Observer> observer) override { list_observer_.remove(observer); }
void Notify() override
{
OutObserverCount();
for (auto &iterator:list_observer_) {
iterator->Update(message_);
}
}
void CreateMessage(std::string message = "NULL msg")
{
this->message_ = message;
Notify();
}
void OutObserverCount()
{
std::cout << "There are " << list_observer_.size() << " observers in the list." << std::endl;
}
private:
std::list<std::shared_ptr<I_Observer>> list_observer_;
std::string message_;
};
//====================================Android系统观察者======================================
class ObserverAndroid : public I_Observer {
public:
ObserverAndroid(std::string name) :name_(name) {}
virtual ~ObserverAndroid(){
std::cout << name_ <<" I was the Observer delete " << std::endl;
}
void Update(const std::string& message_from_subject) override
{
message_from_subject_ = message_from_subject;
PrintInfo();
}
private:
void PrintInfo() {
std::cout << name_ << std::endl;
}
private:
std::string message_from_subject_;
std::string name_;
};
//====================================苹果系统观察者======================================
class ObserverApple : public I_Observer {
public:
ObserverApple( std::string name) : name_(name) {}
virtual ~ObserverApple() {
std::cout << name_ << " Apple was the Observer delete " << std::endl;
}
void Update(const std::string& message_from_subject) override{
message_from_subject_ = message_from_subject;
PrintInfo();
}
private:
void PrintInfo() { std::cout << name_ << " IOS NB" << std::endl; }
private:
std::string message_from_subject_;
std::string name_;
};
//====================================测试======================================
void ClientCode()
{
//创建被观察者
std::shared_ptr<Subject> subject = std::make_shared<Subject>();
//创建两个观察者并注册
std::shared_ptr<ObserverAndroid> observer_xiaomi = std::make_shared<ObserverAndroid>("xiaomi");
subject->Attach(observer_xiaomi);
std::shared_ptr<ObserverAndroid> observer_vivo = std::make_shared<ObserverAndroid>("Vivo");
subject->Attach(observer_vivo);
//被观察者发布消息
subject->CreateMessage("Hello World!");
//移除一个被观察者并发布新消息
subject->Detach(observer_vivo);
subject->CreateMessage("Android NB");
//创建一个新的观察者并注册
std::shared_ptr<ObserverApple> observer_apple = std::make_shared<ObserverApple>("Apple phone");
subject->Attach(observer_apple);
//发布新消息
subject->CreateMessage("Mobile NB");
std::cout << std::endl;
}
int main()
{
ClientCode();
return 0;
}
运行结果
优点:
- 开闭原则,无需修改发布者代码就能引入新的订阅者类
- 可以在运行时建立对象之间的联,如注册或移除等行为
- 观察者和被观察者是松耦合
缺点:
- 订阅者的通知顺序是随机的,很难按照指定顺序执行
- 被观察者对象有很多的直接和间接的观察者的话,通知所有的观察者可能会花费很长的时间
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