设计模式——观察者模式(发布/订阅模式)

观察者模式(发布/订阅模式)

是一种行为模式，允许你定义一种订阅机制，可在对象事件发生时通知多个“观察”该对象的其他对象

观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一主题对象。这个主题对象在状态发生变化时，会通知所有的观察者对象，使得它们能够自动更新自己。

#include <iostream>
#include <list>
#include <string>

using namespace std;

// 观察者(订阅者)的抽象类，里面包含一个Update方法
class IObserver
{
public:
    virtual ~IObserver(){};
    virtual void Update(const string &message_from_subject) = 0;
};

// 主题/通知者(发布者)的抽象类，里面包含add、remove等方法
class ISubject
{
public:
    virtual ~ISubject(){};
    virtual void Attach(IObserver *observer) = 0;
    virtual void Detach(IObserver *onserver) = 0;
    virtual void Notify() = 0;
};

// 发布者在状态发生变化时通知观察者
class Subject : public ISubject
{
public:
    virtual ~Subject()
    {
        cout << "Goodbye, I was the Subject. " << endl;
    }

    void Attach(IObserver *observer) override
    {
        list_observer_.push_back(observer);
    }

    void Detach(IObserver *observer) override
    {
        list_observer_.remove(observer);
    }

    void Notify() override
    {
        auto iterator = list_observer_.begin();
        cout << "There are " << list_observer_.size() << " observers in the list. " << endl;
        while (iterator != list_observer_.end())
        {
            (*iterator)->Update(message_);
            ++iterator;
        }
    }

    // 状态改变，通知订阅者
    void CreateMessage(string message = "Empty")
    {
        this->message_ = message;
        this->Notify();
    }

private:
    list<IObserver *> list_observer_;
    string message_;
};

// 具体的观察者
class Observer : public IObserver
{
public:
    Observer(ISubject *subject) : subject_(subject) // 依赖抽象，而非依赖具体的subject对象
    {
        this->subject_->Attach(this); // 添加到监听列表
        cout << "Hi, I'm the Observer \"" << ++Observer::static_number_ << "\".\n";
        this->number_ = Observer::static_number_;
    }

    virtual ~Observer()
    {
        cout << "Goodbye, I was the Observer \"" << this->number_ << "\".\n";
    }

    void Update(const string &message_from_subject) override
    {
        message_from_subject_ = message_from_subject;
        std::cout << "Observer \"" << this->number_ << "\": a new message is available --> " << this->message_from_subject_ << "\n";
    }

    void RemoveMeFromTheList()
    {
        subject_->Detach(this);
        cout << "Observer \"" << number_ << "\" removed from the list.\n";
    }

private:
    string message_from_subject_;
    ISubject *subject_;
    static int static_number_;
    int number_;
};

int Observer::static_number_ = 0;

// 客户端代码
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;
}

输出：

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. 

• 观察者模式的特点
  观察者模式所做的工作其实就是在解除耦合，让耦合的双方都依赖于抽象，，而不是依赖于具体，从而使得各自的变化都不会影响另一边的变化