Observer Pattern

公众号与订阅者之间的关系，一对多。

// Observer
class Observer
{
public:
    Observer() = default;
    virtual ~Observer() = default;

    virtual void update(const char *msg) = 0;
};

class Subscriber : public Observer
{
public:
    explicit Subscriber(const std::string &id) {}
    ~Subscriber() override {}

    void update(const std::string &msg) override
    {
        // recived a new message
    }

private:
    std::string m_id;
};

// Subject
class Observer;
class Subject
{
public:
    Subject() = default;
    virtual ~Subject() = default;

    virtual void notify() = 0;
    virtual void registerObserver(Observer *) = 0;
    virtual void unregisterObserver(Observer *) = 0;
};
class Service : public Subject
{
public:
    Service() {}
    ~Service() override {}

    void notify() override
    {
        for (Observer *obs : m_observers) {
            obs->update("Hello!");
        }
    }
    void registerObserver(Observer *observer) override
    {
        m_observers.emplace_back(observer);
        observer->update("Subscribe successfully!");
    }
    void unregisterObserver(Observer *observer) override;
    {
        m_observers.remove(observer);
        observer->update("Unsubscribe successfully!");
    }

protected:
    std::list<Observer *> m_observers;
};

// main
#include "observer.h"
#include "subject.h"

int main(int argc, char *argv[])
{
    Service service;
    Subscriber subscriberA("Subscriber A");
    Subscriber subscriberB("Subscriber B");
    Subscriber subscriberC("Subscriber C");

    service.registerObserver(&subscriberA);
    service.registerObserver(&subscriberB);
    service.registerObserver(&subscriberC);
    service.notify();

    service.unregisterObserver(&subscriberC);
    service.notify();

    return 0;
}

• 观察者模式