自由意志

黑人夏川

观察者模式

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定义对象间的一种一对多的依赖关系，以便当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并自动刷新。
观察者模式又叫做发布-订阅(Publish-Subscribe)模式、模型-视图(Model-View)模式、源-监听器(Source-Listener)模式。

实现一个抽象观察者和抽象主题

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class Observer : public std::enable_shared_from_this<Observer>
{
public:
    virtual ~Observer() {}

    virtual void update(int) = 0;
};

class Subject
{
public:
    virtual ~Subject() {}

    virtual void attach(std::weak_ptr<Observer>) = 0;
    virtual void detach(std::weak_ptr<Observer>) = 0;
    virtual void notify() = 0;
};

具体实现

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class PhoneObserver : public Observer
{
public:
    virtual void update(int i) { std::cout << "This is PhoneObserver " << i << std::endl; }
};

class ComputerObserver : public Observer
{
public:
    virtual void update(int i) { std::cout << "This is ComputerObserver " << i << std::endl; }
};

class ConcreteSubject : public Subject
{
public:
    virtual void attach(std::weak_ptr<Observer> observerPtr) { m_Observers.push_back(observerPtr); }
    virtual void detach(std::weak_ptr<Observer> observerPtr)
    {
        m_Observers.remove_if(
            [=](const std::weak_ptr<Observer> &ptr1) { return observerPtr.lock() == ptr1.lock(); });
    }
    virtual void notify()
    {
        for (auto iter = m_Observers.begin(); iter != m_Observers.end();) {
            auto share = (*iter).lock();
            if (share) {
                share->update(m_state);
                iter++;
            } else {
                iter = m_Observers.erase(iter);
            }
        }
    }

    void setState(int state) { m_state = state; }

private:
    std::list<std::weak_ptr<Observer>> m_Observers;
    int m_state = 0;
};

测试

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int main(int argc, char *argv[])
{
    auto phoneObserver = std::make_shared<PhoneObserver>();
    auto computerObserver = std::make_shared<ComputerObserver>();

    auto concreteSubject = std::make_unique<ConcreteSubject>();
    concreteSubject->attach(phoneObserver);
    concreteSubject->attach(computerObserver);

    concreteSubject->setState(1);
    concreteSubject->notify();
    std::cout << "----------------------------" << std::endl;
    concreteSubject->detach(computerObserver);
    concreteSubject->setState(222);
    concreteSubject->notify();
    std::cout << "----------------------------" << std::endl;
    concreteSubject->attach(computerObserver);
    concreteSubject->detach(phoneObserver);
    concreteSubject->setState(333);
    concreteSubject->notify();
    std::cout << "----------------------------" << std::endl;
    concreteSubject->attach(phoneObserver);
    concreteSubject->setState(444);
    concreteSubject->notify();
    std::cout << "----------------------------" << std::endl;
    phoneObserver.reset();
    computerObserver.reset();
    concreteSubject->setState(555);
    concreteSubject->notify();

    return 0;
}

结果

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This is PhoneObserver 1
This is ComputerObserver 1
----------------------------
This is PhoneObserver 222
----------------------------
This is ComputerObserver 333
----------------------------
This is ComputerObserver 444
This is PhoneObserver 444
----------------------------

Observer源码