行为型设计模式-观察者模式的优缺点

• 行为型模式：主要关注的是对象之间的通信
• 观察者-监听者模式（发布-订阅）模式：主要关注的是对象一对多的关系
• 也就是说多个对象都依赖一个对象，当该对象的状态发生改变时，其他对象都能接收到相应通知

class Observer
{
public:
	Observer() { cout << "Observer构造" << endl; }
	virtual ~Observer() { cout << "Observer析构" << endl; }
	//处理消息的接口
	virtual void handle(int msgID) = 0;
};

---

class Subject
{
public:
	Subject()  {
		cout << "Subject的构造函数" << endl;
	}
	~Subject() {

		cout << "Subject的析构函数" << endl;
	}

	//给主题增加观察者对象
	void addObserver(Observer* observer,int msgID) {

		obseverMap_[msgID].push_back(observer);
		/*auto it = obseverMap_.find(msgID);
		if (it != obseverMap_.end()) {
			it->second.push_back(observer);
		}
		else {
			list<Observer*>list;
			list.push_back(observer);
			obseverMap_.insert({ msgID, list });
		}
		*/
	}

	void remove(Observer* observer) {
		
	}
	//主题发生改变，通知相应的观察者对象处理事件
	void dispatch(int msgID) {
		auto it = obseverMap_.find(msgID);
		if (it != obseverMap_.end()) {
			for (Observer* pObserver : it->second) {
				pObserver->handle(msgID);
			}
		}
	}

private:
	unordered_map<int,list<Observer*>> obseverMap_;
};

---

class Observer1:public Observer
{
public:
	Observer1(){

		cout << "Observer1构造" << endl; 
	}
	~Observer1() {
		cout << "Observer1析构" << endl; 
	}

	void handle(int msgID) {
		switch (msgID)
		{
		case 1:
			cout << "Observer1 recv 1 msg!" << endl;
			break;
		case 2:
			cout << "Observer1 recv 2 msg!" << endl;
			break;
		default:
			cout << "Observer1 recv unkonwn msg!" << endl;
			break;
		}

	}
};

---

//观察者2
class Observer2 :public Observer
{
public:
	Observer2() {

		cout << "Observer2构造" << endl;
	}
	~Observer2() {
		cout << "Observer2析构" << endl;
	}

	void handle(int msgID) {
		switch (msgID)
		{
		case 2:
			cout << "Observer2 recv 2 msg!" << endl;
			break;
		
		default:
			cout << "Observer2 recv unkonwn msg!" << endl;
			break;
		}

	}
};

---

/观察者3
class Observer3 :public Observer
{
public:
	Observer3() {

		cout << "Observer3构造" << endl;
	}
	~Observer3() {
		cout << "Observer3析构" << endl;
	}

	void handle(int msgID) {
		switch (msgID)
		{
		case 1:
			cout << "Observer3 recv 1 msg!" << endl;
			break;
		case 3:
			cout << "Observer3 recv 3 msg!" << endl;
			break;
		default:
			cout << "Observer3 recv unkonwn msg!" << endl;
			break;
		}

	}
};

---

int main()
{
	Subject subject;
	Observer* p1 = new Observer1;
	Observer* p2 = new Observer2;
	Observer* p3 = new Observer3;

	subject.addObserver(p1, 1);
	subject.addObserver(p1, 2);
	subject.addObserver(p2, 2);
	subject.addObserver(p3, 3);
	subject.addObserver(p3, 1);
	
	int msgid = 0;
	int conut = 0;
	for (;;) {

		cout << "请输入消息ID:";
		cin >> msgid;
		if (msgid == -1)
			break;
		subject.dispatch(msgid);
		conut++;	
	}

	delete p1;
	p1 = nullptr;
	delete p2;
	p2 = nullptr;
	delete p3;
	p3 = nullptr;

	return 0;
}

优点：

• 观察者和被观察者之间建立了一个抽象的耦合。
• 观察者模式支持广播通信

缺点：

• 观察者之间有过多的细节依赖、提高时间消耗及程序的复杂度。
• 如果在观察者和观察目标之间存在循环依赖，观察目标会触发它们之间进行循环调用，可能导致系统崩溃。