设计模式（4）--对象行为(7)--观察者

1. 意图

定义对象间的一种一对多的依赖关系，

当一个对象的状态改变时，所有依赖于它的对象都得到通知并被自动更新。

2. 四种角色

抽象目标(Subject)、具体目标(Concrete Subject)、抽象观察者(Observer)、

具体观察者(Concrete Observer)

3. 优点

3.1 目标和观察者之间的耦合是抽象的。

3.2 支持广播通信。

4. 缺点

4.1 可能导致意外的更新。

5. 相关模式

当目标和观察者间的依赖关系特别复杂时，需要一个维护这些关系的对象，

这样的对象称为ChangeMananger。

5.1 ChangeManager可使用单例模式来保证它是唯一的并且是可全局访问的。

5.2 ChangeManager充当目标和观察者之间的Mediator

6. 代码示意(C++)

#pragma once
#include <string>
#include <iostream>
#include <vector>
using namespace std;

class Subject;

class Observer
{
public:
	virtual void Update(Subject* pSubject) = 0;
protected:
	Observer() {}
};
class ConcreteObserver :public Observer
{
	string m_state;
	string m_name;
public:
	ConcreteObserver(const string& name) :m_name(name) {
	}
	virtual void Update(Subject* pSubject);
};

class Subject
{
	vector<Observer*> m_observers;
public:
	virtual string GetState() = 0;
	virtual void SetState(const string& state) = 0;
public:
	void Attach(Observer* pObserver) {
		m_observers.emplace_back(pObserver);
		cout << "After attached, observers size is:" << m_observers.size() << endl;
	}
	void Detach(Observer* pObserver) {
		m_observers.erase(std::remove_if(m_observers.begin(), m_observers.end(), [&](Observer* p) { return p == pObserver; }), m_observers.end());
		cout << "After detached, observers size is:" << m_observers.size() << endl;
	}
	void Notify()
	{
		auto it = m_observers.begin();
		while (it != m_observers.end()) {
			(*it)->Update(this);
			++it;
		}
	}
protected:
	Subject(){}
};
class ConcreteSubject :public Subject
{
	string m_state;
public:
	virtual string GetState() {
		return m_state;
	}
	virtual void SetState(const string& state) {
		m_state = state;
	}
};

Observer.cpp:

#include "Observer.h"

void ConcreteObserver::Update(Subject* pSubject) {
	m_state = pSubject->GetState();
	cout << "Observer:" << m_name << ",got state from subject:" << m_state << endl;
}

#include "Observer.h"
int main() {
	Subject* pSubject = new ConcreteSubject();

	Observer* pObserver1 = new ConcreteObserver("obs1");
	Observer* pObserver2 = new ConcreteObserver("obs2");

	pSubject->Attach(pObserver1);
	pSubject->Attach(pObserver2);

	pSubject->SetState("hello1");
	pSubject->Notify();

	pSubject->Detach(pObserver1);
	pSubject->SetState("hello2");
	pSubject->Notify();

	delete pObserver2;
	delete pObserver1;
	delete pSubject;
	return 0;
}

运行结果：

6.1 目标和观察者之间只知道彼此的抽象类(3.1)。

6.2 Subject::Notify里的循环就是广播，观察者自己决定是否处理某一通知(3.2)。

6.3 使用ChangeMananger会使代码复杂些，但简化了Subject，且使更新策略更加灵活。

使用ChangeMananger代码示意：

#pragma once
#include <string>
#include <iostream>
#include <vector>
#include <map>
using namespace std;

class Subject;

class Observer
{
public:
	virtual void Update(Subject* pSubject) = 0;
	virtual string GetName() = 0;
protected:
	Observer() {}
};
class ConcreteObserver :public Observer
{
	string m_state;
	string m_name;
public:
	ConcreteObserver(const string& name) :m_name(name) {
	}
	virtual string GetName() { return m_name; }
	virtual void Update(Subject* pSubject);
};

class ChangeManager;
class SimpleChangeManager;

class Subject
{
	ChangeManager* m_pChangeManager;
public:
	virtual string GetState() = 0;
	virtual void SetState(const string& state) = 0;
	virtual ~Subject();
public:
	void Attach(Observer* pObserver);
	void Detach(Observer* pObserver);
	void Notify();
protected:
	Subject();
};
class ConcreteSubject :public Subject
{
	string m_state;
public:
	ConcreteSubject() {
	}
	virtual string GetState() {
		return m_state;
	}
	virtual void SetState(const string& state) {
		m_state = state;
	}
};

class ChangeManager
{
public:
	virtual void Register(Subject* pSubject, Observer* pObserver) = 0;
	virtual void Unregister(Subject* pSubject, Observer* pObserver) = 0;
	virtual void Notify() = 0;
protected:
	ChangeManager() {}
};

class SimpleChangeManager :public ChangeManager
{
	map<Subject*, vector<Observer*> > m_mapSubjects;
private:
	static SimpleChangeManager* s_instance;
public:
	static ChangeManager* Instance() {
		if (0 == s_instance) {
			s_instance = new SimpleChangeManager();
		}
		return s_instance;
	}
public:
	virtual void Register(Subject* pSubject, Observer* pObserver) {
		vector<Observer*>& observers = m_mapSubjects[pSubject];
		auto it = find(observers.begin(), observers.end(), pObserver);
		if (it == observers.end()) {
			observers.emplace_back(pObserver);
			cout << pObserver->GetName() << " is registered successful" << endl;
		}
		else {
			cout << pObserver->GetName() << " is already registered" << endl;
		}
	}
	virtual void Unregister(Subject* pSubject, Observer* pObserver) {
		auto it = m_mapSubjects.find(pSubject);
		if (it == m_mapSubjects.end()) {
			cout << "No need unregister in map for:" << pObserver->GetName() << endl;
		}
		else {
			vector<Observer*>& observers = m_mapSubjects[pSubject];
			auto itRemove = remove_if(observers.begin(), observers.end(), [&](Observer* p) { return p == pObserver; });
			if (itRemove == observers.end()) {
				cout << "No need unregister in vector for:" << pObserver->GetName() << endl;
			}
			else {
				observers.erase(itRemove, observers.end());
				cout << pObserver->GetName() << " is unregistered successful" << endl;
				if (observers.size() == 0) {
					m_mapSubjects.erase(pSubject);
				}
			}
		}
	}
	virtual void Notify() {
		for (auto& pair : m_mapSubjects) {
			vector<Observer*>& observers = pair.second;
			auto it = observers.begin();
			while (it != observers.end()) {
				(*it)->Update(pair.first);
				++it;
			}
		}
	}
protected:
	SimpleChangeManager() {}
};

Observer.cpp:

#include "Observer.h"

void ConcreteObserver::Update(Subject* pSubject) {
	m_state = pSubject->GetState();
	cout << "Observer:" << m_name << ",got state from subject:" << m_state << endl;
}

Subject::Subject() {
	m_pChangeManager = SimpleChangeManager::Instance();
}
Subject::~Subject() {
	delete m_pChangeManager;
}
void Subject::Attach(Observer* pObserver) {
	m_pChangeManager->Register(this, pObserver);
}
void Subject::Detach(Observer* pObserver) {
	m_pChangeManager->Unregister(this, pObserver);
}
void Subject::Notify()
{
	m_pChangeManager->Notify();
}

SimpleChangeManager* SimpleChangeManager::s_instance = 0;

#include "Observer.h"
int main() {

	Subject* pSubject = new ConcreteSubject();

	Observer* pObserver1 = new ConcreteObserver("obs1");
	Observer* pObserver2 = new ConcreteObserver("obs2");

	pSubject->Attach(pObserver1);
	pSubject->Attach(pObserver2);
	pSubject->Attach(pObserver1);

	pSubject->SetState("hello1");
	pSubject->Notify();

	pSubject->Detach(pObserver1);
	pSubject->Detach(pObserver1);
	pSubject->SetState("hello2");
	pSubject->Notify();

	pSubject->Detach(pObserver2);
	pSubject->Detach(pObserver2);

	delete pObserver2;
	delete pObserver1;
	delete pSubject;
	return 0;
}

运行结果：