What is Observer Pattern?

最新推荐文章于 2021-02-20 21:34:55 发布

原创最新推荐文章于 2021-02-20 21:34:55 发布 · 332 阅读

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文章标签：

#notifications #interface #iterator #class #events #output

本文介绍了一种行为设计模式——观察者模式，它用于解决多个对象同步更新的问题，如Model-View-Controller概念中的通知机制。文章通过一个示例演示了观察者模式的实现方式：包含一个维护观察者列表的Observable类和一个定义更新通知接口的Observer抽象类。

What is Observer Pattern?

* This is a behavioral design pattern.
* Observer pattern is used to solve the problem of notifying multiple objects of a change to keep them in sync like the Model-View-Controller concept.
* Useful for event management kind of scenarios.
* Two classes are involved.
* The Observable class is where the actual data change is occuring. It has information about all the interested objects that need to be notified.
* The Observer is typically an abstract class providing the interface to which concrete classes interested in the events need to be compliant to.

EXAMPLE: Demonstrate the implementation of observer design pattern

#include
#include

using namespace std;

// ---------------- Observer interface -----------------
class MyObserver {
public:
virtual void Notify() = 0;
};

// ---------------- Observable object -------------------
class MyObservable {
static MyObservable* instance;
set observers;
MyObservable() { };
public:
static MyObservable* GetInstance();
void AddObserver(MyObserver& o);
void RemoveObserver(MyObserver& o);
void NotifyObservers();
void Trigger();
};

MyObservable* MyObservable::instance = NULL;

MyObservable* MyObservable::GetInstance()
{
if ( instance == NULL ) {
instance = new MyObservable();
}

return instance;
}

void MyObservable::AddObserver(MyObserver& o)
{
observers.insert(&o);
}

void MyObservable::RemoveObserver(MyObserver& o)
{
observers.erase(&o);
}

void MyObservable::NotifyObservers()
{
set::iterator itr;
for ( itr = observers.begin();
itr != observers.end(); itr++ )
(*itr)->Notify();
}

// TEST METHOD TO TRIGGER
// IN THE REAL SCENARIO THIS IS NOT REQUIRED
void MyObservable::Trigger()
{
NotifyObservers();
}

// ------ Concrete class interested in notifications ---
class MyClass : public MyObserver {

MyObservable* observable;

public:
MyClass() {
observable = MyObservable::GetInstance();
observable->AddObserver(*this);
}

~MyClass() {
observable->RemoveObserver(*this);
}

void Notify() {
cout << "Received a change event" << endl;
}
};

void main()
{
MyObservable* observable = MyObservable::GetInstance();
MyClass* obj = new MyClass();
observable->Trigger();
}

OUTPUT:
Received a change event