Observer em C++
O Observer é um padrão de projeto comportamental que permite que um objeto notifique outros objetos sobre alterações em seu estado.
O padrão Observer fornece uma maneira de assinar e cancelar a assinatura desses eventos para qualquer objeto que implemente uma interface de assinante.
Complexidade:
Popularidade:
Exemplos de uso: O padrão Observer é bastante comum no código C++, especialmente nos componentes da interface de usuário. Ele fornece uma maneira de reagir a eventos que acontecem em outros objetos sem acoplamento às suas classes.
Identificação: O padrão pode ser reconhecido por métodos de assinatura, que armazenam objetos em uma lista e por chamadas para o método de atualização emitido para objetos nessa lista.
Exemplo conceitual
Este exemplo ilustra a estrutura do padrão de projeto Observer. Ele se concentra em responder a estas perguntas:
- De quais classes ele consiste?
- Quais papéis essas classes desempenham?
- De que maneira os elementos do padrão estão relacionados?
main.cc: Exemplo conceitual
/**
* Observer Design Pattern
*
* Intent: Lets you define a subscription mechanism to notify multiple objects
* about any events that happen to the object they're observing.
*
* Note that there's a lot of different terms with similar meaning associated
* with this pattern. Just remember that the Subject is also called the
* Publisher and the Observer is often called the Subscriber and vice versa.
* Also the verbs "observe", "listen" or "track" usually mean the same thing.
*/
#include <iostream>
#include <list>
#include <string>
class IObserver {
public:
virtual ~IObserver(){};
virtual void Update(const std::string &message_from_subject) = 0;
};
class ISubject {
public:
virtual ~ISubject(){};
virtual void Attach(IObserver *observer) = 0;
virtual void Detach(IObserver *observer) = 0;
virtual void Notify() = 0;
};
/**
* The Subject owns some important state and notifies observers when the state
* changes.
*/
class Subject : public ISubject {
public:
virtual ~Subject() {
std::cout << "Goodbye, I was the Subject.\n";
}
/**
* The subscription management methods.
*/
void Attach(IObserver *observer) override {
list_observer_.push_back(observer);
}
void Detach(IObserver *observer) override {
list_observer_.remove(observer);
}
void Notify() override {
std::list<IObserver *>::iterator iterator = list_observer_.begin();
HowManyObserver();
while (iterator != list_observer_.end()) {
(*iterator)->Update(message_);
++iterator;
}
}
void CreateMessage(std::string message = "Empty") {
this->message_ = message;
Notify();
}
void HowManyObserver() {
std::cout << "There are " << list_observer_.size() << " observers in the list.\n";
}
/**
* Usually, the subscription logic is only a fraction of what a Subject can
* really do. Subjects commonly hold some important business logic, that
* triggers a notification method whenever something important is about to
* happen (or after it).
*/
void SomeBusinessLogic() {
this->message_ = "change message message";
Notify();
std::cout << "I'm about to do some thing important\n";
}
private:
std::list<IObserver *> list_observer_;
std::string message_;
};
class Observer : public IObserver {
public:
Observer(Subject &subject) : subject_(subject) {
this->subject_.Attach(this);
std::cout << "Hi, I'm the Observer \"" << ++Observer::static_number_ << "\".\n";
this->number_ = Observer::static_number_;
}
virtual ~Observer() {
std::cout << "Goodbye, I was the Observer \"" << this->number_ << "\".\n";
}
void Update(const std::string &message_from_subject) override {
message_from_subject_ = message_from_subject;
PrintInfo();
}
void RemoveMeFromTheList() {
subject_.Detach(this);
std::cout << "Observer \"" << number_ << "\" removed from the list.\n";
}
void PrintInfo() {
std::cout << "Observer \"" << this->number_ << "\": a new message is available --> " << this->message_from_subject_ << "\n";
}
private:
std::string message_from_subject_;
Subject &subject_;
static int static_number_;
int number_;
};
int Observer::static_number_ = 0;
void ClientCode() {
Subject *subject = new Subject;
Observer *observer1 = new Observer(*subject);
Observer *observer2 = new Observer(*subject);
Observer *observer3 = new Observer(*subject);
Observer *observer4;
Observer *observer5;
subject->CreateMessage("Hello World! :D");
observer3->RemoveMeFromTheList();
subject->CreateMessage("The weather is hot today! :p");
observer4 = new Observer(*subject);
observer2->RemoveMeFromTheList();
observer5 = new Observer(*subject);
subject->CreateMessage("My new car is great! ;)");
observer5->RemoveMeFromTheList();
observer4->RemoveMeFromTheList();
observer1->RemoveMeFromTheList();
delete observer5;
delete observer4;
delete observer3;
delete observer2;
delete observer1;
delete subject;
}
int main() {
ClientCode();
return 0;
}
Output.txt: Resultados da execução
Hi, I'm the Observer "1".
Hi, I'm the Observer "2".
Hi, I'm the Observer "3".
There are 3 observers in the list.
Observer "1": a new message is available --> Hello World! :D
Observer "2": a new message is available --> Hello World! :D
Observer "3": a new message is available --> Hello World! :D
Observer "3" removed from the list.
There are 2 observers in the list.
Observer "1": a new message is available --> The weather is hot today! :p
Observer "2": a new message is available --> The weather is hot today! :p
Hi, I'm the Observer "4".
Observer "2" removed from the list.
Hi, I'm the Observer "5".
There are 3 observers in the list.
Observer "1": a new message is available --> My new car is great! ;)
Observer "4": a new message is available --> My new car is great! ;)
Observer "5": a new message is available --> My new car is great! ;)
Observer "5" removed from the list.
Observer "4" removed from the list.
Observer "1" removed from the list.
Goodbye, I was the Observer "5".
Goodbye, I was the Observer "4".
Goodbye, I was the Observer "3".
Goodbye, I was the Observer "2".
Goodbye, I was the Observer "1".
Goodbye, I was the Subject.
