Design Patterns GOF23 observer mode

Observer Pattern Observer

Broadcast mechanism

Scene: multiple observers - to be notified change

CS when the characters moving coordinate changes, updated coordinates on the map for everyone

Core: When the target object (Subject) status value changes, the need to promptly inform all observers (Observer), to enable them to respond

Development of common scenarios:

- chat room procedures, the server forwards to all clients

- online games (multiplayer), the server will distribute each player's status

-E-mail Subscriptions

-Servlet, the sensing mechanism

The broadcast mechanism -Android

-JDK of AWT event handling model, delegate event model observer mode (Delegation Event Model) based on (event source ------ target object, event listeners ------ observer)

- Jingdong Mall, a mass merchandise discount information

public class Subject {
 //被观察者队伍
  protected List<Observer> observers=new ArrayList<>();
  public void addObserver(Observer obs) {
   observers.add(obs);
  }
  public void removeObserver(Observer obs) {
   observers.remove(obs);
  }
  //通知所有被观察者的状态更新
  public void notifyAllObserve() {
   for(Observer obs:observers) {
    obs.update();
   }
  }
}
class ConcreteSub extends Subject{
 private int state;
 public int getState() {
  return state;
 }
 public void setState(int state) {
  this.state = state;
  this.notifyAllObserve();
 }
}

public interface Observer {
  void update();
}
class ObserverA implements Observer{
 private int myState;
 private Subject sub;
 public ObserverA(Subject sub) {
  this.sub = sub;
 }

 @Override
 public void update() {
  myState=((ConcreteSub)sub).getState();
  System.out.println("状态已更新");
 }
 
}

public class Client {
public static void main(String[] args) {
 ConcreteSub sub=new ConcreteSub();
 ObserverA obs1=new ObserverA(sub);
 ObserverA obs2=new ObserverA(sub);
 ObserverA obs3=new ObserverA(sub);
 //添加到被观察者队伍
 sub.addObserver(obs1);
 sub.addObserver(obs2);
 sub.addObserver(obs3);
 
 sub.setState(1000);
}
}

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java.util.Observable（相当于上面定义的Subject类）和java.util.Observer（相当于上面定义的Observer接口）

public class ConcreteSubject extends Observable{
  private int state;
  public void set(int s) {
   state=s;
   setChanged();
   this.notifyObservers();
  }
public int getState() {
 return state;
}
public void setState(int state) {
 this.state = state;
}
 
}

public class ObserverA implements Observer{
  private int myState;
 @Override
 public void update(Observable o, Object arg) {
  myState=((ConcreteSubject)o).getState();
 }
 public int getMyState() {
  return myState;
 }
 public void setMyState(int myState) {
  this.myState = myState;
 }
 

}

public class Client {
public static void main(String[] args) {
 ConcreteSubject cs=new ConcreteSubject();
 ObserverA obs1=new ObserverA();
 ObserverA obs2=new ObserverA();
 ObserverA obs3=new ObserverA();
 //添加至队伍中
 cs.addObserver(obs1);
 cs.addObserver(obs2);
 cs.addObserver(obs3);
 
 cs.set(100);
 
 System.out.println(obs1.getMyState());
 System.out.println(obs2.getMyState());
 System.out.println(obs2.getMyState());
 
}
}