Observer model of behavioral model

Concept

Observer mode Defines one to many dependencies between objects , thus , When an object changes state , All its dependents are notified and automatically updated . Also known as release order Reading mode .

chart

Role analysis ：

1. Subject: Observed . Define the responsibilities that must be fulfilled by the observed , Must be able to dynamically increase 、 Cancel the observer . Generally, it is an abstract class or interface , Only fulfill the duties that the observed must fulfill ： Manage the observer and inform the observer .
2. Observer The observer ： After the observer receives the message , Process information .
3. ConcreteSubject The specific observed ： Define the business logic of the observed , It also defines which events are notified .
4. ConCreteObserver： The processing logic reaction of each observer after receiving the message is different , Each observer has its own processing logic .

Code

Observed

package com.wlj.observer;

import java.util.ArrayList;

/** * @author wlj * @Classname AbstractSubject * @Description  The observed abstract class  * @Date 7/24/2022 6:06 PM */
public  abstract class ASubject {
    
    //  List of observers 
    private ArrayList<Observer> observerArrayList = new ArrayList<>();
    //  Add an observer 
    public void addObserver(Observer observer){
    
        this.observerArrayList.add(observer);
    }
    //  Delete an observer 
    public void deleteObserver(Observer observer) {
    
        if(observerArrayList.indexOf(observer) >= 0){
    
            this.observerArrayList.remove(observer);
        }
    }
    //  Notify all observers 
    public void notifyObservers(){
    
        for (Observer observer : observerArrayList) {
    
            observer.update();
        }
    }
}
package com.wlj.observer;

/** * @author wlj * @Classname ConcreteSubject * @Description  The specific observed  * @Date 7/24/2022 6:25 PM */
public class ConcreteSubject extends ASubject{
    
    public void notify(){
    
    	//  You can do something else , Notify observer 
        super.notifyObservers();
    }
}

The observer

package com.wlj.observer;

/** * @author wlj * @Classname Observer * @Description  Observer mode  * @Date 7/24/2022 6:07 PM */
public interface Observer {
    
    public void update();
}
package com.wlj.observer;

/** * @author wlj * @Classname ConcreteObserver * @Description  The observer A * @Date 7/24/2022 6:16 PM */
public class ConcreteObserverA implements Observer{
    
    @Override
    public void update() {
    
        System.out.println("A Receive information for processing ");
    }
}
package com.wlj.observer;

/** * @author wlj * @Classname ConcreteObserver * @Description  The observer B * @Date 7/24/2022 6:16 PM */
public class ConcreteObserverB implements Observer{
    
    @Override
    public void update() {
    
        System.out.println("B Receive information for processing ");
    }
}

Use scenarios

package com.wlj.observer;

/** * @author wlj * @Classname Client * @Description  Specific scenarios  * @Date 7/24/2022 6:19 PM */
public class Client {
    
    public static void main(String[] args) {
    
        //  Create an observed 
        ConcreteSubject subject = new ConcreteSubject();
        //  Create observers 
        Observer observerA = new ConcreteObserverA();
        Observer observerB = new ConcreteObserverB();
        //  Add observers 
        subject.addObserver(observerA);
        subject.addObserver(observerB);
        //  Notify all observers 
        subject.notify();
    }
}

As long as it is executed by the observer notify Method , That is, when the observer makes some actions or changes , Will inform the observer .

Pattern analysis

1. When the change of an object needs to be notified to multiple objects , We can use the observer mode .
2. When we need to publish and subscribe , You can use observer mode .
3. MessageQuene and EventBus In fact, it also uses the observer mode ,hook,callback,listener In fact, it can also be generalized as the application of observer pattern .
4. The observer model is often used in conjunction with the chain of responsibility model , Observer mode can only be executed along the task chain , Cannot pass in direction .