Design Pattern 简明教程
Design Patterns - Observer Pattern
观察者模式用于对象之间存在一对多关系的场景，例如如果一个对象被修改，其依赖项会自动被通知。观察者模式属于行为型模式范畴。
Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its depenedent objects are to be notified automatically. Observer pattern falls under behavioral pattern category.
Implementation
观察者模式使用三个参与者类：Subject、Observer 和 Client。Subject 是一个对象，有用于将观察者附加和分离到客户端对象的方法。我们创建了一个抽象类 Observer 和一个扩展类 Observer 的具体类 Subject。
Observer pattern uses three actor classes. Subject, Observer and Client. Subject is an object having methods to attach and detach observers to a client object. We have created an abstract class Observer and a concrete class Subject that is extending class Observer.
我们的演示类 ObserverPatternDemo 将使用 Subject 和具体类对象展示观察者模式的实际场景。
ObserverPatternDemo, our demo class, will use Subject and concrete class object to show observer pattern in action.
Step 1
创建主题类。
Create Subject class.
Subject.java
import java.util.ArrayList;
import java.util.List;
public class Subject {
private List<Observer> observers = new ArrayList<Observer>();
private int state;
public int getState() {
return state;
}
public void setState(int state) {
this.state = state;
notifyAllObservers();
}
public void attach(Observer observer){
observers.add(observer);
}
public void notifyAllObservers(){
for (Observer observer : observers) {
observer.update();
}
}
}Step 2
创建观察者类。
Create Observer class.
Observer.java
public abstract class Observer {
protected Subject subject;
public abstract void update();
}Step 3
创建具体观察者类
Create concrete observer classes
BinaryObserver.java
public class BinaryObserver extends Observer{
public BinaryObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) );
}
}OctalObserver.java
public class OctalObserver extends Observer{
public OctalObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Octal String: " + Integer.toOctalString( subject.getState() ) );
}
}HexaObserver.java
public class HexaObserver extends Observer{
public HexaObserver(Subject subject){
this.subject = subject;
this.subject.attach(this);
}
@Override
public void update() {
System.out.println( "Hex String: " + Integer.toHexString( subject.getState() ).toUpperCase() );
}
}Step 4
使用主题和具体观察者对象。
Use Subject and concrete observer objects.
ObserverPatternDemo.java
public class ObserverPatternDemo {
public static void main(String[] args) {
Subject subject = new Subject();
new HexaObserver(subject);
new OctalObserver(subject);
new BinaryObserver(subject);
System.out.println("First state change: 15");
subject.setState(15);
System.out.println("Second state change: 10");
subject.setState(10);
}
}