設計模式之觀察者模式

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定義：定義了一種一對多的依賴關係，讓多個觀察都對象同時監聽某一個主題對象。這個主題對象在狀態發生變化時，會通知所有觀察者對象，使它們能夠自動更新自己。

結構圖：

範例程式碼：

public interface Observer {public void update();}public class Subject {private List<Observer> observers = new ArrayList<Observer>();public void attach(Observer observer) {observers.add(observer);}public void detach(Observer observer) {observers.remove(observer);}public void notify1() {for (Observer observer : observers) {observer.update();}}}public class ConcreteSubject extends Subject {private String subjectState;public String getSubjectState() {return subjectState;}public void setSubjectState(String subjectState) {this.subjectState = subjectState;}}public class ConcreteObserver implements Observer {private String name;private String observerState;private ConcreteSubject subject;public ConcreteObserver(ConcreteSubject subject, String name) {this.subject = subject;this.name = name;}@Overridepublic void update() {// TODO Auto-generated method stubobserverState = subject.getSubjectState();System.out.println("觀察者" + name + "狀態是" + observerState);}}

用戶端代碼如下：

public class Client {public static void main(String[] args) {ConcreteSubject s = new ConcreteSubject();s.attach(new ConcreteObserver(s, "x"));s.attach(new ConcreteObserver(s, "Y")); s.attach(new ConcreteObserver(s, "Y"));s.setSubjectState("ABC");s.notify1();}}

運行結果：

觀察者x狀態是ABC

觀察者Y狀態是ABC

觀察者Y狀態是ABC

設計模式之觀察者模式