设计模式之观察者模式

1、设计思路

观察者模式又称为发布/订阅(Publish/Subscribe)模式。观察者模式提供了一种对象设计，让主题和观察者之间松耦合。主题只知道观察者实现了某个接口（也就是Observer接口）。主题不需要知道观察者具体类是谁、做了些什么或其他任何细节。主题唯一依赖的东西是一个实现Observer接口的对象列表，所以我们可以随时增加观察者和删除观察者。有新类型的观察者出现时，主题的代码不需要修改。

2、UML类关系图

3、代码实现

最形象的栗子是：杂志订阅，杂志是主题，观察者是订阅者。当出版新杂志时候，这个事件会自动通知所有的订阅者。根据OO基本原则，应该针对接口编程，主题和订阅者一般都作为接口。

/**
 * 出版社主题接口对象  
 *
 */
public interface Subject {

	
	public void registerObserver(Observer observer);
	
	public void removeObserver(Observer observer);
	
	public void notifyObservers();
}

/**
 * 具体的出版社对象
 *
 */
public class ConcreteSubject implements Subject {

	private Boolean changeflag;
	private List<Observer> obsList = new ArrayList<>();
	private String bookname;
	private String author;
	
	
	@Override
	public void registerObserver(Observer observer) {
		obsList.add(observer);
	}

	@Override
	public void removeObserver(Observer observer) {
		if(obsList.contains(observer)){
			obsList.remove(observer);
		}
	}

	@Override
	public void notifyObservers() {
		if(changeflag){
			for (Observer observer : obsList) {
				observer.update(bookname,author);
			}
		}

	}
	
	public void publishBook(String bookname,String author,boolean changeflag){
		this.bookname = bookname;
		this.author = author;
		this.changeflag = changeflag;
		notifyObservers();
	}
	
	

	public Boolean getChangeflag() {
		return changeflag;
	}

	public void setChangeflag(Boolean changeflag) {
		this.changeflag = changeflag;
	}

	public List<Observer> getObsList() {
		return obsList;
	}

	public void setObsList(List<Observer> obsList) {
		this.obsList = obsList;
	}

	public String getBookname() {
		return bookname;
	}

	public void setBookname(String bookname) {
		this.bookname = bookname;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}
	
}

/**
 * 观察者
 *
 */
public interface Observer {

	public void update(String bookname, String author);
	
}

/**
 * 具体的观察者
 *
 */
public class ConcreteObserver implements Observer{

	private ConcreteSubject concreteSubject;
	private String readerName;
	

	public ConcreteObserver(ConcreteSubject subject, String reader) {
		super();
		this.concreteSubject = subject;
		this.readerName = reader;
		subject.registerObserver(this);
	}

	
	@Override
	public void update(String bookname,String author) {
		System.out.println("订阅者获得一本新书《"+bookname+"》，作者是："+author);
		
	}

	public ConcreteSubject getConcreteSubject() {
		return concreteSubject;
	}

	public void setConcreteSubject(ConcreteSubject concreteSubject) {
		this.concreteSubject = concreteSubject;
	}

	public String getReaderName() {
		return readerName;
	}

	public void setReaderName(String readerName) {
		this.readerName = readerName;
	}

}

	public static void main(String[] args) {
		
		ConcreteSubject subject = new ConcreteSubject();
		ConcreteObserver observer1 = new ConcreteObserver(subject,"小明");
		ConcreteObserver observer2 = new ConcreteObserver(subject,"小星");
		ConcreteObserver observer3 = new ConcreteObserver(subject,"小宇");
		ConcreteObserver observer4 = new ConcreteObserver(subject,"小志");
		
		subject.publishBook("追风筝的人", "啊啊", true);
		subject.publishBook("深入理解JVM", "嗯嗯", true);
	}

测试结果：